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Volvox Cellular Reintegration

Volvox Cellular Reintegration



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A couple of days ago, I answered this question over on worldbuilding.SE with some information about volvoxes, and commenter Liam Proven pointed out that one of the claims I made was dubious.

I had first read about the volvox in a book that described their basic biology and life cycle (the book may have been Flatterland by Ian Stewart, but I no longer have a copy to verify). One detail I vaguely remembered was that since volvoxes are essentially colonial organisms, individual cells do not immediately die when the volvox disintegrates, but rather some of these cells can rejoin the cytoplasmic matrix of other colonies, similar to sponges.

Now, I read this a long time ago, and my memory is admittedly foggy, and I never had any university-level training in biology. It's entirely possible I'm conflating this with something completely different I read elsewhere, so Liam's comment sent me searching for more info. And even if I am remembering correctly what I had read, it's possible the original author was mistaken.

The best article I could find on the topic was Cytoplasmic Bridges in Volvox and Its Relatives by Hoops, Nishii, and Kirk, but it doesn't appear to support my recollection (it talks mostly about the inversion process in embryos).

So I thought I'd bring the question to a wider audience. Do individual volvox cells reintegrate after disintegration, or do the mother's cells simply die?


Volvox

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Volvox, genus of some 20 species of freshwater green algae (division Chlorophyta) found worldwide. Volvox form spherical or oval hollow colonies that contain some 500 to 60,000 cells embedded in a gelatinous wall and that are often just visible with the naked eye.

Volvox colonies were first recorded by Dutch microscopist Antonie van Leeuwenhoek in 1700 and are widely studied as a genetic model of morphogenesis (how organisms develop specialized cells and tissues). Volvox also exhibit differentiation between somatic (non-sex cells) and reproductive cells, a phenomenon considered by some biologists to be significant in tracing the evolution of higher animals from microorganisms.

The somatic cells of a Volvox colony each feature two flagella (whiplike appendages), several contractile vacuoles (fluid-regulating organelles), a single chloroplast (the site of photosynthesis), and an eyespot used for light reception. Neighbouring cells are often joined together by strands of cytoplasm, which enable cell-to-cell communication, and the colony moves through water by the coordinated movement of the flagella. The photosynthetic colonies are usually organized so that cells with larger eyespots are grouped at one side to facilitate phototaxis (movement toward light) for photosynthesis, and the reproductive cells are grouped at the opposite side.

Most species of Volvox reproduce both asexually and sexually, and some, such as Volvox carteri, switch primary modes of reproduction at least once each year. Asexual colonies have reproductive cells known as gonidia, which produce small daughter colonies that are eventually released from the parent as they mature. In sexual colonies, developing ova or spermatozoa replace gonidia, and fertilization results in zygotes that form a cyst and are released from the parent colony after its death. Thick-walled zygotes formed late in the summer serve as winter resting stages.

Volvox can be found in ponds, puddles, and bodies of still fresh water throughout the world. As autotrophs, they contribute to the production of oxygen and serve as food for a number of aquatic organisms, especially the microscopic invertebrates called rotifers. One of the most-common species, V. aureus, can form harmful algal blooms in warm waters with a high nitrogen content.

This article was most recently revised and updated by Melissa Petruzzello, Assistant Editor.


Volvox Reproduction

Volvox can reproduce both asexually and sexually. In asexual reproduction, the gonidia develop into new organisms that break out of the parent (which then dies). In sexual reproduction, the presence of an inducing chemical causes

  • The gonidia of the males to develop into clusters of sperm.
  • The gonidia of the females to develop into new spheres each of whose own gonidia develops into a pair of eggs.
  • The sperm break out of the male parent and swim to the female where they fertilize her eggs.
  • The zygotes form a resting stage that enables Volvox to survive harsh conditions.

The genome of Volvox carteri consists of 14,560 protein-encoding genes - only 4 more genes than in the single-celled Chlamydomonas reinhardtii! Most of its genes are also found in Chlamydomonas. The few that are not encode the proteins needed to form the massive extracellular matrix of Volvox.


Unicellular to multicellular: What can the green alga Volvox tell us about the evolution of multicellularity and cellular differentiation?

