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Why would a flower evolve such that a particular pollinator cannot pollinate it?

Why would a flower evolve such that a particular pollinator cannot pollinate it?



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I went on an interpretive trail today and came across a wild flower sign. It said that some flower species evolved such that they can not be pollinated by humming birds as they are shaped such that only an insect proboscis could get into it. Why would a flower evolve in such a way? Wouldn't it be advantageous to take any pollinator?

An interpreter in the visitor center suggested it might be that the flower wants to save it's limited pollen for pollinators most likely to visit other families of its species. I'm not completely sure I buy this, as a pollinator willing to visit one flower of a species is likely to visit another flower of the same species. That said, I think he might have been on to something. Alternatively, this link made me think perhaps certain pollinators might damage the flower and it would want to discourage them.

This question goes beyond a flower co-evolving such that a particular organism could specialize in pollinating it; I'm curious why a flower would specifically evolve such that it could not be pollinated by a particular organism.


I think the explanation by the interpreter is most appropriate. The definition of species is a particular category of organisms capable of reproduction among themselves. Thus pollen of a particular species must fall on the stigma of the same species. But pollinators visit all species of flowers (as they never visit Biology Stock Exchange, thus ignorant of the definition of species) which bears nectar and the entrance is big enough to let them in.

The flowers in your question increase their chance of pollination by reducing the varieties of flowers a particular pollinator can visit. Lets make it simpler (or even more difficult for those who don't like mathematics!)…

Suppose there lives 4 humming birds and 4 bees in a oasis… both are too lazy to visit other oasis in search of nectar. One morning 4 flowers bloom in that oasis… 2 wild flowers you mentioned and 2 wide mouthed flower of some other species. Now 1 flower of the wild species can be visited by 4 bees (as humming birds can't enter the flower). One flower of the other species can be visited by 4 bees and 4 humming birds. This large number of pollinators for the second type of flower reduces its chance to be visited by a pollinator that visited a flower of the same species. Whereas for the wild type flower the possible number of pollinators are very few (only 4)… so the chance of one bee visiting two flowers of the same species is very common and so is the chance of it getting pollinated.

In this way these flowers increase their chance of pollination by modification of their shape.


Blooms on ‘chocolate’ tree are crazy-hard to pollinate

The seeds that give the world chocolate come from coy, fussy flowers that make pollination very difficult.

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February 8, 2018 at 9:50 am

It’s a wonder chocolate exists. Talk about plants that resist help. Cacao trees provide the seeds from which chocolate is made. But those seeds develop only once the trees’ blooms have been pollinated. The trees’ fruit — known as pods — are created by dime-sized flowers. And those blooms are difficult. They make pollination barely possible.

Growers of other commercial fruits expect 50 to 60 percent of the flowers on their crop plant to make seeds, notes Emily Kearney. And some cacao trees manage those rates. Kearney knows. She works at the University of California, Berkeley. A biologist there, she focuses on the pollination of cacao. The problem: Pollination rates in these plants tends to be much lower — as in closer to 15 to 30 percent. But in the South American country of Ecuador, traditional plantings may contain a mix of species. There, Kearney has seen cacao pollination rates of just 3 to 5 percent.

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The first sight of a blooming cacao tree (Theobroma cacao) can be “disconcerting,” she says. That’s because flowers don’t sprout from branches as in many other trees. Instead, they emerge directly from the trunk. They burst into little pink-and-white constellations of five-pointed starry blooms. Some trunks, Kearney says, “are completely covered with flowers.”

Pretty as they are, these flowers make nothing easy. Each petal curves into a tiny hood. This hood fits down around the plant’s male, pollen-making structure. To reach that pollen, a honeybee would be a useless giant blimp. So tiny flies step up to the task. Each of them is little bigger than a poppy seed. Known as chocolate midges, they are part of a family called biting midges.

After crawling up into the flowers’ hoods, they do — something.

But what? The flower offers those midges no nectar to drink. So far, researchers haven’t even shown that some scent lures in the midges. Some biologists have mused that reddish parts of the flower offer nutritious nibbling for the bugs. But Kearney knows of no tests that have confirmed this.

Another hitch to pollination: One cacao pod (resembling a wrinkled, swollen cucumber in shades of brown, purple or orange) requires from 100 to 250 grains of pollen to fertilize its 40 to 60 seeds. Yet midges typically emerge from a flower hood speckled with just a few to maybe 30 grains of the sticky white pollen. (Kearney says those pollen grains look like “clumpy sugar.”)

Story continues below image.

Pods, here, from Theobroma cacao trees are plump (with dozens of seeds) and vary a lot in color. E. Kearney

What’s more, the midge can’t just hike over to the female part of the same bloom. The female part sticks up in the very center of the flower, like some white-bristled paintbrush. Yet pollen is useless for any blooms on the tree it came from. That pollen won’t even work for close relatives.

To better understand cacao pollination, Kearney doesn’t suggest looking for answers at cacao farms. She says, “I think it’s the wild individuals that are going to open up the field.”

These trees evolved mostly in the Amazon Basin. There, cacao trees often grow in clusters of siblings that a monkey might have accidentally planted (while sucking pulp from a pod, dropping seeds as it fed).

To Kearney, dot-sized midges seem unlikely to fly the distance from clusters of cacao siblings to unrelated trees where cross-pollination chances would be better. So she wonders: Could the cacao with its elaborate reproductive system have a stealth, strong-flying native pollinator species that has to date escaped scientists’ notice?

