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Are there any examples of a group of micro-organisms where two different, established species are designated and these two groups meet all aspects of the definition of species (perhaps can not reproduce with each other?), but it turns out that there is a continuous distribution of occurences of this group such that they can reproduce "locally" with nearby members?
I lack the sophistication in biology to know the best possible words to use here so some guidance is welcome, and since the concept of species classification has been around much longer than genome sequencing and analysis, the topic itself draws on concepts from several different centuries.
So I've draw a simple diagram with a 1-D simplified "spectrum" of genetic difference, whereas of course it would likely be multi-dimensional.
Symbiotic Relationship between Organisms | Microbiology
The following points highlight the seven main types of symbiotic relationship that exists between organisms. The types are: 1. Mutualism 2. Parasitism 3. Amensalism 4. Competition 5. Predation 6. Proto-Cooperation 7. Commensalism.
Mutualism describes a relationship in which both associated partners derive some benefit, often a vital one, from their living together.
Table 33.1 is an attempt to summarise the main kinds of mutualistic associations some of which are trivial and of scientific interest only but others such as Rhizobium-legume association, mycorrhizae, coral-microbial association, herbivore-microbial association and lichens are very important, or indispensible, both to the local ecosystem and on a world scale.
The most important and the best studied mutualistic association between microorganisms and plants is undoubtedly that between Rhizobium spp. and various legumes.
Mycorrhizae (Sing. Mycorrhiza):
Mycorrhizae represent a mutualistic symbiosis between the root system of higher plants and fungal hyphae. Frank, who first noted the existence of such a characteristic association in the roots of Cupulifereae in 1885, coined the term ‘mycorrhiza’. Over the last 20 years, basic works conducted by hundreds of researchers from different countries has shown that this association is fundamental and universally occurring.
Among the different symbiotic associations between the soil microorganisms and root of plants, mycorrhizae are the most prevalent as they occur on more than 90% of the vascular plants.
However, Kumar and Mahadevan (1984) have studied a large number of mycorrhizal associations and found that they are highly influenced by the toxic substances that, when present, are essentially concentrated in the root of plants. Such substances may be alkaloids, phenolics, terpenoids, tannis, stilbenes, etc.
The mycorrhizae are advantageous because:
(i) The fungus derives nutrients via the root of the plant. Sugars formed in the leaves move down the stem as sucrose. Sucrose itself never accumulates in the fungus it is converted into isomers such as ‘trehalose’ thus resulting in the low sugar concentration,
(ii) The fungal hyphae act like a massive root hair system, scavenging minerals from the soil and supplying them to the plant, and
(iii) Due to this associationship the plant partner, in addition to the nutritional benefits, develops drought resistance, tolerance to pH and temperature extremes, and greater resistance to pathogens due to ‘phytoalexins’ released by the fungus.
Mycorrhizae are generally classified into two types, although a third type that is more or less a combination of the first two is recognized by some. The two major types are termed Ectomycorrhizae and Endomycorrhizae while the third one, however, is referred to as Ectendomycorrhizae.
Ectomycorrhizae (Ectotrophic mycorrhizae):
Ectomycorrhizae (Fig. 33.2) are common on many forest trees, particularly pines, beech and birch which are of much economic value. The fungal hyphae form a sheath over the outside of the roots which is generally called ‘mantle of hyphae’. From this mantle, a hyphal network called hartignet extends into the first few layers of the cortex or rarely deeper and then reaches the endodermis.
Root hair formation is suppressed in the infected root and the root morphology is changed by the repeated formation of short branches with blunt tips and limited growth. Common ectomycorrhizal genera are Basidiomycetes, particularly Agaricales such as Amanita, Tricholoma, Russula, Lactarious, Suillus, Leccinum and Cortinarius’, some Ascomycetes such as the truffles have also been reported.
The fungi of ectomycorrhizae secrete various growth promoting substances such as auxins, cytokinins and gibberellic acids. Nevertheless, they produce some antimicrobial substances which protect the host plant against soil-borne pathogens.
Fungi derive their carbon from the host in the form of glucose, fructose or sucrose which is ultimately converted to manitol, trehalose, and glycogen. These mycorrhizae are known to stimulate plant growth and nutrient uptake in soils of low to moderate fertility.
Ectomycorrhiza (Ectotrophic mycorrhiza):
The mycorrhizae in which the fungal hyphae invade the root cells without forming any external sheath, mantle of hyphae, are called endotrophic mycorrhizae. Usually, some part of invading fungal hyphae lie externally as a loose mass of hyphae but they do not form mantle.
Three types of endomycorrhizae are recognised:
(i) Vasicular arbuscular (VA),
(i) Vesicular-arbuscular (VA) mycorrhizae:
Vesicular-arbuscular (VA) mycorrhizae (Fig. 33.3) represent associations between fungi, mostly the members of Zygomycetes, and a great number of angiosperms such as tropical forest trees, almost all agricultural crops (except rice in paddy fields), and most of the herbs and grasses of tropical and temperate natural ecosystems.
Fungi forming VA mycorrhizae are restricted to only one family, Endogonaceae, of Zygomycetes with two genera, Endogone and Glomus, forming associations with a huge variety of distantly related plants. VA mycorrhizae are especially important because of their widespread occurrence and association with agricultural crops.
In VA mycorrhizae the fungal hyphae develop some special organs, called vesicles and arbuscules, within the root cortical cells. Vesicles are thick-walled, spherical to oval in shape, borne on the tip of the hyphae either in intercellular spaces or in the cortical cells of the root. These vesicles are food storage organs of the fungus.
However, the arbuscules are brush-like dichotomously branched (extensively) haustoria developed within the cortical cells. Though widely distributed geographically, the VA mycorrhizae are not of usual occurrence in continuously flooded sites (Keeley, 1980).
The importance of VA mycorrhizae is in the effects that they have on plant nutrition, especially the immobile elements such as phosphorus. The external hyphae greatly increase the volume of soil and translocate the phosphorus to the roots. Plants are heavily infected with VA mycorrhizal fungi in phosphorus-deficient soils and mycorrhizae are poorly developed when the phosphorus supply is adequate.
It is thus a self-regulating system, increasing phosphorus uptake when this element is in short supply. The phosphorus so absorbed is converted into polyphosphate granules in the hyphae and passed to the arbuscules for ultimate transfer to host plant. Gianinazzi et al. (1979) have demonstrated that transfer of polyphosphate occurs in the presence of acid phosphatase during the life span or scenescence of arbuscule.
In addition to stimulation of phosphorus uptake, mycorrhizal fungi stimulate rooting, growth, and survival of the transplant. Lambert et al. (1979) have studied that the VA mycorrhizae stimulate uptake of zinc, copper, sulfur, and potassium by the plant enhances nodulation in legumes decreases rots caused by fungal pathogen, and root penetration and larval development of nematodes.
(ii) Orchidaceous mycorrhizae:
Orchidaceous mycorrhizae (Fig. 33.4) are very different from VA mycorrhizae. Here the higher plant is temporarily or permanently parasitic on the fungus the latter are mostly from the genus Rhizoctonia with the perfect stages occurring in basidiomycetes and ascomycetes. Orchid seeds are minute (0.3-14 μg), without any significant food reserve.
Some fail to germinate at all unless infected by fungus others germinate, but, development soon ceases unless the seedling becomes infected by the fungus. The hyphae of the fungus penetrate the cells of the cortex and form coils within the cortical cells. These nutrient rich hyphal coils, generally called peletons, then break down making food available to the plant.