Despite being 2mm in diameter and only having 2 cell types, green alga Volvox have fascinated biologists for over 300 years and are a model organism for developmental, physiological and evolutionary research. Published today in BMC Biology new research analyzes the whole transcriptome of Volvox carteri by RNA sequencing. Here, lead author of the study, Armin Hallmann, explains how this impacts our understanding of evolution from unicellular to multicellular organisms.

In unicellular organisms, all tasks to survive and reproduce have to be performed by one and the same cell because only one cell forms the entire organism. One of the greatest achievements in the evolution of complex life forms was the transition from unicellular organisms to multicellular organisms with different cell types.

Because the step from unicellular to multicellular life was taken early and frequently, the selective advantage of multicellularity seems to be quite large. In multicellular organisms like humans, a large number of cells form a cooperating cell community with specialized cell types and a division of labor among the various cells. Finding out how unicellular organisms can develop into multicellular organisms over the course of evolution is a central issue in biological research.

The green alga Volvox

The situation appears different for volvocine green algae, such as Volvox carteri, in which multicellularity is a relatively recent innovation.

What’s so interesting about the tiny, spherical green alga Volvox carteri in this context? Even though complex multicellularity evolved several times in eukaryotes, in most lineages it is quite challenging to investigate its molecular background because the transition happened too far in the past and, in addition, these lineages evolved a large number of cell types.

However, the situation appears different for volvocine green algae, such as Volvox carteri, in which multicellularity is a relatively recent innovation. In terms of cellular composition, V. carteri is about as simple as a multicellular organism can be, yet it shares many features that characterize the life cycles and developmental histories of much more complex, higher organisms.

V. carteri only has two cell types: 2000-4000 small, terminally differentiated, biflagellate somatic cells near the surface of the spheroid, and approximately 16 large, potentially immortal reproductive cells just internal to the somatic cell layer. More than 95% of the volume of such a spheroid consists of a complex, but transparent extracellular matrix.

In nature, Volvox lives in freshwater ponds, puddles and ditches. With a diameter of up to 2 mm, you can even recognize the alga with the naked eye. Incidentally, the name Volvox comes from the Latin word volvere, to roll, and -ox, as in atrox, means “fierce”. Thus, in a nutshell, it is the ”fierce roller”.

Volvox and its relatives

Since its discovery just over 300 years ago by Antoni van Leeuwenhoek, Volvox not only fascinated biologists but it also has long since become a model organism for developmental, physiological and evolutionary research.

Its current status was also supported by the fact that Volvox has quite interesting relatives within the volvocine lineage. Several genera of this lineage can be arranged in a conceptual series according to increasing developmental complexity from unicellular Chlamydomonas, to colonial organisms without a division of labor, such as Gonium, Pandorina, Yamagishiella and Eudorina, to multicellular organisms with a partial or full germ–soma division of labor, such as Pleodorina and Volvox, respectively.

In this series there are progressive increases in cell number, organismal polarity, volume of extracellular matrix per cell, size of adult organisms, and the tendency to produce sterile, terminally differentiated somatic cells. The evolution of multicellularity is also considered to be associated with the stepwise transition from isogamy to anisogamy/oogamy (sexual reproduction through fusion of gametes of similar size versus fusion of gametes of dissimilar size).

Thus, the volvocine green algae and particularly V. carteri provide a unique opportunity to study multicellularity and cellular differentiation at the molecular level and to discover universal rules that characterize the transition to differentiated multicellularity.

Sequencing of the nuclear genomes and comparison of genomic features of V. carteri and two of its relatives, Chlamydomonas and Gonium, revealed that a surprisingly low amount of genomic innovation seems to be required for the evolutionary transition from unicellular to complex multicellular algae.

Large scale molecular analyses in Volvox

The genome of V. carteri was shown to have about fourteen thousand genes. The expression of these genes dictates how its cells function and, among quite a few other tasks, it regulates cell differentiation and the separation of germ and soma. Gene expression can be experimentally determined by measuring the amount of transcripts of a certain gene. If this is done not only for a single gene but for all genes at a time, it’s a transcriptome analysis.

In a recent study published in BMC Biology, my colleagues and I analyzed the whole transcriptome of V. carteri by RNA sequencing (RNA-Seq). This allowed us to measure how strong the expression of each gene is or whether it is turned off. Because we separated the cell-types of Volvox before we did the analysis, we were also able to compare the expression of all genes between the two cell types, i.e., somatic cells and reproductive cells.