Power Words

basin (in geology) A low-lying area, often below sea level. It collects water, which then deposits fine silt and other sediment on its bottom. Because it collects these materials, it&rsquos sometimes referred to as a catchment or a drainage basin.

biology The study of living things. The scientists who study them are known as biologists.

bloom A flower. Or the term for the emergence of flowers.

bug The slang term for an insect.

cacao The name of a tropical tree and of the tree&rsquos seeds (from which chocolate is made).

commercial (in research and economics) An adjective for something that is ready for sale or already being sold. Commercial goods are those caught or produced for others, and not solely for personal consumption.

constellation Patterns formed by prominent stars that lie close to each other in the night sky. Or something that has some starlike pattern.

crop (in agriculture) A type of plant grown intentionally grown and nurtured by farmers, such as corn, coffee or tomatoes. Or the term could apply to the part of the plant harvested and sold by farmers.

develop (in biology) To grow as an organism from conception through adulthood, often undergoing changes in chemistry, size and sometimes even shape.

fertilize (in biology) The merging of a male and a female reproductive cell (egg and sperm) to set in create a new, independent organism.

field An area of study, as in: Her field of research was biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory.

fruit A seed-containing reproductive organ in a plant.

insect A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.

midges Any of many types of small flies that often live around water. Some are blood-sucking insects others can derive their energy from eating plants. Frequently mistaken for mosquitoes, midges can transmit disease or move pollutants through an ecosystem.

native Associated with a particular location native plants and animals have been found in a particular location since recorded history began. These species also tend to have developed within a region, occurring there naturally (not because they were planted or moved there by people). Most are particularly well adapted to their environment.

nectar A sugary fluid secreted by plants, especially by flowers. It encourages pollination by insects and other animals. It is collected by bees to make into honey.

pollen Powdery grains released by the male parts of flowers that can fertilize the female tissue in other flowers. Pollinating insects, such as bees, often pick up pollen that will later be eaten.

pollinator Something that carries pollen, a plant&rsquos male reproductive cells, to the female parts of a flower, allowing fertilization. Many pollinators are insects such as bees.

pulp The fibrous inner part of a vegetable or fruit (such as an orange).

sibling An offspring that shares the same parents (with its brother or sister).

species A group of similar organisms capable of producing offspring that can survive and reproduce.

Citations

Website. U.S. National Park Service. Pollinators &mdash Chocolate midge.

About Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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The little things add up

Taken together, the characteristics of a flower make it more or less attractive to a certain kind of pollinator, which is why we see different pollinators on different flowers. As an example of a plant characteristic, think of the length of the corolla — the tube made by a whorl of petals. Some plants, like honeysuckle (Lonicera), have a long and deep flower. The only creatures that can pollinate this type of flower are those with equally long tongues or with very tiny bodies that can crawl down to the nectar despite their tongue length.

Conversely, some of the disk-shaped flowers such as those in the family Asteraceae, have very short flowers which are preferred by short-tongued animals, but can also be used by others.

As the name suggests, a syndrome is not one characteristic but “a set of concurrent things.” In addition to corolla length, these characteristics may include the presence or absence of nectar, landing platforms, and aromas. In addition, visual patterns known as nectar guides — sometimes evident only to those that can see UV light — may point to the flower’s sweet reward.

Other characteristics of a syndrome may include flower color, the amount of pollen, the size of the pollen grains, the sweetness of the nectar, and whether the flower opens during the day or in the evening. Just as an individual goes to the restaurant that carries his favorite dishes, a pollinator goes to the flower where conditions are just right and the food is perfect.


Year of Pollination: Most Effective Pollinator Principle and Beyond, part one

Have you ever considered the diversity of flowers? Why do they come in so many different shapes, sizes, and colors? And why do they produce so many different odors – or none at all? Flowering plants evolved around 140 million years ago, a fairly recent emergence evolutionarily speaking. Along with them evolved numerous species of insects, birds, and mammals. In his book, The Triumph of Seeds, Thor Hanson describes the event this way: “In nature, the flowering plants put sex, seeds, and dispersal on full display, spurring not only their own evolution but also that of the animals and insects with which they became so entwined. In most cases, the diversity of dispersers, consumers, parasites – and, most especially, pollinators – rose right alongside that of the plants they depended upon.”

Speaking of dependence, most flowering plants depend upon pollinators for successful reproduction – it is, for the most part, a mutually beneficial relationship. Even the casual observer of flowers will note that a large portion of the creatures that visit them appear to be pollinators. Thus, it is no wonder that pollination biologists have given pollinators so much credit in shaping the flowers that we see today.

Consider G. Ledyard Stebbins and his Most Effective Pollinator Principle which he defined in a paper published in 1970: “the characteristics of the flower will be molded by those pollinators that visit it most frequently and effectively in the region where it is evolving.” He then goes on to reference pollination syndromes, a phenomenon that describes how the traits of flowers are best suited for their “predominant and most effective vector[s].” In my post about pollination syndromes a few months ago, I discussed how a strict adherence to this concept has waned. In the next two posts, I discuss how the Most Effective Pollinator Principle (MEPP) may not be the best way to explain why flowers look the way they do.





To make this argument I am drawing mainly from two chapters in the book Plant-Pollinator Interactions: From Specialization to Generalization. The first is “Ecological Factors That Promote the Evolution of Generalization in Pollination Systems” by Jose M. Gomez and Regino Zamora, and the second is “The Evolution of Specialized Floral Phenotypes in a Fine-grained Pollination Environment” by Paul A. Aigner.

According to Aigner the MEPP “states that a plant should evolve specializations to its most effective pollinators at the expense of less effective ones.” And according to Gomez and Zamora it “states that natural selection should modify plant phenotypes [observable characteristics derived from interactions between a plant’s genes and its surrounding environment] to increase the frequency of interaction [between] plants and the pollinators that confer the best services,” and so “we would expect the flowers of most plants to be visited predominantly by a reduced group of highly effective pollinators.” This is otherwise known as adaptive specialization.

Specialization is something that, in theory, plants are generally expected to evolve towards, particularly in regards to plant-pollinator relationships. Observations, on the other hand, demonstrate the opposite – that specialization is rare and most flowering plants are generalists. However, the authors of both chapters advise that specialization and generalization are extreme ends to a continuum, and that they are comparative terms. One species may be more specialized than another simply because it is visited by a smaller “assemblage” of pollinators. The diversity of pollinators in that assemblage and the pollinator availability in the environment should also be taken into consideration when deciding whether a relationship is specialized or generalized.