How the orchid persuades the fungus to undergo this bizarre self-sacrifice is not known, though the role of “phytoalexin orchinol” has been implicated. However, the degenerating peletons supply the orchid with carbohydrate and probably vitamins and hormones obtained by saprophytic action of fungus outside the root.
Most orchids eventually become green, and so the association between the orchid and the fungus may shift from parasitism to mutualism. Other orchids, e.g., Neottia nidus-avis (bird’s nest orchid) remain acholorophyllus and parasitic on the fungus throughout their life.
(iii) Ericaceous mycorrhizae:
Ericaceous mycorrhizae (Fig. 33.5) are associated with the two families, ericaceae and epacridaceae, in which the fungus forms dense intracellular coil in the outer cortical cells. Earlier, it was believed that the fungus was Phoma sp. but cultural studies proved that it is Pezizella ericae (an ascomycete).
This fungus has tremendous capacity of mineralization and absorption of organic nitrogen, thus it greatly stimulates nitrogen uptake and plant growth even in infertile peat soil. Englander and Hull (1980) have suggested Clavaria spp. as the mycorrhizal fungus of Rhidodendron spp. and Azalea spp. However, this mycorrhizal association result in no development of root hairs as well as the absence of epidermal cells of the root.
Ectendomycorrhizae (Ectendotrophic mycorrhizae):
The mycorrhizae which bear the characteristics of both, ecto- and endomycorrhizae, are categorised by some as ectendomycorrhizae. The fungal partner establishes mantle of hyphae on the surface of the root as well as hyphal coils and haustoria within the invaded cortical cells of the root.
In the forests, ‘Conifer-Boletus-Monotropa’ association represents a well studied example of ectendomycorrhizae. Monotropa, a nonchlorophyllous plant, usually grows near the roots of the conifers in the forests.
It is now well established that a fungus called Boletus forms a common mycorrhizal association between the conifer and Monotropa, though the nature of the association differs with the two plants. Boletus forms ectomycorrhiza with Monotropa and endomycorrhiza with the conifer. The fungus forms a bridge between the two plants.
Syntrophy (Gk. syn = together trophe = nourishment) is such mutualistic interrelationship between two different microorganisms which together degrade some substances (and conserve energy doing it) that neither could degrade separately.
In most cases of syntrophism the nature of a syntrophic reaction involves H2 gas being produced by one partner and being consumed by the other. Thus, syntrophy has also been called interspecies hydrogen transfer.
Following are some examples of syntrophic associations:
(i) Ethanol fermentation to acetate and eventual production of methane (Fig. 33.6.) is a good example. Ethanol oxidizing bacterium ferments ethanol producing H2 which is a valuable electron donor for methanogenesis hence used by a methanogen. When both these reactions are summed, the overall reaction is exergonic (i.e., energy releasing).
Actually, the oxidation of ethanol to acetate plus H2 is energetically unfavourable, the reaction becomes favourable when H2 produced during it is consumed by the methanogens. In this way, both partners thus use the energy released in the coupled reaction of syntrophic association.
(ii) Oxidation of butyric acid to acetic acid plus H2 by the fatty acid-oxidizing syntroph Syntrophomonas is another good example. Syntrophomonas does not grow in a pure culture on butyric acid as the energy released during butyric acid oxidation to acetic acid is highly unfavourable to the bacterium. But, if the hydrogen produced in the reaction is immediately utilized by a syntrophic partner (e.g., methanogen), Syntrophomonas grows luxuriantly in mixed-culture with the H2 consumer.
Butyrate – + 2H2O → 2 Acetate – + H + + 2H2 + Energy (+ 48.2 kJ)
(iii) Syntrophobacter degrades low molecular weight fatty acids (e.g., propionic acid) to produce H2 which, if not consumed, inhibits the growth of the producer. When this hydrogen is immediately consumed by Methanospirillum, the syntrophic partner, the growth rates of both Syntrophobacter and Methanospirillum is stimulated, i.e., both partners are benefited.
Corals are highly productive and yet live in waters that are very poor in nutrients the open ocean may have a net productivity of 50 g cm -2 year -1 whereas coral reefs may produce up to 2500 g cm -2 year -1 . The reasons for this are still not clear.
It is considered that the dinoflagellate symbionts, Gymnodinium microadriaticum and Amphidinium species, are ubiquitous in reef-building corals and they pass atleast 25%, and probably as much as 60 to 70% of their fixed carbon to the animal as glycerol and glucose.
They may also take up nitrate from the water and pass it to the coral in a utilizable form alanine. Cyanobacteria are important on reefs in fixing nitrogen, and they may be free living or symbiotic.
Bacterial symbionts living on the outside of the coral in mucilage layer have also been implicated in the conservation and rapid recycling of phosphorus and nitrogen to the corals. There are, therefore, a number of potential or actual symbiotic microorganisms which could account for the productivity of coral reefs.
Apart from the productivity aspects, Gymnodinium is also very important in depositing skeletal calcium as a result of photosynthesis.
The dinoflagellates apparently have to reinfect each generation of corals for they are not passed on during reproduction. How they do this is not known for they have never been found free living in the environment, though they can grow independently in culture.
Plants contain about 30% cellulose (dry weight), the large insoluble inert polysaccharide. It would be very much to the advantage of any herbivore to digest the chemical but, however, the only herbivore to possess the appropriate digestive enzyme, cellulase, are snails.
All others, from insects to mammals, do not possess this enzyme and they establish mutualistic associationship with cellullose-splitting bacteria and protozoa. These microorganisms generally occupy one of the several sites in the gut, the most advanced condition being that in ruminants.
Ruminants, such as cow and sheep, have evolved a unique four chambered ‘stomach’ that has helped establish them as extremely successful herbivores (Fig. 33.7). The rumen volume is large compared with size of the mammal (in the cow it is 80-100 L) so that there being a long resistance time for cellulose decomposition. Plant material is chewed, mixed with saliva and passed to rumen.
The rumen contains very numerous microorganisms of which about 90% are cellulase secretors. These may be 10 4 to 10 5 protozoa, 10 10 -10 11 bacteria and 4 x 10 4 fungi. The contents of the rumen are continually mixed by slow contractions of the wall at 1-2 minute intervals.
Microbial Action in the Rumen of Ruminants:
The cellulases hydrolyse the cellulose to glucose, and the microorganisms then ferment the glucose to a variety of organic acids such as acetic acid (ethanoate), butyric acid (butyrate) and propionic acid (propionate), so providing energy for their own growth (Fig. 33.8).
About 10 3 dm 3 per day of CO2 and CH4 are produced as waste products, which are burped out by the animal. Thus the microorganisms present in rumen convert largely indigestible plant material into low molecular weight carbon compounds which can be utilized by the herbivore.
Lichens are remarkable in that under natural conditions the algal-fungal or cyanobacterial-fungal association behaves as a single organism. The fungus (mycobiont) is usually an ascomycete and about 20,000 lichen fungi have been described which is approximately 25% of all known fungi. There are only some 30 genera of algae (photobiont earlier called phycobiont) and cyanobacteria (cyanobiont) known to form lichens.
The relationship between the two associates of the lichen thallus is still not fully confirmed, though lichens have been the classic material for the study of microbial mutualistic symbiosis. The phycobiont/cyanobiont supplies carbohydrate to the mycobiont and the latter may supply minerals to the former.
