Tag Archives: gender conflict

Loneliness made me female: more on the hermaphrodite’s dilemma

by Piter Kehoma Boll

Some time ago I wrote about the conflicts of hermaphrodite organisms while they have sex, i.e., how both could be the male and the female at the same time, but that is usually not of their interest, especially if playing the female role would force you to end up with low-quality sperm for your eggs.

Two banana slugs ready to copulate. Photo by Andy Goryachev.

But sex is usually much more complex and how it occurs is usually shaped by environmental conditions, especially by the presence of competitors. In dioecious species, males usually compete for the females but is there a similar behavior applied to hermaphrodites?

According to the sex allocation theory, hermaphrodite organisms have to choose how much they invest in the male versus the female function. If you produce more eggs, thus preferring the female side, you end up producing less sperm and vice versa. So what should hermaphrodites do?

Sometimes there is competition. Photo by Wikimedia user Miekemuis.*

One way to try to take the best of this situation is allocating resources to the female or the male role according to what will give you more advantages in the current scenario. This would be determined mainly by the number, or the density, of individuals in the population.

When there are a lot of individuals, there is a lot of sperm, and being able to fertilize the eggs becomes more difficult. Thus, hermaphrodites would increase their investment in the male function to have a better chance against the sperm of the others. In other words, it is better to be a male when there are too many guys around you.

On the other hand, when finding a mate is rare, there is little sperm competition, so focusing on being a female is more advantageous. After all, the little sperm you produce is enough to fertilize the eggs of the few other individuals around there.

Most studies looking for evidence of the sex allocation theory found conflicting results. In many organisms, only one of the sexual functions changes according to population density, with either the number of eggs or the amount of sperm remaining the same and sometimes both functions are enhanced at the same time, going against the idea of a trade-off that the sex allocation theory predicts.

The problem may be simply a matter of how to look at things. Most studies focused on gamete production only, but sex is much more than that. One important part that has been neglected is sexual behavior. In order to test whether behavioral investment may show sex allocation differences, a recent study compared the investment of the hermaphrodite polychaete worm Ophryotrocha diadema in a female-related and a male-related behavior. According to their hypothesis, a low density of organisms would increase parental care, a female-related behavior, while a high density of organisms would increase motility in order to find a mate, a male-related behavior.

Two individuals of Ophryotrocha diadema. The yellow marks on the upper one are eggs. Photo by Viriginie Boutias. Extracted from http://leec.univ-paris13.fr/new/animals_en.html

And their hypothesis proved to be correct! Worms kept in pairs, i.e., with few mating opportunities due to the low density of individuals, moved less but took more care of the eggs. On the other hand, worms kept in groups, i.e., with more mating opportunities, moved more and did not take so much care of their eggs.

More than being nice evidence for the sex allocation theory, this study highlights the need to look beyond gamete production to assess sex allocation not only in hermaphrodites but in all organisms.

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Picchi L & Lorenzi MC 2019. Gender-related behaviors: evidence for a trade-off between sexual functions in a hermaphrodite. Behav Ecol. doi: 10.1093/beheco/arz014

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Male resistance: when females disappear and hermaphrodites don’t like you

by Piter Kehoma Boll

During the evolution of life, sex was certainly a great innovation. It allows organisms to reproduce while mixing their genes with that of another individual. Although it usually makes your offspring to have only half of your genes, which does not seem to be as great as an offspring that carries you as a whole into the next generation, there are certainly advantages in mixing. The most evident advantage is that your genes can combine with other versions and, as a result, produce a better team of genes than the one that you had. Even though each of your children carries only half of you, that half is more likely to survive than a child that carries you as a whole. In other words, sex gives the possibility for a population of genes (those that make up an individual) to get rid of some of the less efficient ones and replace them with better copies.

As you know, most sexual organisms make such a recombination by fusing two sexual cells, the gametes, and those are usually of two different kinds: a small one (the male) and a large one (the female).

In some species, each individual can only produce either male or female gametes, therefore being either a male organism or a female organism. In such species, sexual reproduction requires a male to mate with a female. This is the pattern found, for example, in most vertebrates and arthropods.