That pollinators can be agents in shaping floral forms and that flowering plant species can become specialized in their interactions with pollinators is not the question. There is evidence enough to say that it occurs. However, that the most abundant and/or effective pollinators are the main agents of selection and that specialization is a sort of climax state in the evolutionary process (as the MEPP seems to suggest) is up for debate. Generalization is more common than specialization, despite observations demonstrating that pollinators are drawn to certain floral phenotypes. So, could generalization be seen as an adaptive strategy?

In exploring this question, Gomez and Zamora first consider what it takes for pollinators to act as selective agents. They determine that “pollinators must first benefit plant fitness,” and that when calculating this benefit, the entire life cycle of the plant should be considered, including seed germination rate, seedling survival, fecundity, etc. The ability of a pollinator species to benefit plant fitness depends on its visitation rate and its per-visit effectiveness (how efficiently pollen is transferred) – put simply, a pollinator’s quantity and quality during pollination. There should also be “among-pollinator differences in the evolutionary effect on the plant,” meaning that one species or group of pollinators – through being more abundant, effective, or both – contributes more to plant fitness compared to others. “Natural selection will favor those plant traits that attract the most efficient or abundant pollinators and will also favor the evolution of the phenotypes that cause the most abundant pollinators to also be the most effective.” This process implies possible “trade-offs,” which will be discussed in part two.

When pollinators act as selective agents in this way, the MEPP is supported however, Gomez and Zamora argue that this scenario “only takes place when some restrictive ecological conditions are met” and that while specialization can be seen as the “outcome of strong pollinator-mediated selection,” generalization can also be “mediated by selection exerted by pollinators…in some ecological scenarios.” This is termed adaptive generalization. In situations where ecological forces constrain the development of specialization and pollinators are not seen as active selection agents, nonadaptive generalization may be occurring.

Gomez and Zamora spend much of their chapter exploring “several causes that would fuel the evolution of generalization” both adaptive and nonadaptive, which are outlined briefly below.

  • Spatiotemporal Variability: Temporal variability describes differences in pollinator assemblages over time, both throughout a single year and over several years. Spatial variability describes differences in pollinator assemblages both among populations where gene flow occurs and within populations. Taken together, such variability can have a measurable effect on the ability of a particular pollinator or group of pollinators to act as a selective agent.
  • Similarity among Pollinators: Different pollinator species can have “equivalent abundance and above all comparable effectiveness” making them “functional equivalents from the plant perspective.” This may be the case with both closely and distantly related species. Additionally, a highly effective pollinator can select for floral traits that attract less effective pollinators.
  • The Real Effects on Plant Fitness: An abundant and efficient pollinator may select for one “fitness component” of a plant, but may “lead to a low overall effect on total fitness.” An example being that “a pollinator may benefit seed production by fertilizing many ovules but reduce seedling survival because it causes the ripening of many low-quality seeds.” This is why “as much of the life cycle as possible” should be considered “in assessing pollinator effectiveness.”
  • Other Flower Visitors: Pollinators are not the only visitors of flowers. Herbivores, nectar robbers, seed predators, etc. may be drawn in by the same floral traits as pollinators, and pollinators may be less attracted to flowers that have been visited by such creatures. “Several plant traits are currently thought to be the evolutionary result of conflicting selection exerted by these two kinds of organisms,” and “adaptations to avoid herbivory can constrain the evolution of plant-pollinator interactions.”

This, of course, only scratches the surface of the argument laid out by Gomez and Zamora. If this sort of thing interests you, I highly encourage you to read their chapter. Next week I will summarize Aigner’s chapter. If you have thoughts on this subject or arguments to make please don’t hesitate to comment or contact me directly. This is a dialogue, dudes.


Venus flytraps tend not to eat their pollinators

The notch-tipped flower longhorn beetle was one of the most common species found on Venus flytrap flowers.

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A Venus flytrap requires a pollinator to make seeds and reproduce. But who those pollinators are have long remained a mystery. One reason: Scientists and gardeners tend to focus on the carnivorous plant’s clever traps. Now researchers have finally looked at which species visit the plant’s delicate white flowers. And they’ve found little overlap with species that serve as the plant’s meals.

There are hundreds of species of carnivorous plants found across the planet. Few, however, attract quite as much fascination as the Venus flytrap. The plants are native to just a small section of North Carolina and South Carolina. But these tiny plants can now be found around the world. They’re a favorite among gardeners, who grow them in homes and greenhouses.

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Although scientists have extensively studied the famous trap, they’ve largely ignored the flower that blooms atop a stalk 15 to 35 centimeters (9 to 14 inches) high. That means they’ve also largely missed seeing what pollinates that flower.

“The rest of the plant is so incredibly cool that most folks don’t get past looking at the active trap leaves,” says Clyde Sorenson. He’s an entomologist, or insect biologist, at North Carolina State University in Raleigh.

Because flytraps grow in such a small area of the Carolinas, field studies can be difficult, adds Elsa Youngsteadt. She’s an insect ecologist, also at NCSU. And there’s another problem when studying flytrap pollination. Most people who raise these plants cut off their flowers. Why? It helps the plant put more of its energy into making traps.

Sorenson and Youngsteadt realized that the answer to what pollinates these flytraps was sitting almost literally in their backyard. So they started sleuthing.

Tallying pollinators and prey

Their team collected flytrap flower visitors and prey on four days in May and June 2016. All came from three sites in Pender County, N.C.

The flower of a Venus flytrap sits well above the carnivorous trap. C. SORENSON

“This is one of the prettiest places where you could work,” Youngsteadt says. Venus flytraps are habitat specialists. They’re found only in certain spots of longleaf pine savannas in the Carolinas.

“They need plenty of sunlight but like their feet to be wet,” notes Sorenson. In May and June, the spots where the flytraps grow are “just delightful,” he says. Other carnivorous plants show up there, too. These include pitcher plants and sundews.