We have no experimental confirmation that the mycobiont supplies minerals to its associates also, the phycobiont may be able to absorb its own minerals from the substrate. ‘Good’ laboratory conditions cause the association to break down, whilst adverse conditions help to maintain it. This indicates that the association probably enables the associates to exploit habitat which would be unsuitable when they grow apart.
Lichens are considered the ‘pioneer organisms’ as they have been claimed to be important in increasing the rate of soil formation from bare rock. They may accelerate physical destruction of the rock by shrinkage and expansion of the thallus, may decompose the rock by wide range of chemical substances such as carbon dioxide (acting as H2CO3), various organic acids, and chelating agents.
Lichens may accumulate minerals and nitrogen which are eventually released to the primitive soil when the lichen thallus is decayed. Lichens are greatly effected (even killed) by the level of SO2 present in the atmosphere their abundance can be used as an indicator of atmospheric pollution. They or their products may be used as food dyes, and indicators (litmus).
Parasitism represents the symbiotic associationship between two living organisms and is of advantage to one of the associates (parasite) but is harmful to the other (host) to a greater or lesser extent. The parasites may be destructive or balanced. The former destroy the host cells in their later stages of development whereas the latter fulfill their demands from the host in such a way that the host cells are not destroyed but continue to live.
Facultative and Obligate Parasites:
Associations would be easy to describe if organisms always behaved in the same way. Unfortunately, they do not. Many microorganisms, for instance, can survive as both parasites and saprophytes.
The fungus Ceratocystis ulmi, which causes Dutch elm disease, kills the tree and then lives saprophytically on its dead remains. Such an organism which mostly lives as saprophyte but seldom holds the charge of a parasite is referred to as facultative parasite.
In contrast, downy mildews, powdery mildews, etc. only grow on live protoplasm of the host plant in nature. Such as organism which cannot live elsewhere except on the living protoplasm of its host in nature is called obligate parasite (biotroph). Facultative and obligate parasites often differ in their pathogenic effects, i.e., in their ability to injure the host.
Since obligates are restricted to living organisms, their effects on the host are often less severe, although the host may show less vigorous growth. In contrast, facultative parasites which have only recently acquired a host, tend to be more damaging.
When one fungus parasitizes the other, the act is referred to as ‘mycoparasitism’. This term has been generally used interchangeably with ‘hyperparasitism’, ‘direct parasitism’ or ‘interfungus parasitism’. The incitant is generally called ‘mycoparasite’ or ‘hyperparasite’.
Mycoparasitism has been classified into two main groups on the basis of nutritional relationship of parasite with host:
The necrotrophic (destructive) parasite makes contact with its host, excretes a toxic substance which kills the host cells and utilizes the nutrients that are released. The biotrophic (balanced) parasite is able to obtain its nutrients from the living host cells, a relationship that normally exists in nature.
The mycroparasitism is of common occurrence and examples can be found among all the groups of fungi from chytrids to higher basidiomycetes. Few examples are as follows.
A three member mycoparasitic associationship has also been reported in which Chytridium parasiticum is parasite on Chytridium subercrelatum which, too, parasitizes Rhizidium richmondense, another chytrid.
The biological control of plant diseases has recently become an area of intensive research in view of the hazardous impact of pesticides and other agro-chemicals on the ecosystem. Amongst the biological agents, the mycoparasites have attained a significant position.
It has been suggested that efforts should be made to investigate the biological control of plant diseases through parasitism and predation. Therefore, the mycologists and plant pathologists are searching for new mycoparasites because the greater number of these the greater would be the chance of exploiting them as agents for biological control. Trichoderma is an important example.
Amensalism (from the Latin for not at the same table) refers to such an interaction in which one micro­organism releases a specific compound which has a negative effect on another microorganism. That is, the amensalism is a negative microbe-microbe interaction.
Some important examples are the following:
(i) Antibiotic production by a microorganism and inhibiting or killing of other microorganism susceptible to that antibiotic is the most important example of amensalism. Concentrations of such antibiotics in the bulk of soil or water are certainly small, though there could be a large enough quantity on a micro-habitat scale to give inhibition of nearby microorganisms.
The antibiotics reduce the saprophytic survival ability of pathogenic microorganisms in soil. The attini ant- fungal mutualistic relationship is promoted by antibiotic producing bacteria (e.g., Streptomyces) that are maintained in the fungal gardens (see box). In this case, Streptomyces produces an antibiotic which controls Escovopsis, a persistent parasitic fungus, which can destroy the ant’s fungal garden (Fig. 33.9).
(ii) Production of ammonia by some microbial population is deleterious to other microbial populations. Ammonia is produced during the decomposition of proteins and amino acids. A high concentration of ammonia is inhibitory to nitrite oxidizing populations of Nitrobacter.
In contrast to the positive interactions of mutualism and synergism, competition represents a negative relationship between two populations in which both populations are adversely affected with respect to their survival and growth. In this case, the microbial populations compete for a substance which is in short supply.
Competition results in the establishment of dominant microbial population and the exclusion of population of unsuccessful competitors. During decomposition of organic matter the increase in number and activity of microorganisms put heavy demand on limited supply of oxygen, nutrients, space, etc.
The microbes with weak saprophytic survival ability are unable to compete with other soil saprophytes for these requirements and either perish or become dormant by forming resistant structures.
Predation typically occurs when one microorganism, the predator, engulfs and digests another microorganisms, the prey, and the former derives nutrition from the latter. In microbial fraternity, however, the distinction between predation and parasitism is not sharp.
The interaction between Bdellovibrio bacteria and other small gram-negative bacteria is considered by some as predation but by others as parasitism. Bdellovibrio is apparently quite widespread in aquatic habitats and attacks other bacteria, normally gram-negative ones, by boring a hole in the wall, entering the bacterium and causing lysis with the eventual release of many small vibrio-shaped bacteria.
The major microbial predators are the protozoa which may engulf bacteria and more rarely algae and other protozoa. These systems have been used extensively in models and simulations of predator-prey-relationship. In the simplest form the protozoan population (e.g., Tetrahymena) is limited by its bacterial food (e.g., Klebsiella) and numbers of both prey and predator show cyclic oscillations.
Another such example is of Didinium-Paramecium (both protozoa) relationship. Didinium preys on the Paramecium until the population of the later becomes extinct. Lacking a food source, the Didinium population also becomes extinct.
If a few members of the Paramecium population are able to hide and escape predation by the Didinium, then the Paramecium population recovers following the extinction of the Didinium. Thus, a cyclic oscillation can occur in the population of these two protozoans.
Predatory fungi exist and have been considered as possible bio-control agents for some diseases of plants caused by soil microorganisms. Nematodes and protozoa may be trapped by a variety of net-like-hyphae, sticky surfaces and nooses. The organism is then invaded by hyphae and digested.
6. Proto-Cooperation (Synergism):
Proto-cooperation (or synergism), like mutualism, represents an association between two microbial populations in which both populations benefit from each other, but it differs from the mutualism in that the association is not ‘obligatory’.
Both synergistic populations of microbes are able to survive in their natural environment on their own. Proto-cooperation or synergism allows microbial populations to perform metabolic activities such as synthesis of a product which neither population could perform alone.