Our results demonstrate a surprisingly extensive compartmentalization of the transcriptome between cell types: More than half of all genes show a clear difference in expression between somatic and reproductive cells. This high degree of differential expression indicates a strong differentiation of cell types in spite of the fact that V. carteri diverged relatively recently from its unicellular relatives. The analysis of cell-type specific gene expression also made it possible to provide new information about the expression pattern of previously investigated Volvox genes (roughly four hundred genes in number).

The analysis of the whole Volvox transcriptome of separated cell types is a major step towards understanding the molecular “toolbox” underlying the evolution from unicellular to multicellular organisms. In the long term, the study of molecular processes in simple organisms should lead us to a better understanding of the developmental history and key functions of far more complex life forms.


Volvox: Occurrence, Features and Life Cycle

Volvox is a colonial alga, it grows in fresh water of pools, ponds etc. It is represented by about 20 species. Single colony looks like a small ball about 0.5 mm in diameter. In rainy season the colour of the ponds becomes greenish due to rapid growth of Volvox.

Plant Body of Volvox:

Plant body of Volvox (L. volvere, the roll) is a coenobium, like a hollow sphere of gelatinous substance (Fig. 3.52A, B). In the hollow sphere, huge number of cells are arranged towards periphery in a single layer (Fig. 3.52C, D). The number of cell varies from species to species (500-1,000 in K aurens, 2,000-3,000 in V. rousseletii) and it ranges from 500-60,000.

Individual cell is typically like Chlamydo­monas (except a few like V. globator and V. rousseleti, those are Sphaerella type). The cells are spherical in shape having cup-shaped chloroplast, with one or more pyrenoid, an eye-spot, 2-6 con­tractile vacuoles and a single nucleus.

Each cell has two equal flagella placed anteriorly (Fig. 3.52D). Thus the coenobium is the aggregation of a number of Chlamydomonas-like cells. But indi­vidual cell performs its own metabolic functions like photosynthesis, respiration, nutrition, excre­tion etc.

Adjacent cells remain connected by cyto­plasmic strands formed during cell division (Fig. 3.52C). In some species like C. tertius, C. mononae, cytoplasmic thread is absent. The central region of the coenobium is generally hollow but in some cases it is filled with gelatinous material (V. aureus) or water (V. globator).

The cells of the anterior region have large eye-spots than the posterior region, indicating the clear polarity in the coenobium.

Important Features of Volvox:

1. Plant body is coenobium and consists of large number of biflagellate, pear-shaped cells.

2. The cells of the coenobium are connec­ted together by means of protoplasmic strands.

3. Young coenobia consist of only vegetative cells and are concerned with locomotion and food production.

4. Older coenobium consists of vegetative cells, daughter coenobia and antherozoid mother cells and/or ovum mother cells.

5. Sexual reproduction is oogamous and the coenobia may be monoecious or dioe­cious.

6. The female gametes or ova are large and non-motile, produced singly inside the oogonium.

7. The male gametes or sperms are spindle- shaped, narrow with a pair of apical cilia and are produced in bunch inside the antheridium.

8. The result of sexual union is the zygote, which on germination develop into new coenobium either directly or by the forma­tion of single biflagellate zoospore.

Reproduction of Volvox:

Volvox reproduces both asexually and sexu­ally. Asexual reproduction takes place during favourable condition, but the sexual reproduc­tion occurs during unfavourable condition i.e., towards the end of the summer months.

Asexual Reproduction:

A few cells at the posterior side of the coenobium enlarge about 10 times. The cells withdraw their flagella and become more or less round. They are pushed inside the colony during their development. These cells are called gonidia (Fig. 3.53A) or parthenogonidia or autocolony initials. The gonidium is separated from the vegetative cells by its position and size.

Development of Daughter Colony:

The goni­dium undergoes repeated divisions of about 15 or more times and can develop more than 3,200 cells. Those cells ultimately form a colony.

Initially the gonidium undergoes longitudi­nal division with respect to the colony and form 2 cells (Fig. 3.53B), The second division is at right angle to the first one and forms 4 celled stage (Fig. 3.53C). These ceils again divide longi­tudinally (3rd division) and form 8 celled stage. The cells are arranged in such a pattern that their concave inner surface faces towards the outer side of the colony.