The researchers brought their finds back to the lab for identification. They also catalogued what kind of pollen the blooms’ visitors picked up — and how much.

Arthropods include a range of invertebrate animals, including insects, spiders and centipedes. Nearly 100 different arthropod species visited the flowers, the team reports February 5 in American Naturalist. “The diversity of visitors,” says Youngsteadt, “was surprising.” However, only three species — a sweat bee and two beetles — appeared especially important. They were either the most frequent visitors or had carried the most pollen.

Only 13 species were found both in a trap and on a flower. And of nine potential pollinators in that group, none was trapped in high numbers. For a carnivorous plant, “you don’t want to eat your pollinators,” Sorenson says. His team’s data now suggests flytraps do a good job of sparing their helpers.

Keeping pollinator and prey separate

There are three ways that a plant can keep pollinators and prey separate, the researchers note.

Flowers and traps could exist at different times of the year. However, that’s not the case with Venus flytraps. Traps develop earlier but stick around and are active during plant flowering.

The traps and blooms could also be spaced far apart. Pollinators tend to be fliers while prey were more often crawlers, such as spiders and ants. This matches up with the high flowers and low traps. But the researchers would like to do some experiments that manipulate the heights of the structures, Youngsteadt says. That would let them see just how much that separation matters.

The third option is that different scents or colors might lure different species to the flowers and traps. That’s another area for future study, Youngsteadt says. While scent and color are known attractants for the traps, little is known about whether those factors lure in pollinators.

Venus flytraps are considered vulnerable to extinction, Sorenson notes. The plant’s habitat is being destroyed as housing and other developments expand into their areas. What is left of that habitat is being degraded as fires are suppressed. (Fires help clear vegetation and keep sunlight shining on the flytraps.) Finally, people steal flytraps from the wild by the thousands.

While research into flytrap pollinators won’t hold off any of those threats, it could aid in future conservation efforts. “Anything we can do to better understand how this plant reproduces will be of use down the road,” Sorenson says.

But what really excites the scientists is that they discovered something new so close to home. “One of the most thrilling parts of all this,” Sorenson says, “is that this plant has been known to science for [so long], everyone knows it, but there’s still a whole lot of things to discover.”

Power Words

arthropod Any of numerous invertebrate animals of the phylum Arthropoda, including the insects, crustaceans, arachnids and myriapods, that are characterized by an exoskeleton made of a hard material called chitin and a segmented body to which jointed appendages are attached in pairs.

beetle An order of insects known as Coleoptera, containing at least 350,000 different species. Adults tend to have hard and/or horn-like &ldquoforewings&rdquo which covers the wings used for flight.

carnivorous plant A plant that trap animals, usually insects, as food.

conservation The act of preserving or protecting something. The focus of this work can range from art objects to endangered species and other aspects of the natural environment.

diversity A broad spectrum of similar items, ideas or people. In a social context, it may refer to a diversity of experiences and cultural backgrounds. (in biology) A range of different life forms.

ecology A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

entomology The scientific study of insects. One who does this is an entomologist. A paleoentomologist studies ancient insects, mainly through their fossils.

extinction The permanent loss of a species, family or larger group of organisms.

generation A group of individuals (in any species) born at about the same time or that are regarded as a single group. Your parents belong to one generation of your family, for example, and your grandparents to another. Similarly, you and everyone within a few years of your age across the planet are referred to as belonging to a particular generation of humans.

greenhouse A light-filled structure, often with windows serving as walls and ceiling materials, in which plants are grown. It provides a controlled environment in which set amounts of water, humidity and nutrients can be applied &mdash and pests can be prevented entry.

habitat The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

insect A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.

native Associated with a particular location native plants and animals have been found in a particular location since recorded history began. These species also tend to have developed within a region, occurring there naturally (not because they were planted or moved there by people). Most are particularly well adapted to their environment.

naturalist A biologist who works in the field (such as in forests, swamps or tundra) and studies the interconnections between wildlife that make up local ecosystems.

pitcher plant A carnivorous plant that traps bugs in traps filled with fluid and shaped like pitchers.

pollen Powdery grains released by the male parts of flowers that can fertilize the female tissue in other flowers. Pollinating insects, such as bees, often pick up pollen that will later be eaten.

pollinate To transport male reproductive cells &mdash pollen &mdash to female parts of a flower. This allows fertilization, the first step in plant reproduction.

pollinator Something that carries pollen, a plant&rsquos male reproductive cells, to the female parts of a flower, allowing fertilization. Many pollinators are insects such as bees.

population (in biology) A group of individuals from the same species that lives in the same area.

prey (n.) Animal species eaten by others. (v.) To attack and eat another species.

savanna A grassland sometimes also populated with trees. Most are fairly dry for part or much of the year.

species A group of similar organisms capable of producing offspring that can survive and reproduce.

threatened (in conservation biology) A designation given to species that are at high risk of going extinct. These species are not as imperiled however, as those considered &ldquoendangered.&rdquo

vegetation Leafy, green plants. The term refers to the collective community of plants in some area. Typically these do not include tall trees, but instead plants that are shrub height or shorter.

Citations

Journal:​ E. Youngsteadt et al. Venus flytrap rarely traps its pollinators. American Naturalist. Published online February 5, 2018. doi: 10.1086/696124.

About Sarah Zielinski

Sarah Zielinski is managing editor of Science News for Students. She has degrees in biology and journalism and likes to write about ecology, plants and animals. She has two cats, Oscar and Saffir.