Following are few examples:
(i) The Desulfolvibrio bacteria supply H2S and CO2 to Chlorobium bacteria and, in turn, the Chlorobium bacteria make sulphate (SO4 – ) and organic material available to Desulfovibrio. Thus the mixture of the two bacterial populations produce much more cellular material than either alone (Fie 33.10A).
(ii) Nocardia populations metabolize cyclohexane resulting in degradation products that are used by Pseudomonas population. The Pseudomonas species produce biotin and growth factors that are required for the growth of Nocardia (Fig. 33.1 OB).
(iii) Azotobacter populations present in soil fix atmospheric nitrogen if they have a sufficient source of organic compounds. Other soil bacterial populations such as Cellulomonas are able to utilize the fixed form of nitrogen and provide the Azotobacter populations with needed organic compounds (Fig. 33.10C).
Commensalism represents a relationship between two microbial populations in which one is benefited and the other remains unaffected (i.e., neither benefited nor harmed). Thus the commensalism is an unidirectional relationship between two microbial populations. It is quite common, frequently based on physical or chemical modifications of the habitat, and is usually not ‘obligatory’ for the two populations involved.
Commensalistic association is often established when one microbial population, during the course of its normal growth and metabolism, modifies the habitat in such a way that the other population is benefited.
Following are some examples:
(i) A disease causing microbial population when opens a lesion on the host surface, if creates an entry- passage for other microbial population that otherwise could not enter and grow in the host tissues. For convenience, Mycobacterium leprae, the causative agent of leprosy, opens lesions on the body- surface and thus allows other pathogens to establish secondary infections.
(ii) When facultative anaerobes utilize oxygen and lower the oxygen content, they create anaerobic habitat which suits the growth of obligate anaerobes because the latter benefit from the metabolic activities of the facultative anaerobes in such a habitat.
On the contrary, the facultative anaerobes remain unaffected. The occurrence of obligate anaerobes within habitats of predominantly aerobic character, such as the oral cavity, is dependent on such commensal relationship.
(iii) Population of Mycobacterium vaccae, while growing on propane cometabolizes (gratuitously oxidizes) cyclohexane to cyclohexanone which is then used by other bacterial population, e.g., Pseudomonas (Fig. 33.11). The latter population is thus benefited since it is unable to oxidise cyclohexane to cyclohexanone. Mycobacterium remains unaffected since it does not assimilate the cyclohexanone.
(iv) Some microbial populations create commensalistic habitat by detoxifying compounds by immobilization. Leptothrix bacteria deposite manganese on their surface. In this way, they reduce manganese concentration in the habitat thus permitting the growth of other microbial populations. If Leptothrix do not act so, the manganese concentration would be toxic to other microbial populations.
MORPHOLOGY AND EVOLUTION OF THE EGG OF OVIPAROUS AMNIOTES
James R. Stewart , in Amniote Origins , 1997
The prevalence of viviparity among lizards and snakes has stimulated interest in placental structure among Squamata. As a result, the development of the extraembryonic membranes is known for a variety of species, most of which are viviparous. Major features of the pattern of development do not differ for the oviparous species that have been studied.
The development of the yolk sac consists of the three phases characteristic of other Reptilia. The primary yolk sac is a bilaminar omphalopleure. A trilaminar omphalopleure is formed as a layer of mesoderm proliferates between the ectodermal and endodermal layers of the bilaminar omphalopleure. The nonvascular trilaminar omphalopleure is vascularized shortly thereafter to form the choriovitelline membrane. Transformation of the yolk sac is progressive as modifications associated with each phase spread outward over the yolk mass. The concentric growth of the choriovitelline membrane over the surface of the yolk is delimited by a vascular ring, the sinus terminalis.
In contrast to all other Reptilia, the course of mesodermal growth in advance of the choriovitelline membrane deviates from the bilaminar omphalopleure and extends into the yolk ( Stewart, 1993 ). This intravitelline mesoderm most commonly forms a sheet that is several cell layers thick and which continues to grow to ultimately form a continuous membrane within the yolk across the entire abembryonic pole. These cells undergo a morphogenetic process similar to that of the mesoderm of the choriovitelline membrane and an extraembryonic coelom, the yolk cleft, is formed. The yolk cleft is lined by splanchnopleure. The outer compartment of yolk that is separated from the yolk mass by this process is termed the isolated yolk mass ( Fig. 3 ). The isolated yolk mass is lined internally by splanchnopleure and externally by the bilaminar omphalopleure. The pattern of growth of the extraembryonic mesoderm has an important effect on the early distribution of blood vessels at the abembryonic pole of the egg. Blood vessels that subsequently form in the abembryonic region of the yolk sac do so in association with intravitelline cells, whereas the bilaminar omphalopleure remains intact and nonvascular at the abembryonic pole. The formation of an isolated yolk mass has been documented in all species of Squamata that have been studied ( Stewart, 1993 ). The subsequent development of this structure following its formation varies among species, but this structural variation has not been linked to specific functional characteristics ( Stewart, 1993 ).
Figure 3 . Extraembryonic membranes of Squamata, based on Elgaria multicarinata ( Stewart, 1985 ).
Development of the region of the yolk sac that transforms into a choriovitelline membrane is similar to other Reptilia. The mesodermal layer separates and an extraembryonic coelom lined externally by somatopleure and internally by splanchnopleure is formed. The vitelline vessels remain associated with the yolk sac splanchnopleure following disruption of the choriovitelline membrane.
The few studies of amniogenesis among squamates are reviewed by Fisk and Tribe (1949) . The most detailed work is on several species of Chamaeleo that are unusual compared to other reptiles in that amniogenesis proceeds concentrically and early in development. The composition and development of the amniochorion for the few species of snakes and lizards that have been described is similar to that of Sphenodon ( Fisk and Tribe, 1949 ).
The allantois of squamates expands to fill the extraembryonic coelom and its outer membrane fuses with the chorion ( Fig. 3 ). Growth of the allantois toward the abembryonic pole of the egg is dependent on the fate of the isolated yolk mass ( Stewart, 1993 ). In some species, the allantois expands as the isolated yolk mass regresses and eventually surrounds the entire yolk mass. In other species, the allantois extends into the yolk cleft.
Notes on Microorganisms | Biology
In this article we have compiled various notes on microorganisms. After reading this article we will have a basic idea about:- 1. Meaning of Microorganisms 2. Origin of Microorganisms 3. Distribution 4. Nature 5. Nutrition 6. Classification 7. Reproduction 8. Importance of the Study.
- Notes on the Meaning of Microorganisms
- Notes on the Origin of Microorganisms
- Notes on the Distribution of Microorganisms
- Notes on the Nature of Microorganisms
- Notes on the Nutrition of Microorganisms
- Notes on the Classification of Microorganisms
- Notes on the Reproduction of Microorganisms
- Notes on the Importance of the Study of Microorganisms
Note # 1. Meaning of Microorganisms:
Microorganisms are microscopic forms of life, include bacteria, fungi, algae, protozoa, and the infectious agents at the borderline of life that are called viruses which are not cellular organisms. They are also known as microbes. The term microbe is taken from the French and means a microscopic organism or microorganism, being usually applied to the pathogenic forms.
Again the term germ, in popular usage, refers to any microorganism but especially to one of the pathogenic or disease-producing bacteria. Hence the terms microbe and germ are probably synonymous with bacterium (pi. bacteria). A microorganism that harms its host is also called a pathogen. Microorganisms differ widely in form and life cycle.