A female (large) and a male (small) of the tick Ixodes ricinus mating. Image by Jana Bulantová.*

In other species, each individual can produce both male and female gametes, therefore being called a hermaphrodite. The advantage of such a system is that hermaphrodites can mate with any individual of their species, sometimes even with themselves! One of the main problems with hermaphroditism is when you decide to play only one role, which may lead to conflict during sex.


Two snails Helix pomatia (hermaphrodites) making love. Photo by Wikimedia user Jangle1969.**

Now what evolved first? Dioecious species (those having male and female individuals) or hermaphrodites (allso called monoecious species)? It’s hard to tell, but we can be sure that during evolution many lineages switched from one system to the other and back. And the coolest part is that such switches still happen today.

You may know that most flowering plants are hermaphrodites. Flowers usually have both male and female organs, although they are rarely able to fertilize themselves (self-fertilization). Among plants, the cases of dioecious species seem to be mainly due to some mutation that ended up partially sterilizing an individual. For example, a mutation could appear that makes the plant unable to produce male organs, thus becoming only female. Other individuals in the population that lack this mutation continue to be hermaphrodites, so we have an “unbalanced” species with two sexes, females and hermaphrodites, but no males. Although unusual at first, such a system can remain stable if reproduction occurs through cross-fertilization and not self-fertilization. As both females and hermaphrodites need pollen (which produces the male gametes) from other plants, they can coexist as long as the pollinator carries pollen to both sexes. The same happens if the sexes are male and hermaphrodite. As long as the pollinator carries the male’s pollen to hermaphrodite flowers, both sexes can do just fine.


The plant Geranium sylvaticum includes hermaphrodites and females, but no males. Photo by Enrico Blasutto.**

Species composed of males and hermaphrodites are called androdioecious (from Greek andro-, man, male + di-, two + oikos, home, house; therefore “male in two “houses”, i.e., in two different kinds of organisms), while those composed of females and hermaphrodites are called gynodioecious (from Greek gyno-, woman, female; therefore “female in two different kinds of organisms).

Androdioecious and gynodioecious species occur among animals as well, but in this case their existance indicates something happening in the other direction, i.e., it is a transition from a dioecious species (with males and females) to a hermaphrodite species. And this is much more complicated that the other way round. Actually, it can get really, really bad for the “single-sex sex”.

This unbalanced sexual system in animals usually happens like this. There is a happily dioecious species with male and female individuals, but one day a new mutation appears and allows one of the sexes to produce both male and female gametes, thus becoming an hermaphrodite. However, such hermaphrodites are usually unable to play the role of the new sex while mating, i.e., they have the gametes, but not the tool to mate using them. Thus, the only way to use both gametes is to fertilize themselves.

One problem that comes from doing that is inbreeding. When you fertilize yourself, you are not increasing genetic diversity. On the contrary, you have very high chances of producing offspring with two copies to the same gene, thus decreasing genetic diversity. In order to continue to have recombination, you must mate with the single-sex individuals, which means you can only play the role of your original sex and your hermaphroditism is irrelevant. You are producing useless gametes. Or are you?


A male and a hermaphrodite of the nematode Caenorhabditis elegans an androdioecious species. Credit to Worm Atlas.

The problem with inbreeding happens when an organism ends up with two copies of a deleterious gene, which is fairly common in species where cross-fertilization is the rule and such deleterious genes are maintained in the population through individuals with a single copy that is not enough to cause any trouble. That is why having kids with your parents, children of siblings is usually a bad idea. When a species evolves from a system of cross-fertilization to one of self-fertilization, inbreeding can be a serious problem at first, producing many descendants that will die soon. However, eventually this will “purge” the set of genes. If individuals only mate with themselves, the number of deleterious genes will sharply decrease after some generations and inbreeding will not be such a big problem anymore.

When this happens in a species with unbalanced sex, the single-sex individuals will be in trouble. Two androdioecious animals have been studied regarding this conflict, the nematode and model organism Caenorhabditis elegans and clam shrimps of the genus Eulimnadia, such as Eulimnadia texana. In both groups, the hermaphrodites do not seem to be very interested in mating with males. They have even lost most phenotypic clues that help males identify them as potential mates. The only thing left for the males is to insist, to look for hermaphrodites and force them to mate with them, but it is a hard battle. Even when mating does occur, the hermaphrodite usually discards the male’s sperm.