Classroom Resources for This Article Learn more

Free educator resources are available for this article. Register to access:


3 DO POLLINATOR IMMUNE BEHAVIORS IMPACT PLANT FITNESS?

3.1 Self-medication

3.2 Disease avoidance

While there is no direct evidence of the impact of floral parasite presence on plant fitness, parasite presence on flowers seems to reduce the relative attractiveness of their reward (Fouks & Lattorff, 2011 , 2013 ). Moreover, pollinator disease avoidance can lead to a reduction in the overall pollinator visitation rates (Yousefi and Fouks, 2019 ). Since reduced pollinator visitation leads to pollen limitation (Knight et al., 2005 ) and decreased plant reproductive success (Irwin & Brody, 1998 ), the presence of pollinator parasites on flowers is likely to negatively impact plant fitness. In addition, the positive correlation between pollinator visit duration and plant fitness (Ivey et al., 2003 ) suggests that reduced pollinator visit duration on parasite-contaminated flowers may lead to reduced plant fitness. In Mimulus aurantiacus, presence of bacteria but not yeast in the nectar resulted in decreased pollination success and a reduction in seed set (Vannette et al., 2013 ).

One might object that for pollinator parasites to be present on flowers, they need to be deposited by pollinators, which may result in flower pollination. Nevertheless, one pollinator visit may not be sufficient for a successful pollination. Furthermore after the deposition of pollinator parasites, the subsequent avoidance of the flower by pollinators may lead to a reduced pollen dispersal, as many flowers are hermaphrodites. In addition, most flowering plant species are visited by multiple pollinator species (Fontaine et al., 2006 ) and not all pollinator species provide efficient pollination service (Koski, Ison, Padilla, Pham, & Galloway, 2018 ). Therefore, it is possible that pollinator parasites can be deposited by inefficient pollinators and consequently lead both inefficient and efficient pollinators to avoid contaminated flowers. In such scenario, plant fitness may drastically be reduced, as both female and male fitness will be impacted.

3.3 Immune grooming

Pollinators primarily groom in order to gather pollen as a food source (Harder, 1990 ). It is generally understood that grooming reduces pollen dispersal and that the reduction in pollen carryover depends on the timing and intensity of grooming (Castellanos, Wilson, & Thomson, 2003 Harder & Wilson, 1998 Rademaker, de Jong, & Klinkhamer, 1997 Thomson, 1986 ). Grooming immediately following removal of pollen from a donor flower should reduce pollen carryover considerably because the largest loads of pollen from a particular donor are usually deposited on the first few recipient flowers (Castellanos et al., 2003 Rademaker et al., 1997 Thomson, 1986 ). Most pollen-foraging bees groom to some extent after every flower visited, packing most of the removed pollen into their corbiculae, which reduces the amount of pollen available for transfer to stigmas. These grooming events vary considerably in relative intensity, with the intensity and frequency of grooming increasing as bees accumulate pollen on their bodies during foraging (Harder, 1990 ). Grooming behavior and its influence on pollinator-mediated gene dispersal have primarily been studied in bees (Holmquist, Mitchell, & Karron, 2012 Thomson, 1986 ). Bees are known to groom when infested with mites (Peng et al., 1987 Sammataro et al., 2000 ), and may therefore increase their grooming intensity when encountering parasites on flowers. Theoretically, the presence of pollinator parasites on flowers could intensify pollinator grooming and thus drastically influence pollen dispersal (Box 3, Figures 2 and 3a).

Box 3. Pollen dispersal model in relation to floral pollinator parasites presence.

We modeled pollen dispersal by a pollinator from one donor flower to consecutively visited flowers using the two-compartment mathematical model developed by Harder and Wilson ( 1998 ). In this model, the flower contains R pollen grains which are available for pollinators to pick up. Pollinators pick up pollen either on safe sites of their body at the rate of πs or on exposed sites of their body at the rate of πe. Pollen in exposed sites of a pollinator's body has two non-mutually exclusive fates: a fraction of pollen can be deposited on recipient stigmas (ρe) or pollen can be displaced. Displaced pollen is either moved to safe sites of a pollinator's body at a rate of Гys or lost at a rate of ГL, where Г = grooming intensity. Pollen in safe areas of a pollinator's body is deposited on recipient stigmas at a rate of ρs. Using this model, pollen dispersal across different grooming intensities was plotted (Supporting Information Figure S1a). As illustrated above, high grooming intensity leads to a rapid depletion of pollen, resulting in a small percentage of donor pollen being dispersed to the next five flowers visited by the pollinator. After the 10th consecutive flower visited by the pollinator, pollen deposition does not differ significantly between pollinators exhibiting high and low grooming intensities. This model incorporates various grooming intensities, however in our case we expect to have only high intensity of grooming when flowers are contaminated by a parasite (intensive grooming as immune behavior). The pollen dispersal of a donor flower by a pollinator was then calculated as a function of flower contamination (Figure 2). Most of the pollen is deposited or lost on the first five flowers visited by the pollinator, regardless of overall parasite prevalence. Thus, the contamination status of the first five flowers is decisive for the fate of most pollen. Due to this variation of pollen dispersal depending on the contamination status of the first flowers, the pollen dispersal was averaged (on 100 random runs) between different flower orders bearing pollinator parasites of various quantities. The highest and lowest average of pollen deposited on each flowers were drawn (from 1,000 averages) in relation to parasite prevalence (Fig. 3a). Parasite prevalence on consecutive flowers affects the magnitude of pollen dispersal (as illustrated in Supporting Information Figure S1a). The presence of pollinator parasites on flowers could hinder pollen dispersal if pollinators exhibit increased grooming as a response to parasites. It is possible that flowers have adapted different strategies for pollen dispersal in the presence of pollinator parasites. For example, some morphological floral traits could be selected to increase the exposure of pollen to safe sites on the pollinator's body. Here, we assumed that πe and πs are the proportion of the overall π pollen picked up by pollinators, where π is a constant. Flowers could adapt a longer anther (safe strategy: πe = 0.35, πs = 0.15 70/30) to increase the number of pollen grains deposited on safe sites of pollinators. For flowers without a long anther (lax strategy: πe = 0.45 and πs = 0.05 90/10) we modeled a higher rate of πe with the same overall π (Supporting Information Figure S1b). While the safe strategy for a flower increases its pollen dispersal when there is high grooming intensity, this benefit is less obvious when grooming intensity is low (Supporting Information Figure S1b). In the same manner, high parasite prevalence makes the safe strategy of a flower more beneficial for pollen dispersal than lax strategy flowers (Figure 3b). However, when parasite prevalence is low, this strategy does not significantly increase the number of pollen grains dispersed. The net cumulative gain of pollen dispersed (total number of pollen donated to flowers beyond the donor) was accounted for using the minimum and maximum difference in pollen dispersed between the two strategies. According to the model, the safe strategy is only beneficial in environments with high parasite prevalence (Figure 3c). Variation of pollen dispersal (due to the order of contamination status on following flowers) demonstrates that the safe strategy of flowers in environments of low parasite prevalence is highly variable, and can be negative for pollen dispersal compared to a flower with lax strategy (Figure 3c). This model demonstrates that the presence and high prevalence of pollinator parasites on flowers favor the adaptation of long anthers or other traits similarly capable of increasing the proportion of pollen deposited on safe sites of pollinator bodies.