Some are single-celled, but others are multicellular. Some of them have no well-organized nucleus, but others do. Some of the microorganisms are predominantly plant-like, others are animal-like, again others share characteristics common to both plants and animals. They grow rapidly and reproduce at an unusually high rate, within 24 hours some of them com­plete almost 100 generations.
Microorganisms have pattern of metabolic processes like that of higher plants and animals. Regardless of the complexity of structure of a microorganism, the cell is the basic structural unit of life. All living cells are funda­mentally similar.
Microorganisms may or may not contain chlorophyll and can use inorganic or organic carbon as the source of carbon. Some have the unique ability of utilizing either radiant or chemical energy and may be autophytes or heterophytes.
Again some are able to utilize atmospheric nitrogen for the synthesis of proteins and other complex organic nitrogenous compounds. Some microorganisms synthesize all their vitamins, while others need to be furnished with vitamins.
Microorganisms play an important and often dominant role in all fields of human endeavour like industry, agriculture, problems connected with food, shelter and clothing, and in the conser­vation of human health and combating diseases.
Note # 2. Origin of Microorganisms:
Microorganisms originate from the earth’s surface and are dispersed in nature through various agencies like air, water, rain, insects and various other media. They occur nearly everywhere in nature where they find food, moisture, and a temperature suitable for their growth and multiplication.
Some are found in the bottom of the ocean and others miles away on the mountain heights, thereby can stand wide range of temperature conditions. Microorganisms occur in water (fresh and marine), stagnant and/or free flowing.
Since they thrive under conditions suitable for human survival, it is inevitable that we live among a multitude of microorganisms. They infest the air we breathe and the food we eat.
Note # 3. Distribution of Microorganisms:
Microorganisms inhabit on our body surfaces, in our alimentary tract, in our mouth, nose and all other body orifices. Fortunately most microorganisms are harmless to us and we have means of resisting invasion by those that are potentially harmful.
The microbial population in our environment is both large and complex. For example, a single sneeze may disperse approximately from 10,000 to 100,000 bacteria. One gram of faces may contain millions of bacteria. Our environment—air, soil, water—likewise consists of a menagerie of bacteria and other microbes.
Soil micro­organisms include many species of bacteria, fungi, algae, protozoa and viruses. A study of the microorganisms in these habitats requires specialized knowledge of the specific microbes present.
The bacterial population of the soil exceeds the population of all other groups of microorganisms in both number and variety. Several billions of bacteria per gram of soil have been reported by direct count method.
Note # 4. Nature of Microorganisms:
All microorganisms of the aquatic environment have patterns of seasonal growth which again are controlled by the nature of water— fresh or marine and stagnant or free flowing. As such it is extremely difficult to enlist the nature of aquatic microorganisms.
Still a very broad idea is given in the following manner:
i. Fresh water planktonic microorganisms include primarily the producer orga­nisms (photosynthetic algae) —green and blue-green algae, diatoms, pigmented bacteria, etc.
Whereas, marine planktonic ones are: diatoms, blue-green algae, dinoflagellates, chlamydomonads species of the genera Pseudomonas, Vibrio, Flavobac- terium, and Achromobacter which give protection against lethal portion of solar radiation protozoa—species of Foraminifera and Radiolaria, and many flagellated and ciliated species.
The characteristic colour of the Red Sea is associated with heavy blooms of a blue-green alga, Oscillatoria erythraea. The chytrid Rhizophidium, small animals and viruses parasitize on planktonic algae causing profound effect on the algal population.
ii. In the benthos epilithic forms include many diatoms (e.g., species of the genera Synedra, Meridion, Licmorphora) species of Chamaesiphon, Rivularia, Chaetophora, Cladophora, Sphacelaria and many microscopic Rhodophytes. Some produce mucilage in which filaments are embedded whilst others precipitate Calcium carbonate around themselves and build up nodular growths.
Again some small algal cells are attached amongst the coating of bacteria on both fresh water and marine sand grains— epiplasmmic flora—the majority belong to the Bacillariophyta, though some of Cyanophyta and Chlorophyta. Besides this, small Crustacea bear epizooic algal populations.
iii. Both pigmented and nonpigmented bacteria including species of Pseudomonas, Chromobacterium, Achromobacter, Flavobacterium and Micrococcus are very common in less polluted water. Bacteria washed into water are mainly species of Bacillus (aerobic) and Clostridium (anaerobic).
In water polluted with animal or human excreta some of the common bacteria are: Escherichia coli, Streptococcus faecalis, Proteus vulgaris, Clostridium perfringens, Salmonella typhi and Vibrio cholerae. Where oxygen is available, a wide range of aquatic fungi is available of which chytrids are most important.
Some more common ones are the species of Mycosphaerella, Ceriosporopsis, Saprolegnia, Monoblepharis. Where oxygen is scarce, species of Clathrosphaerina and Helicodendron may be present. Aquatic protozoa are common in both fresh are salt waters. Planktonic protozoa (ciliates, flagellates, and the Heliozoa) and grazing species of other animals fluctuate with the nature of phytoplankon.
Pollution of Natural Waters:
The increasing use of fertilizers and disposal of wastes in streams are resulting in rapidly increasing in nutrient content in many natural water bodies. Enormous algal crops result, followed by their decay giving massive pollution of water. Similar problems of excessive algal growth arise in industrial plants, e.g., in power station cooling towers, and ponds.
The degree of pollution of both fresh and marine waters can be determined by studying the algal growth. Benthic algae are very sensitive indicators. In the extremely polluted zones only algae such as: Oscillatoria chlorina, and Spirulina jenneri occur and in the slightly less polluted zones the diatoms Nitzschia palea and Gamphonema parvulum appear.
The recent growth in number of nuclear power stations, from which large amount of excess heat has to be dissipated, is giving cause for concern over ‘heat pollution’ of rivers and estuaries. It causes direct danger to fish and microorganisms, thereby may disrupt the food chain.
Note # 5. Nutrition of Microorganisms:
Microorganisms are autotrophs and heterotrophs mesophiles, thermnphiles, and psychrophiles aerobes and anaerobes cellulose digesters and sulfur oxidizers nitrogen fixers and protein digesters and other kinds of bacteria in soil which are yet to be discovered.
Large number of actinomycetes, as many as millions per gram, is present in dry warm soils. The most predominant genera of this group are Nocardia, Streptomyces, and Micromonospora which are responsible for the characteristic musty or earthy odour of a freshly ploughed field. They are capable of degrading many complex organic substances and consequently play an important role in building soil fertility.
Fungi are most abundant near the soil surface where an aerobic condition is likely to prevail. They exist in both the mycelial and spore stage. It is difficult to estimate their numbers. Fungi are active in decomposing the major constituents of plant tissues, namely, cellulose, lignin, and pectin. The physical structure of soil is improved by the accumulation of fungal mycelium within it.
The population of algae in soil is generally smaller than that of either bacteria or fungi. The major types present are the green algae and diatoms. Their photosynthetic nature accounts for their predominance on the surface and just below the surface layer of soil. On barren and eroded lands algae may initiate the accumulation of organic matter because of their ability to carry out photosynthesis and other metabolic acti­vities.
The blue-green algae are known to grow on the surfaces of freshly exposed rocks where the accumulation of their cells results in simultaneous deposition of organic matter. This establishes a nutrient base that will support bacterial species. The growth and activities of the initial algae and bacteria pave the way for the growth of other bacteria and fungi.