This stage is called plakea stage or cruciate plate (Fig. 3.53D). The 4th divi­sion forms 16 celled stage (Fig. 3.53E) and at that time it becomes a hollow sphere with an open­ing towards the outer side, called phialopore.

The division of cells continues up to the number specific for a particular species. The cells now face towards the centre (Fig. 3.53F). This group of cells then undergoes inversion through the phialopore, by which normal pattern of the colony is achieved.

During inversion a constriction appears at a point opposite to phialopore. This constricted region becomes pushed gradually towards the phialopore (Fig. 3.53G). Simul­taneously the phialopore becomes enlarged, through which the lower part comes out and the edges of phialopore hang backwards.

With the help of inversion the anterior side of the cells changes their position from inner to the outer side and the position of phialopore becomes reversed i.e., changes its position from outer to inner side (Fig. 3.53H).

The phialopore gradually closes down and a complete hollow sphere is formed. After completion of inversion, the cells secrete their own gelatinous cell wall and each develops two flagella. Thus the daughter colony is formed.

Many such colonies may develop in a coenobium and they swim freely inside the gela­tinous matrix of the mother coenobium (Fig. 3.52B). Later on the daughter coenobia come out by rupture or disintegration of the mother colony. In some species like V. carteri and V. africanus daughter colonies of 2-4 generations may remain within the mother coenobium.

Sexual Reproduction:

Volvox reproduces sexually during unfavou­rable condition i.e., towards the end of growing season (late summer). The sexual reproduction is oogamous. Some species (V. globa­tor) is monoecious and others (V. aureus) are dioecious.

Most of the monoecious species are of protandrous type (i.e., antheridia develop and mature earlier than oogonium). Some cells of the posterior region of the colony withdraw their flagella and develop into reproductive bodies called gametangia. The male gametangia are called antheridia and the female as oogonia.

Development of Antheridium:

During deve­lopment of antheridium, an antheridial initial becomes differentiated from the colony. It is like a gonidium, which is aflagellated, larger in size than vegetative cells and contains dense cyto­plasm with a single nucleus.

The cell undergoes repeated longitudinal divisions like the asexual stage and forms generally about 64-128 cells (though the number varies from 16-512, depen­ding on species). Like the asexual stage the cells are arranged in groups and then undergo inver­sion by which the anterior side of the cells faces towards outer side (Fig. 3.54).

Each cell develops into unicellular, elonga­ted, fusiform, naked and biflagellate anthero­zoid. The antherozoids are released individu­ally. In some species they are also released in groups.

Development of Oogonium:

Single vege­tative cell of the colony at the posterior side withdraws its flagella, enlarges in size and become a more or less flask-shaped oogonium. The entire protoplast without undergoing any division, forms an uninucleate non-flagellated egg or female gametophyte (Fig. 3.55A).

The egg or female gametophyte is spherical, uninucleate, non-flagellated, green in colour and has parietal chloroplast. It has many pyrenoids and large amount of reserve food. The mouth of the flask- shaped oogonium opens towards the outer surface of the colony.

After maturation, the anthrozoids (= sper- matozoids) are liberated from the antheridium either singly or in mass. They move in water and get attracted by the chemotactic stimu­lation to the surface of the oogonium.

A few antherozoids enter near the egg (Fig. 3.55B) by breaking the oogonial wall with the help of proteolytic enzyme probably secreted by the antherozoids. Out of many antherozoids
entered into the oogonium only one succeeds to fertilise the egg and forms a zygote.

The zygote secretes a thick wall around itself (Fig. 3.55C). It accumulates the haematochrome and becomes red in colour. The wall of the zygote may be smoothly, (V. monanae, V. globator etc.) or spiny (V. spermatophora etc.). Zygote is liberated by the disintegration of the mother wall and remains dormant for a long period.

Germination of Zygote:

During favourable condition the zygote germinates. Before germination, the diploid (2n) nucleus (Fig. 3.56A) of the zygote undergoes meiotic division and forms 4 haploid cells (Fig. 3.56B, C).

Further development of zygote varies with species:

1. In V. minor and K aureus, after meiotic division the cells undergo repeated mitotic division and form a new colony as formed during asexual reproduction (Fig. 3.56D, E and F).