Sexual Reproduction In Plants

Human beings have a close relationship with flowers. Flowers are related to social, religious, aesthetic and cultural values – they have always been worn as symbols to convey important human emotions such as happiness, love, affection, and grief.

Even before the flower appears on the plant, the decision that plant will bear the flower has taken place. A number of hormonal and structural variations begin which lead to the differentiation and development of the flower. The floral buds are borne by inflorescences which further develop into a flower. You will find the reproductive organs of the angiosperms in flower. The male reproductive organ is stamen (androecium), and female reproductive organ is pistil (gynoecium).

The flower can be unisexual, example – watermelon, papaya when it consists of either carpels or stamens or bisexual, example – mustard or Hibiscus when it consists of both carpels and stamens.

Carpel

  • It is present in the centre portion of the flower.
  • It is a female reproductive organ
  • It is further divided into three parts –
    Ovary – the bottom swollen part. It contains ovule which further consists of an egg cell. The pollen grain produces male germ cells which fuse with the egg cell in the ovule. This fusion gives rise to zygote which grows into a new plant.
    Style – Middle and an elongated portion
    Stigma – It is the terminal portion. It can be sticky.

In order to do fertilization, it is essential to transfer pollen grains from stamen to stigma.

There are two main types of pollination in general terms

  • Self-pollination – Transfer of pollen from the anther to the stigma of the same flower
  • Cross – Pollination – If the transfer of pollen is transferred from one flower to another, it is known as cross-pollination. The transfer of pollen grains can be attained by agents like water, wind and animals.

Kinds of Pollination

On the basis of the source of pollen, it can be categorised into three kinds:

  • Autogamy – In this, the pollination is attained within the same flower. There is a transfer of pollen grains from the anther to the stigma of the same flower. However, in normal conditions, complete autogamy is very difficult. Autogamy in such flowers needs coordination in the release of pollen and stigma receptivity. In order to achieve autogamy every time, the stigma and anthers need to lie in proximity to each other. Some plants like Oxalis, Viola, and Commelina bear two types of flowers:
    • Chasmogamous flowers – These flowers are similar to the flowers of other species with exposed anthers and stigma.
    • Cleistogamous flowers – The flowers do not open at all which is why the anthers and stigma remain nearby. When anthers burst open in the flower buds, the pollen grains come in contact with the stigma and completes pollination. Therefore, these flowers are truly autogamous as the alien pollen cannot reach the stigma, and no cross-pollination can occur.

    Agents of Pollination

    There are abiotic (wind and water) and biotic (animals) agents that help in pollination. Most of the plants use animals for pollination. Only a small group of plants use wind and water as their agents. In the case of wind and water, pollen grains come in contact with stigma only by chance. To compensate for this situation of uncertainty and loss of pollen grains, the flowers bear numerous pollen grains.

    Pollination by Wind

    Pollination by wind is the most common method in abiotic pollinations. Wind usually carries lightweight and non-sticky pollen grains with it. The stamens of the flowers that use wind pollination are generally well exposed so that the pollens can be dispersed easily. The female counterpart contains feathery stigma that can easily trap the pollen grains moving in the air. These kinds of flowers usually contain a single ovule in each ovary, and numerous flowers are packed to form an inflorescence. Example – corn. Wind pollination is most commonly seen in grasses.

    Pollination by Water

    Water is not the most preferred agent in the plant kingdom for pollination. It is mostly seen in monocotyledons. However, the main mode of transport when we see the lower plant groups like algae, pteridophytes, and bryophytes. The distribution of these plants is limited due to the need for water for the transfer of male gametes and fertilization. Some examples are Hydrilla and Vallisneria. Some seagrasses such a Zostera also pollinate and fertilize through the water.

    Note – Not all aquatic plants pollinate through the water. Example – Water hyacinth and water lily. Also, both water and wind-pollinated flowers are not quite attractive and colourful.

    Pollination by Animal (Insects)

    Most of the flowering plants use insects or other animals as pollinating agents. Bees, flies, butterflies, wasps, moths, beetles, bats and birds are the most common pollinating agents. Bees are the most dominating biotic pollinating agents. In some cases, large animals like tree-dwelling rodents or reptile, primates (lemurs) have also been reported to be involved in pollination.

    Flowers are usually adapted to in a certain way for a particular species of animal. Most of the insect-pollinated flowers are colourful, fragrant, large and rich in nectar so that they can attract insects or animals.

    Fertilization in flower

    After the pollen reaches a suitable stigma, it tries to reach the female germ cells present in the ovary. For this, a small tube-like structure appears that grows out and enters the style to finally reach the ovary. After fertilization, the egg and the male gamete forms zygote which divides many times to form an embryo inside the ovule. A tough coat is developed around the ovule and is the whole organ converts into a seed. There is rapid development of ovary that forms a fruit. Meanwhile, the sepals, petals style, stigma, and stamens shrivel and fall off leaving only fruit behind. However, in some cases, sepals can be seen even after the formation of fruit.