The mineral nutrients of the rock are slowly dissolved by acids resulting from microbial metabolism. This process continues with a gradual accumu­lation of organic matter and dissolved minerals until a condition results that supports growth of lichens, then mosses, then higher plants. The blue-green algae play a key role in the transformation of rock to soil, a first step in rock-plant succession.
Most soil protozoa are flagellates or arnoebas. Their number per gram of soil ranges from a few hundred to several hundred thousand in moist soils rich in organic matter. Their mode of nutrition involves ingestion of bacteria with some selectivity. Hence protozoa is one of the factors in maintaining equilibrium of microorganisms in soil.
Plant and animal viruses, as well as bacterial viruses (bacteriophages) gain entry into soil microbial population through plant and animal wastes.
In general, some microorganisms survive utilizing solar energy, others use various chemical substances as their fuel, again others have special mechanism of livelihood. As such, microorganisms may behave either as autophytes, as saprophytes, as parasites, or as symbionts.
Microorganisms bring about in nature many changes some desirable (beneficial) and others undesirable (harmful) to us. The diversity of their activities ranges from the enhancement of soil fertility by decomposing dead organic matter, transforma­tion and deposition of minerals in the soil, formation of coal, production of various useful substances, causing diseases of humans and animals and plants to reducing soil fertility and various other activities.
Note # 6. Classification of Microorganisms:
The subject of microbial classification has a long history. One of the first systems was proposed in 1773, and many more appeared since then. Until the eighteenth century, the living organisms were classified into two Kingdoms: plant and animal. Since there are organisms which are typically neither plants nor animals, naturally they do not fall into either plant or the animal Kingdom.
Hence it was proposed that new kingdoms be established to include these organisms. In 1866 E. H. Haeckel, a German zoologist suggested that a third kingdom, Protista, be formed to include those unicellular microorganisms that are typically neither plants nor animals.
These organisms, the protists, include bacteria, algae, fungi and protozoa. Since viruses are not cellular organisms, hence they were not included with the protists.
He referred bacteria to as lower protists and called algae, fungi and protozoa higher protists. But Haeckel’s concept of Kingdom Protista failed to put forward criteria that could be used to distinguish a bacterium from a yeast or certain microscopic algae. Late in the 1940s with the help of powerful magnification provided by electron microscopy it was possible to make more definitive observation of internal cell structure.
It was dis­covered that in some cell, particularly in bacteria, the nuclear substance was not enclosed by a nuclear membrane. Whereas, in other cells, for example typical algae and fungi, the nucleus was enclosed in a membrane. This discovery enabled to sepa­rate one group of protists (bacteria) without membrane bound nucleus from all the others (algae, fungi, protozoa) which possess membrane bound nucleus.
These two cell types have been designated as: procaryotic and eucaryotic and organisms of each cell type are called procaryotes and eucaryotes respectively. Hence bacteria are procaryotic organisms and eucaryotic organisms include algae, fungi and protozoa.
A comprehensive system of classification, designated as five-Kingdom system, was proposed by R. H. Whittaker (1969). This system of classification is based on three levels of cellular organization which evolved to accommodate three principal modes of nutrition: photosynthesis, absorption, and ingestion. The procaryotes are included in the Kingdom Monera, they lack the ingestive mode of nutrition.
Unicellular eucaryotic microorganisms are placed in the Kingdom Protista, all nutritional types are represented here. It includes microalgae whose mode of nutrition is photosynthetic and protozoa with ingestive mode of nutrition.
The multicellular and multinucleate eucaryotic organisms are found in the Kingdoms Plantae (multicellular green plants and higher algae), Animalia (multicellular animals), and Fungi (higher fungi). The diversified modes of nutrition lead to a more diversified cellular organization of the five kingdoms: Monera (bacteria and cyanobacteria), Protista (microalgae and protozoa) and Fungi (yeasts and molds).
The scheme of classification is pre­sented below:
AA. Multicellular organisms
B. Higher algae and green plants.. Plantae
Bergey’s Manual of Determinative Bacteriology in 8th edition (1974) has recogni­zed the Kingdom Monera, but has called it Kingdom Procaryotae because of the procaryotic nature of the cells. This Kindgom is divided into two divisions: the blue- green algae or cyanobacteria (some microbiologists consider the blue-green algae as bacteria) and bacteria (procaryotic organisms other than blue-green algae).
In 1984 the scope of Bergey’s Manual was greatly broadened by bringing together various other information concerning bacterial classification and identification.
It was published under a new name—Bergey’s Manual of Systematic Bacteriology in which all bacteria were placed in the Kingdom procaryotae which has been divided into 4 divisions as follows:
Procaryotes with a complex cell wall structure -Gram-negative bacteria
Procaryotes with a cell wall structure characteristic of Gram-positive bacteria.
Procaryotes that lack a cell wall.
Procaryotes with characteristics of an earlier phylogenetic origin than above Divisions.
The study of cell organization by the electron microscope and by advanced bio­chemical techniques has revealed fundamental resemblances and differences between various microorganisms and offers a sound basis for their division into major groups (i.e. procaryotes and eucaryotes, and viruses) and for their farther subdivision.
L. E. Hawker and A. H. Linton (1978) included under the Kingdom Protista the so-called microorganisms (both procaryotic and eucaryotic) having relatively lowly differentiated body and subdivided it in the following manner.
A. Organization subcellular Viruses
AA. Thallus unicellulr, multicellular, or plasmodial
B. Nucleoplasm not bounded by a membrane Prokaryota
C. Chlorophyll absent, or if present of type different from that of plants
CC. Chlorophyll present, together with charac­teristic blue-green pigment, pigments not located in discrete plastids
BB. Cells or plasmoclia containing one or more discrete membrane-bounded nuclei
D. Cell(s) of vegetative thallus (with a few exceptions possessing a cell wall(s)
E. Chlorophyll present and located in discrete chloroplasts
DD. Gell(s) of vegetative thallus lacking true cell walls
F. Thallus unicellular, remaining so Protozoa
FF. Thallus unicellular at first, becoming a plasmodium or pseudoplasmodium and even­tually forming a fructification Slime molds
In Bergey’s Manual of Systematic Bacteriology (1984) the Cyanophyta (Blue- green algae) have been placed in volume 3—Bacteria with unusual properties in the Section: Oxygenic Phototrophic Bacteria. They were designated as the Cyanobacteria as they contain chlorophyll, can use light as an energy source, and evolve O2 in a manner similar to that of green plants.
This arrangement has not been accepted by the algologists. According to them, the Cyanophyta (Blue-green algae) have closer affinities with algae than bacteria and as such they should be placed under the group Algae and not under Bacteria.
Note # 7. Reproduction of Microorganisms:
Microorganisms reproduce by the simple method of binary fission. Besides this, there are other methods by which they reproduce.
Note # 8. Importance of the Study of Microorganisms:
Microorganisms though microscopic in size are most interesting life forms which attracted attention of workers through ages. They possess immense potentialities which are continuously being explored for the welfare of human society. Not only that they are most fascinating group of organisms, but also have a very simple life form, easy to handle, and are pliable.
With increase in know ledge of the microbial world, the task of befriending the microbial regime—particularly bacterial has now become an urgent agendum in the struggle for existence of the human beings together with the necessary organisms. Following are some of the essential aspects of human society that have opened up based on the study of microorganisms and have emphasized the importance of their study.