2. In V. rousseletii, out of 4 haploid cells generally only one survives. The outer wall (exospore) of the zygote breaks and the inner wall (endospore) comes out in the form of vesicle containing a single biflagellate meiospore.

The meiospore is then liberated in the water by breaking the inner wall i.e., endospore. The biflagellate meiospore then undergoes divisions like the development of daughter colony during asexual process and forms new coenobium.

3. In V. campensis, out of many zoospores formed in the oogonium by zygotic division only one survives and others degenerate. The surviving one comes out and by repea­ted mitotic division it forms a new colony like asexual reproduction.

V. aureus, V. merrille, V. rousseleti, V. africanus, V. globator and V. prolificus are very com­mon.


Volvox Cellular Reintegration - Biology

Volvox is a genus of multicellular plants. Each individual volvox is a hollow sphere of cells.
Each cell has flagella, and the coordinated beating of these flagella cause the Volvox to swim.
The next generation of volvox (the 'babies' of the next generation) develop from special cells, and grow as smaller, inside-out spheres inside each parent. They are inside-out in the sense that the flagellar end of each cell points inward. When they are mature, they invert right-side-out and break through their mother's surface to be released out into the world.

Notice the similarity between Volvox and a genus of one-celled algae named Chlamydomonas .
These live as individuals, with each cell having two flagella at one and, which swim in a pattern resembling a "breast stroke". There are many different species of Chlamydomonas, and also many different species of Volvox, and there are also many species of some morphological intermediates (that look sort of like smaller Volvox) such as those of the genus Eudorina. DNA sequencing of parts of the genomes of many of these species seems to prove that certain species of Eudorina evolved from certain species of Chlamydomonas,
and they in turn begat Volvox (my use of the verb begat is to assuage Creationists), and that this evolutionary sequence has happened several times, separately. (" Convergent Evolution ")

In other words, species X of Volvox has gene sequences that are more similar to species Q of Chlamydomonas, and sometimes to species B of Eudorina. Whereas species Y of Volvox has gene sequences that are more similar to species R of Chlamydomonas, and to species C of Eudorina.

That is strong evidence that the "Volvox" morphology, shape, appearance and way of life has evolved over and over again from Chlamydomonas (in the distant past, of course), and that the species classified as Eudorina and Pandorina are living examples of intermediate stages in this evolutionary achievement of multicellularity (this use of the word "achievement" may reflect multicellular chauvinism). Different species of Volvox evolved separately, three or more separate times, always from unicellular algae that would be classified as Chlamydomonas. Nobody watched this happen, any more than they made a time-lapse video of the erosion of the Grand Canyon, but the evidence from the genetic base sequences is clear.

A very different way to become multicellular occurs in a group of "animals" (?) called " Cellular Slime Molds ". Are they fungus-like amoeboid animals? Or are they animal-like ameboid fungi? The best-studied species in this group is Dictyostelium discoideum, which was discovered by a UNC graduate named Kenneth Raper in the 1930s. Hundreds of different research laboratories, all over the world, have concentrated on studying this one species.
They are small unicellular amoebae , that eat bacteria by phagocytosing them.
When they run out of food, these amoebae begin to attract one another by chemotaxis .
These amoebae aggregate into " slugs ", which may contain any where from hundreds of thousands of cells per slug, down to as few as a hundred cells or less. (These are not the same as the snail-like animals called slugs, which are molluscs). You can call one a "pseudoplasmodium" or a "grex". These slugs elongate and crawl around for a few hours, or less, until they find a good spot, and then some of the cells differentiate into stalk cells, and the majority differentiate into spore cells. This process of "fruiting" produces a tall stalk, with an approximately spherical mass of spores at its top end. "Grex" is Latin for herd (gregarious).