    Ecosystem Services☆

    Pollination Services Provided by Bees

    Pollination , the movement of genetic material in the form of pollen grains, is a key step in the development of most food crops. Even crops that do not rely on insect pollination – wind pollinated or self-pollinated crops – are sometimes more productive when visited by an insect pollinator. Bees are a particularly important group of insect pollinators, responsible for pollinating 60–70% of the world's total flowering plant species, including nearly 900 food crops worldwide, such as apples, avocados, cucumbers, and squash. These crops comprise 15–30% of the world's food production, and bees are credited with $4.2 billion in annual crop productivity in California alone. Bees are especially important pollen vectors in part because physical adaptations, such as hairs designed to pick up pollen, and behavioral adaptations, such as fidelity to a single species of plant on each pollen-gathering trip, ensure good pollen transport and cross-pollination ( Kremen et al., 2002 ).

    In the US, most major agricultural enterprises that rely on bee pollination import managed bees, almost always the European honeybee Apis mellifera. The available stock of managed honeybees has declined dramatically, however, dropping by over 50% in the last 50 years, while demand for pollination services has increased in many areas. This decline in managed bee populations has many causes, including increased pesticide use, disease in the hives, and downsizing of stocks that have hybridized with Africanized bees, introducing traits that make managed bees more aggressive and thus a liability to the farmer.

    The contribution of native, wild bees to agricultural pollination was ignored, and assumed to be negligible, until the early 2000s. Since then, research has shown that native bees serve an important role in pollination, picking up slack when managed bee pollination is insufficient and enhancing crop production in general. Farms with generous native bee habitat nearby may be able to fully or partially replace pollination by managed bees. In some cases, native bees are more efficient pollinators than European honeybees. The variety of wild bees, with distinct physical and behavioral traits, allows them, as a group, to pollinate a wide variety of flowering plants. Tomatoes, for example, have pollen that is accessible only by vibrating the flower, which bumble bees and some other native bees can, while honeybees cannot. Though tomatoes are self-pollinating and do not require an insect vector, native bees promote cross-pollination, which, for example, significantly increases the fruit set and size of Sungold cherry tomatoes.

    The contributions of native bees to crop production are usually undocumented and underestimated, and they are always unpaid, at least directly. Though hives of managed honeybees must be rented or maintained, wild bees pollinate at no cost to the farmer. Populations of native bees are under great threat, however, by land management practices that promote the use of pesticides and the loss, fragmentation, and degradation of habitat. Protecting native bees without protecting the ecosystems in which they live is impossible. Native habitat, unlike agricultural monocropping, provides the year-round supply of blooming plants that wild bees require for sustenance. Native habitat also provides nesting areas most wild bees are solitary, laying a single egg in a nest cavity dug into the ground or into dead wood, not forming social hives. In order to reap the benefits of native pollinators, food resources and nesting habitat must be available within a short distance of crops, possibly as hedgerows, in ditches, or around water ponds. A study of wild bee pollination of coffee in Costa Rica showed that farms closer to tropical forest remnants were visited by many more species of wild bees than those further away. Had the far sites been adequately pollinated, coffee yield would have been increased by nearly 20% and misshapen coffee beans reduced by 27%. A lower-bound estimate of the pollination services from these patches is US $62 000 per year (in the early 2000s) ( Ricketts et al., 2004 ).

    The diversity of the native bee population is one of its strengths. Many species of bees participate in pollination, and the abundance of different species varies year by year. This diversity allows the native pollinator community to be both resistant, maintaining functionality in the face of environmental upheaval, and resilient, able to reestablish itself in the wake of a destructive event. When the population of Apis declined dramatically in the second year of the Costa Rica study, sites close to forest fragments showed minimal loss of pollination while pollinator visits dropped by nearly 50% further away. Thus, as well as enhancing pollination services in conjunction with managed bees, native bee populations provide important insurance against the possibility that managed bee populations could fail because of disease, hybridization, or other causes.


    Background

    Understanding and predicting the evolutionary responses of phenotypes to selection remains a major challenge in evolutionary biology. This undertaking is not trivial because phenotypes are often complex traits co-evolving with each other underlain by complex genetic architectures. Yet, understanding how such co-evolutionary units evolve under natural selection is important to understand how species may respond to changes in their environment. Flowers are complex organs with enormous diversity in morphology, color and scent, and thus comprise a complex set of interrelated traits. These visual and olfactory components, which characterize the radiation of angiosperms, are recognized to evolve as a means of interaction with their biotic environment [1, 2]. One key driver, the pollinators, has been emphasized to be important for floral trait evolution since long [3, 4]. Yet, only a handful of studies have attempted to predict evolutionary responses of floral traits to pollinator selection [5,6,7,8,9] . Moreover, these studies only examined one or a few morphological traits at a time, whereas interactions of flowers with other organisms are typically mediated by a combination of traits of morphological and/or olfactory nature [10, 11]. A multivariate approach can, therefore, help to unravel the genetic architecture of floral traits and predict their joint evolution.

    A great number of empirical studies have documented significant heritability and genetic (co)variance of diverse floral traits [12,13,14,15], as well as phenotypic selection acting on them [16,17,18,19,20,21,22,23,24,25,26,27]. Among those traits, floral scents have started drawing more and more attention. Floral scents are usually highly variable and diverse on all taxonomic levels [28], and many studies have documented natural selection on scent [22,23,24,25, 29, 30]. Earlier studies have shown that scent phenotypic variation has a significant heritable genetic component in fast-cycling Brassica rapa (20–45%, [15]). In the same species, Gervasi and Schiestl [25] showed in a greenhouse experiment that bumblebee pollinator selection resulted in taller plants with higher UV-reflecting flowers and increased amounts of several floral scents, presumably used by the bees as visual and olfactory signals. However, the selection intensities acting on each trait alone could not fully explain the realized evolutionary changes [25]. Phenotypic trait responses to selection are known to depend on the pattern of genetic variance-covariance among them [31, 32]. In particular, traits that are genetically correlated because of a shared genetic basis (e.g., pleiotropic genes) will indirectly respond to selection on linked traits, which may mask the effect of direct selection on them. Therefore, the targets of direct selection cannot be well characterized unless the selection responses are decomposed into their direct and indirect components. This is best done using a multivariate quantitative genetics framework [31, 33].