A hermaphrodite (left) and a male (right) of the clam shrimp Eulimnadia texana. Credits to arizonafairyshrimp.com

The persistence of males in the population depends basically on their ability to fertilize hermaphrodites against their will and the sex-determination system of the species. When hermaphrodites produce males by self-fertilization, they are destined to remain for at least some time even if they cannot fertilize that much. Now if self-fertilization only produce hermaphrodites, the poor males have to be really persistent or otherwise they will soon perish.

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You may also like:

Having more females makes you gayer… if you are a beetle

Endosperm: the pivot of the sexual conflict in flowering plants

Gender Conflict: Who’s the man in the relationship?

Male dragonflies are not as violent as thought

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References and further reading:

Chasnov JR 2010. The evolution from females to hermaphrodites results in a sexual conflict over mating in androdioecious nematode worms and clam shrimp. Journal of Evolutionary Biology 23: 539–556.

Ellis RE & Schärer L 2014. Rogue Sperm Indicate Sexually Antagonistic Coevolution in Nematodes. PLoS Biol 12: e1001916.

Ford RE & Weeks SC 2018. Intersexual conflict in androdioecious clam shrimp: Do androdioecious hermaphrodites evolve to avoid mating with males? Ethology 124: 357–364.

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Half male, half female: the amazing gynandromorph animals

by Piter Kehoma Boll

In dioic species, i.e., those in which males and females are separate organisms, sexual dimorphism is very common. It is usually possible to say whether an individual is male or female through external caracteristics, such as color pattern, size or proportion of different body parts.


Male (left) and female (right) of Malurus cyaneus, the superb fairy-wren. A case of striking sexual dimorphism. Photo by Wikimedia user Benjamint444.*

Vertebrates and arthropods are certainly the two phyla in which sexual dimorphism is best known and found very often. See, for example, the birds above and the spiders below.


A female (left) and a male (right) of the spider Argiope apensa. The difference in size is more than evident. Photo by Wikimedia user Sanba38.*

The mechanisms that lead to sexual dimorphism are usually the same that lead to the differences in sex by itself. In mammals, birds and arthropods, it is usually due to differences in chromosomes. In other groups, such as crocodiles and snakes, it may be simply a matter of incubation temperature. It is not uncommon to find deviations from this “ideal” dichotomy, with organisms showing unusual chromosome combinations or other features that originate intermediate forms, such as hermaphrodites or androgynous individuals. We have a lot of this in our own species!

There is, however, a much more intriguing and astonishing male-female blend that is often found in arthropods. Known as gynandromorphism, this phenomenon creates specimens with mixed male and female characters forming a mosaic in which one part of the body is male and the other is female. And this distribution is usually bilateral, with one side of the body being male and the other being female.


Gynandromorph of the common blue (Polyommatus icarus). Male on the left side and female on the right side. Photo by Burkhard Hinnersmann.*


Gynandromorph of the Malaysian stick insect (Heteropteryx dilatata). Male on the left side and female on the right side. Photo by Wikimedia user Acrocynus.*

A recent paper by Labora & Pérez-Miles (2017) describes the first report of gynandromorphism in a mygalomorph spider (i.e., a tarantula). As the images are not distributed in an open access or creative commons licese, I cannot publish them here, but you can read the article for free thanks to our most beloved god, SciHub.

The causes of gynandromorphism are not always clear, but most of the times it seems to be due to a chromosome impairment in mitosis during the first stages of embryonic development. Thus, it is more likely to occur in inviduals that were originally heterogametic, i.e., they had two different sex chromosomes in their zygote.


A gynandromorph cardinal (Cardinalis cardinalis). Photo by Gary Storts.**

Gynandromorphism should not be confused with chimerism, a somewhat similar phenomenon in which an individual is the result of the fusion of two different embryos.

Now tell me, isn’t nature fascinating in every single detail?

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References and further reading:

Jones, S. R.; Philips Jr., S. A. (1985) Gynandromorphism in the ant Pheidole dentata Mayr (Hymenoptera: Formicidae). Proceedings of the Entomological Society of Washington, 87(3): 583–586.