Volvox

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  • Publisher: Cambridge University Press
  • Online publication date: December 2009
  • Print publication year: 1997
  • Online ISBN: 9780511529740
  • DOI: https://doi.org/10.1017/CBO9780511529740
  • Subjects: Plant Sciences, Life Sciences, Cell Biology and Developmental Biology
  • Series: Developmental and Cell Biology Series (33)

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Book description

The central thesis of this book is that Volvox and its unicellular and colonial relatives provide a wholly unrivalled opportunity to explore the proximate and ultimate causes underlying the evolution, from unicellular ancestors, of multicellular organisms with fully differentiated cell types. A major portion of the book is devoted to reviewing what is known about the genetic, cellular and molecular basis of development in the most extensively studied species of Volvox: V. cateri, which exhibits a complete division of labour between mortal somatic cells and immortal germ cells. However, this topic has been put in context by first considering the ecological conditions and cytological preconditions that appear to have fostered the evolution of organisms of progressively increasing size and with progressively increasing tendency to produce terminally differentiated somatic cells. The book concludes by raising the question of whether the germensoma dichotomy may have evolved by similar or different genetic pathways in different species of Volvox. Biologists and phycologists interested in development, genetics and cellular evolution will find this a fascinating work.

Reviews

‘I recommend [this book] unreservedly to all phycologists there is something for everyone.’

John Raven Source: European Journal of Phycology

‘This book will be the source to scoop plentifully for a long time.’

Lothar Jaenicke Source: SGM Quarterly

‘In this excellent, well-written and well-documented monograph, Kirk has made a critical compilation of the accumulated literature on Volvox and has encompassed the whole field, the pinnacle of which is his own work.’


The relationship between cell size and cell fate in Volvox carteri

In Volvox carteri development, visibly asymmetric cleavage divisions set apart large embryonic cells that will become asexual reproductive cells (gonidia) from smaller cells that will produce terminally differentiated somatic cells. Three mechanisms have been proposed to explain how asymmetric division leads to cell specification in Volvox: (a) by a direct effect of cell size (or a property derived from it) on cell specification, (b) by segregation of a cytoplasmic factor resembling germ plasm into large cells, and (c) by a combined effect of differences in cytoplasmic quality and cytoplasmic quantity. In this study a variety of V. carteri embryos with genetically and experimentally altered patterns of development were examined in an attempt to distinguish among these hypotheses. No evidence was found for regionally specialized cytoplasm that is essential for gonidial specification. In all cases studied, cells with a diameter > approximately 8 microns at the end of cleavage--no matter where or how these cells had been produced in the embryo--developed as gonidia. Instructive observations in this regard were obtained by three different experimental interventions. (a) When heat shock was used to interrupt cleavage prematurely, so that presumptive somatic cells were left much larger than they normally would be at the end of cleavage, most cells differentiated as gonidia. This result was obtained both with wild-type embryos that had already divided asymmetrically (and should have segregated any cytoplasmic determinants involved in cell specification) and with embryos of a mutant that normally produces only somatic cells. (b) When individual wild-type blastomeres were isolated at the 16-cell stage, both the anterior blastomeres that normally produce two gonidia each and the posterior blastomeres that normally produce no gonidia underwent modified cleavage patterns and each produced an average of one large cell that developed as a gonidium. (c) When large cells were created microsurgically in a region of the embryo that normally makes only somatic cells, these large cells became gonidia. These data argue strongly for a central role of cell size in germ/soma specification in Volvox carteri, but leave open the question of how differences in cell size are actually transduced into differences in gene expression.


Reviews & endorsements

'I recommend [this book] unreservedly to all phycologists there is something for everyone.' John Raven, European Journal of Phycology

'This book will be the source to scoop plentifully for a long time.' Lothar Jaenicke, SGM Quarterly

'In this excellent, well-written and well-documented monograph, Kirk has made a critical compilation of the accumulated literature on Volvox and has encompassed the whole field, the pinnacle of which is his own work.' Lothar Jaenicke, Trends in Microbiology


What Are Some Characteristics of a Volvox?

Characteristics of a Volvox include the fact that each cell contains two flagella, which aids in movement through water, and the colonies reproduce asexually. The cells of a Volvox are held together by protoplasm strands.

The Volvox is a mobile colony of green algae. There are between 500 and 50,000 cells within the colony. These colonies are spherical or oval hollow in nature, and may be larger than a pinhead. The cells within a colony are contained within a gelatinous wall. For movement, each cell contains two flagella, which allows the organism to move through the water. In terms of reproduction, the colonies are asexual and produce daughter colonies within the parent colony. Once the daughter colony is sufficiently mature, the parent colony opens up, releasing the daughter colonies. For food, the Volvox cells contain chlorophyll and are able to make their own food through photosynthesis.


Watch the video: Live embryo turning itself inside out - Volvox embryos morphogenesis (August 2022).