    Quantitative genetics theory provides a means to make such evolutionary predictions in the form of the multivariate breeder’s equation (or Lande’s equation), △ z = [31]. Lande’s equation predicts the per-generation change in a set of quantitative traits in a population ( △ z) as the product of their genetic variance-covariance matrix (G-matrix) with the vector of selection gradients acting on them (β). The components of Lande’s equation can be estimated from phenotypic and individual pedigree relationship data in an experiment by using the classical tools of quantitative genetics [32, 34]. More importantly, this multivariate approach can help distinguish between the direct and indirect responses to pollinator selection. The direct component of the response to selection is obtained by multiplying the diagonal elements of the G-matrix (Gii, the additive genetic variance of the traits) with the β vector, which holds, for a single trait i: △ zi direct = Gii*βi, while its indirect component is the product of the off-diagonal elements of G (genetic covariance: Gij) with β, summed over all traits ji: △ zi indirect = ∑Gij*βj. The total response is the sum of these two components. Traditionally, the distinction between direct and indirect selection has been made by comparing the selection differential and the selection gradient acting on each trait separately [32]. A selection differential (S) is the phenotypic covariance between relative fitness w and the trait z: S = Cov(w, z), while β is the coefficient of regression of relative fitness on the trait value z: β = S/VP (with VP the phenotypic variance) [32]. S includes the direct and indirect effects of selection on the trait but it cannot distinguish between them (unless in artificial selection where the directly imposed selection is known). Under this approach, a trait is said to be under direct selection if its selection gradient estimate β is significant. Otherwise, the selection represented by a significant selection differential is interpreted as a mix of direct and indirect selection. However, the relative importance of direct and indirect selection can be established when comparing the direct and indirect components of the predicted selection response obtained from Lande’s equation. In that case, if the total predicted response of a trait is opposed to or smaller than its direct component, then evolution of that trait will be said to be constrained by the genetic correlation among the traits. Furthermore, if the observed response is well predicted by the total response and the direct selection component is in the same direction as the observed response, then the trait can be said to be a target of direct selection. Otherwise, if observed and direct responses are opposed, the trait response is more influenced by indirect selection than direct selection and is thus likely evolutionarily constrained. Because these inferences use the G-matrix, they are less influenced by non-genetic causes of association among traits than inferences based on the selection differentials. Finally, one key evolutionary insight we can get from the multivariate quantitative genetic framework presented here is that traits may deviate from their predicted changes under direct selection (β) because of indirect selection pressures caused by selection on the other traits and their genetic correlation with them. In other words, traits may deviate from their expected evolutionary trajectory given by β, and thus be constrained by genetic correlations with traits under a different set of selection pressures [35, 36].

    In this study, we predict floral traits evolution under artificial and pollinator selection from estimates of the G-matrix and the selection gradients of the traits. We tested how the evolutionary trajectory of a trait is affected by genetic correlations among traits by dissecting the total responses to selection into their direct and indirect components. By comparing the direction of the observed trait responses with the directions of their direct and indirect predicted responses, we tested whether traits were targets of direct selection in the pollinator experiment. We also assessed the evolution of genetic architectures (G-matrices) during the artificial selection process. We used data from two forward-in-time experimental evolution experiments that documented genetic co-variation and evolutionary responses in floral traits of fast cycling Brassica rapa plants. The G-matrix of the plant population was estimated from a three-generation bi-directional artificial selection experiment on plant height [14]. In that study, tall- and short-plants were selected artificially for building the two directional lines, plus randomly selected plants for an additional control line. Four morphological floral traits and 12 floral volatiles were measured in each generation. Control lines in this experiment were used to estimate the G-matrix. The selection gradients β were estimated in four evolutionary experiments: two for the tall- and short-selection lines in the artificial selection experiment mentioned above [14] the other two from a 9-generation pollinator selection experiment [25]. The pollinator selection experiment was carried out with bumblebees and hoverflies as the selection agents separately. The same set of floral traits were measured, and the parental plants were from the same seed bank as in the artificial-selection experiment.


    Conclusions

    The presence of a floral scent signal clearly affects pollinator behaviour and, therefore, is likely to be a crucial part of floral signalling involved in plant reproductive success. Pollinator olfactory sensory acuity and olfactory learning are an important part of the selective environment that shapes the evolution of floral signals through their impact on plant fitness, but our understanding of the exact means by which olfactory learning shapes the expression of this floral trait remains relatively poor. It will be important in future studies to test exactly how floral scent complexity affects a pollinator’s ability to discriminate between flowers and to test how information about nectar reward quality influences what is learned about floral scent. Furthermore, testing how much variation in floral scent naturally occurs in plant populations relative to plant mating strategies (see Ashman, this special feature) may elucidate how floral scent has evolved. In particular, studies of this kind should focus on providing explicit examples of scent polymorphism within species and its influence on pollinator behaviour and pollen deposition. By measuring variation in scent production in plant populations and by using techniques which measure the hypothetical (using fluorescent dye) or realized (using molecular markers) forms of gene flow (see Whitehead & Peakall, this special feature), it should be possible to predict how changes in floral scent affect pollinator behaviour in specific ecological contexts. In particular, the recent advance in molecular gene silencing techniques in planta also offers exciting possibilities for field-based experiments of changes in floral scent and its influence on pollinator behaviour ( Kessler, Gase & Baldwin 2008 ).


    Watch the video: Film 4. Laurits dyrker kæmpe-græskar. Micki Cheng. GoCook by Coop (August 2022).