Plant Genetic Engineering Towards the Third Millennium
A strategy for the search of other Bacillus thuringiensis strains carrying novel δ-endotoxin genes was designed in our laboratory. Introduction of δ-endotoxin genes coding for insecticidal proteins differing in their RWW midgut receptor might both enhance efficiency of control and delay appearing of insect resistance. The introduction into rice of insecticidal genes with different mode of action, such as protease inhibitors or lectins, might also contribute to this purpose.
Since some pests of economical importance for rice in Cuba are not of the chewing type, eg. the stinkbug Oebalus insularis, the Homopteran Togasodes oryzicola, and the mite Steneotarsonemus spinki, the potential of insecticidal molecules different from δ-endotoxins of B. thuringiensis need to be explored.
Besides chitinase, glucanase and PRs genes, other antifungal genes are envisaged to be introduced into rice for ensuring a longer lasting effectiveness of transgenic plants in the field. Among these, genes coding for systemic acquired resistance (SAR) would be of outstanding importance.
The development of expression systems for specific expression of the desired proteins in particular tissues, organs, cell compartments, and plant developmental stages will be necessary to cope with the expectations created around transgenic rice plants.
In particular, it would be very convenient to set up systems capable of diminish the risk of gene escaping through the pollen from transgenic plants to sexually compatible wild or relative rice species, and/or controlling the viability of potential undesired hybrids. These is a technical development specially appropriated when creating transgenic plants carrying genes and could confer competitive advantage if unintentionally bred into natural populations.
Transformation of local rice cultivars with genes targeted to improve the defense of the plant against the main environmental stresses limiting rice yields in Cuba (cold, drought and salinity) would enable biotechnology to help breeders to complement rice breeding for attaining genetic goals not feasible using conventional techniques.
The introduction of genes capable of simplifying the technology for production of hybrid seed or providing rice cultivars with the uniqueness of perpetuating the advantages of hybrids will means a breakthrough for rice production in Cuba.
4. Levels in Biological Theory
Besides the more philosophical debates discussed above, levels of organization also play an important conceptual role in biological research and theory. Interestingly, this growing body of literature on levels in evolutionary biology is almost entirely disconnected from the debates on levels in philosophy of science discussed above.
A prominent example is the issue of levels of selection. In this debate, the hierarchical organization of nature into levels is an important background assumption, as the aim is to find out at which level(s) of the biological hierarchy natural selection is taking place (Griesemer 2000 Okasha 2006). Although Darwin&rsquos original account was focused on evolution at the level of organisms, arguably the conditions for natural selection can be formulated abstractly without referring to any specific kinds of entities, which allows for natural selection to operate at any level where the conditions are satisfied (Griesemer 2000 Lewontin 1970). Since the 1970s, the debate on levels of selection has kept on growing and extending to different areas, though no precise consensus has been reached. Positions range from the gene-centered view, where natural selection is taken to operate almost exclusively at the level of genes (e.g., Dawkins 1976 Williams 1966), to the pluralistic multilevel selection theory, which allows for natural selection to operate on any level of the biological hierarchy where we find the right kind of units (e.g., Sober & Wilson 1998 Wilson & Wilson 2008).
One branch of the levels of selection debate that is particularly interesting from the point of view of levels of organization is the issue of evolutionary transitions. Here the focus is on the emergence of new levels of organization through evolutionary processes (Buss 1987: ch. 5 Griesemer 2000 Maynard Smith & Szathmáry 1995 Okasha 2006). The background idea is that the complex hierarchical organization of nature that we observe today must itself be a result of evolution, and therefore requires an evolutionary explanation. For example, somehow prokaryotes evolved to eukaryotic cells, single-celled organisms evolved to multicellular organisms, individual animals evolved to colonies, and so on. In their highly influential book, Maynard Smith and Szathmáry (1995) proposed that the characteristic feature of major evolutionary transitions is that entities that were capable of replicating independently before the transitions are only capable of replicating as parts of higher-level wholes after the transitions (see also Buss 1987). For example, after a single-celled organism has evolved into a multicellular organism, the cells of the organism can no longer replicate independently of the organism as a whole. For more on levels of selection and evolutionary transitions, see Okasha (2006) and the entry units and levels of selection.
In these debates, the notion of levels of organization is typically used as a primitive term that is assumed to be clear enough and is therefore left undefined (Griesemer 2005). A notable exception is Okasha (2006), who puts forward a proposal for understanding levels of organization in natural selection, building on earlier work by McShea (2001). The starting point is the part-whole relationship, which is the standard definitive feature of levels of organization, but taken alone is insufficient for defining levels of organization: A big heap of sand is made up of smaller heaps of sands, but does not constitute a higher level of organization. Therefore, Okasha (2006) and McShea (2001) propose two further conditions: The parts that form higher-level wholes must interact with each other, and they must be homologous with organisms in a free-living state. For example, the cells that compose organisms interact with each other and are homologous to free-living unicellular organisms, and therefore constitute a level of organization. On the other hand, candidates such as heaps of sand are ruled out, as they do not significantly interact with each other, nor are they homologous to free-living organisms.
Another context in biology where the nature of levels has received explicit attention is the &ldquohierarchy theory of evolution&rdquo developed by Niles Eldredge and colleagues (Eldredge et al. 2016, Vrba & Eldredge 1984). In this theoretical framework, levels and hierarchies are taken to be fundamentally important ontological features of nature:
Biological evolutionary theory is ontologically committed to the existence of nested hierarchies in nature and attempts to explain natural phenomena as a product of complex dynamics of real hierarchical systems. (Tëmkin & Eldredge 2015: 184)
In the hierarchy theory of evolution, a distinction is made between two types of hierarchies and the corresponding levels (Eldredge 1996 Vrba & Eldredge 1984): The ecological and the genealogical hierarchy. In both kinds of hierarchies, higher-level things are formed through specific interactions among lower-level things. In the ecological hierarchy, these interactions are exchanges of matter and energy, such as consuming and gathering resources, and in this hierarchy, we find things such as cells, organisms, avatars and ecosystems. In the genealogical hierarchy, the defining activity through which levels are formed is the transmission of information through replication, and the hierarchy includes things such as cells, organisms, demes and species. Note that cells and organisms appear in both hierarchies, but in the ecological hierarchy they are seen as interactors, whereas in the genealogical hierarchy they are seen as replicators.
Accession codes: Geographic range data for New World snakes are archived at Dryad (http://dx.doi.org/10.5061/dryad.qk300) and nucleotide sequences for all newly acquired Sonorini species are deposited in the Genbank nucleotide database under accession codes KU859402 to KU859865.
How to cite this article: Davis Rabosky, A. R. et al. Coral snakes predict the evolution of mimicry across new world snakes. Nat. Commun. 7:11484 doi: 10.1038/ncomms11484 (2016).
Fungi are important organisms that are so distinct from plants and animals that they have been allotted their own classifications of life on earth. Fungi are tremendously important to human society and the planet we live on. They provide fundamental products including foods, medicines, and enzymes important to industry. They are also the unsung heroes of nearly all ecosystems, hidden from view but inseparable from the processes that sustain life on the planet.
In 1969 fungi were first officially recognized as a distinct group. And more recently, using DNA sequences and comparisons of cell structure, we have learned that fungi are in fact more closely related to animals than they are to plants. Superficially, they remind us more of plants than animals because they don't move, but scratch the biological surface just a little and that's just about the only thing they have in common.
No one knows for sure how many species of fungi there are on our planet at this point in time, but what is known is that at least 99,000 species of fungi have been described, and new species are described at the rate of approximately 1200 per year.
Fungi come in many different sizes and shapes, and are divided into three main groups depending on the shape.
A unicellular fungus which includes baker's yeast. Yeast can also be found in pharmacies as probiotic which can help prevent diarrhea. There is also yeast that can be damaging to the human body. When present in the mouth, esophagus, bowel and vagina, it can cause yeast infections in people with low immune systems. If it invades the blood yeast can be fatal.
A multicellular fungi and appear as fuzzy growths. Mold can be both harmful and beneficial. For example, mold was used to produce the antibiotic penicillin. Mold is used to produce cheese. Mold commonly contaminates starchy foods and when certain types of this contamination are ingested, it can cause miscarriages, birth defects, and some cancers. Most commonly, mold appears on old bread, and decaying fruit. Mildew is a mold growth that is visible on plants, walls, leather, paper, cloths, and damp areas. It is easy to see that mold is a fungus that can be both helpful and harmful.
A fleshy, spore-bearing fruiting body of a fungus, typically produced above ground on soil or on its food source. It typically consists of a stem, cap and gills. Some are harmful and some are not. Some mushrooms are edible and have successfully been cultivated for human consumption. A mushroom develops from a nodule, or pinhead, less than two millimeters in diameter. Many species of mushrooms seemingly appear overnight, growing or expanding rapidly. In reality all species of mushrooms take several days to form primordial mushroom fruit bodies, though they do expand rapidly by the absorption of fluids.
Table 1 summarizes our results, and Fig. 2 maps them. Details of the statistical fits, along with maximum-likelihood confidence intervals (CIs) of the estimates, are provided in Materials and Methods and Dataset S1. Overall, in the analysis we present here, we predict 21% of species are missing from our sample of approximately 108,000 species. This estimate is similar to estimates produced for all the monocots (17%) and for a combination of the taxonomically revised nonmonocots (13%) (10).
Currently known patterns of flowering plant species richness as a percent of all species (Left) and patterns when corrected for species predicted to be missing from the taxonomic record (Right). They are broadly similar differences are noted in the text. Although our results do not take area into account, we plot the results in an equal-area projection to help visualize the sometimes confusing relationship between undiscovered species and region. We excluded gray regions (Saharan Africa and the Arabian Peninsula) from the analysis as a result of low numbers of endemic species. Original TDWG regions are shown in Fig. S1. Fig. S3 shows raw estimates of numbers of missing species in each region.
Forty of 50 regions show a marked reduction over time in the number of species described per taxonomist per 5-y interval. This leads to low ratios of numbers of predicted species over numbers of presently known species (Fig. 1 provides the example for Mexico and Central America, where the ratio is 1.17, meaning we predict an additional 17%, a percentage close to the numbers for all species combined). Our model suggests other regions with floras that are relatively well known. These include some biodiversity hotspots including Cuba, southeastern Brazil, India and Sri Lanka, and much of mainland tropical Asia.
Do our estimates change our understanding of the rankings of global biodiversity? Table 1 shows the percentage distribution of known species, as well as for when we include the predicted numbers of species. For ease, we have also added the change in rank. Arrows indicate the direction of this change.
Fig. 2 maps the current (Fig. 2, Left) and predicted (Fig. 2, Right) overall percentages of endemic species around the world. The similarities between panels in Fig. 2, along with the rank change column in Table 1, indicate that existing conservation priorities would not be changed significantly by correcting for missing species (rs = 0.97, P < 0.001). For example, the northern Andes and Atlantic Coast forests of South America, Southern Africa, Australasia, and the islands of tropical Asia will remain major centers of plant endemism, as well as areas under threat.
That said, there are geographic differences between what is currently known and what we predict. The region including Ecuador and Peru houses more than 5% of the species in our sample and is currently ranked as the second richest single area. However, this region is projected to contain 29% of the world's missing species, and by incorporating this datum, we expect this region to overtake the area currently ranked first (Mexico to Panama). Further, we predict that Tanzania and the southern cone of South American countries will become relatively much richer in species when missing species are included.
Additionally, it is of conservation interest to note that South American regions will, on average, increase their ranking by 16% (based on the percentage of total possible moves in rank) and African regions by 10%. In contrast, the tropical mainland Asian regions will fall in rankings on average by 6% and the tropical Asian islands by more than 10%. We note that, although increasing taxonomic knowledge generally implies increasing numbers of known species, there has been at least one exception to the rule. In the Eastern Arc Mountains, better information on species ranges and synonymies caused a downward revision of regional endemic plant species, and necessitated a regrouping of the hotspot region (21). Generally, correcting for missing species, as we do here, will increase the number of species endemic to a region.
Finally, there are other species not endemic to the regions we describe, although, given the rather large ranges of these taxa, it is likely that the pool of unknown species is quite small. Approximately 25% of the species we analyzed occur in two or more of the regions we define, and, as one would expect, such generally widespread species are well known and the rate of taxonomic description is now low (Table 1). Species endemic to oceanic islands also show only slow rates of discovery, perhaps because such places are well explored and have been for a long time (Table 1).
Distribution and abundance
Of the more than 65,000 species, about 30,000 are marine, 5,000 live in fresh water, and 30,000 live on land. In general, oceanic gastropods are most diverse in number of species and in variety of shell structures in tropical waters several hundred species (each represented by a small number of individuals) can be found in a single coral reef habitat. This is in contrast to the Arctic or subarctic coasts, where the few species present are represented by many individuals. A number of deep-sea species are known, and a significant snail fauna is associated with hydrothermal vents. Most marine species have large ranges.
Freshwater snails are common in ponds, streams, marshes, and lakes. Usually only a few species are found at one place, but each species will have a rather wide range. Most species are common and feed on algae or dead plant matter. In a few relatively old river systems and lakes—in particular, Lake Baikal in Siberia, Lake Titicaca in South America, Lake Ohrid on the North Macedonia–Albania border, the Mekong basin in Southeast Asia, and the African Rift lakes—extensive and complex radiations of snails have occurred in recent geologic time, producing a large number of species.
Land snails are marginally, but very successfully, terrestrial. When actively moving, they continuously lose water. During periods when water is unavailable, they retreat into their shells and remain inactive until conditions improve. They hibernate during winter periods, when water is locked into snow or ice, and estivate during periods of summer drought. Land snails have been found above the snow line species of Vitrina crawl on snowbanks in Alpine meadows. Other species inhabit barren deserts where they must remain inactive for years between rains.
Fewer than 10 species live in the same area together across most of North America. On the other hand, in such favourable areas as New Zealand, Jamaica, northeastern India, and the wet forests of Queensland (Australia) 30 to 40 different species can be found together. In some parts of western Europe 20 species can be found together. Only one or two species are found in many desert regions, and they have dramatic feeding specializations.
The local abundance of snails and slugs can be spectacular. Millions of some brackish-water and freshwater species can live on small mud flats. An acre of British farmland may hold 250,000 slugs, and a Panamanian montane forest was estimated to have 7,500,000 land snails per acre. Despite this abundance, snails and slugs often pass unobserved. Land and freshwater species often stay hidden during the day and are active at night. Most marine species as well are nocturnal, and the shells of many of these species are so heavily covered with algae and other encrusting organisms that they may be mistaken for bits of rock.