Laborda, A.; Pérez-Miles, F. (2017) The first case of gynandry in Mygalomorphae: Pterinochilus murinus, morphology and comments on sexual behavior.  Journal of Arachnology, 45(2): 235–237. https://doi.org/10.1636/JoA-S-049.1

Labruna, M. B.; Homem, V. S. F.; Heinemman, M. B.; Ferreira Neto, J. S. (2000) A case of gynandromorphism in Amblyomma oblongoguttatum (Acari: Ixodidae). Journal of Medical Entomology, 37(5): 777–779.

Olmstead, A. W.; LeBlanc, G. A. (2007) The environmental-endocrine basis of gynandromorphism (intersex) in a crustacean. International Journal of Biological Sciences 3(2): 77–84.

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Gender Conflict: Who’s the man in the relationship?

by Piter Kehoma Boll

Everyone with some sort of knowledge on evolution has heard of sexual conflict, how males and females have different interests during reproduction, and sexual selection, i.e., how one sex can influence the evolution of the other.

Sexual organisms are almost always defined by the presence of two sexes: male and female. The male sex is the one that produces the smaller gamete (sexual cell) and the female sex is the one that produces the larger gamete. The male gamete is usually produced in large quantities because as it is small, it is cheaper to produce. On the other hand, the female gamete is produced in small quantities, because its large size makes it an expensive gamete.

A classical image of a male gamete (sperm) reaching a female gamete (egg) during fertilization. See the astonishing difference in size.
A classical image of a male gamete (sperm) reaching a female gamete (egg) during fertilization. See the astonishing difference in size.

As one can clearly see, the female puts a lot more resources in the production of a single descendant than a male does. As a result, females are usually very choosy regarding who will have the honor to fertilize her eggs. Males need to prove that they are worth the paternity, and female choice, through generations, increase male features that they judge attractive. A classic example is the peacock.

The peacock is one of the most famous examples of how sexual selection can drive the evolution of dioecious species. Photo by Oliver Pohlmann.
The peacock is one of the most famous examples of how sexual selection can drive the evolution of dioecious species. Photo by Oliver Pohlmann.

There are a lot of exceptions, of course, most of them driven by the social environment of the species or due to an unusual natural environment which may increase male investment. But all of this stuff refers to dioecious species, i.e., species in which male and females are separate organisms. But what happens if you are part of a hermaphroditic species, therefore being male and female at the same time? Do you simply mate with anyone? Is everyone versatile every time they get laid?

Well, there is a lot of diversity in these organisms, but all the principles of sexual conflict are still valid. Even if you are male and female at the same time, you still have the desire to fertilize as many eggs as possible with your cheap sperm while choosing carefully who is worth fertilizing your own eggs. The main problem is that anyone else wants the same.

- Come on, darling. Let me fertilize you. - Will you let me fertilize you too? Photo by Jangle1969, Wikimedia user.*
“Come on, darling. Let me fertilize you.”
“Will you let me fertilize you too?”
Photo by Jangle1969, Wikimedia user.*

Imagine that you are a hermaphrodite with a handful of expensive eggs and lots of cheap sperm. You are willing to mate and you go on a hunt. Eventually, you find another individual with the same intentions. You look at each other in the eyes, get closer and start a conversation. Let’s assume that you didn’t find the other one very attractive to be the father of your children, but you want to be the father of their children.

“So, what are your preferences?” you ask.
“Right now, I wanna be the male” the other one answers.

“Damn!”, you think. Both of you want the same thing. You guys want to play the same sexual role, so there’s a conflict of interests, or, as it is called, a “gender conflict”. In this case, regarding sexual behavior in biology, the word gender refers to the role you play during sex. Who will be the man in the relationship?

In face of this conflict, this hermaphrodite’s dilemma, you both have to find a solution. There are four possible outcomes:

1. You insist on being the male and your partner agrees to play the female against their will. You win, the other one loses.
2. Your partner insists on being the male and you agree to play the female against your will. The other one wins, you lose.
3. Both of you insist on being the male. Sex doesn’t happen and both of you go home without having got laid.
4. Both of you agree to play both roles. Sex happens and you successfully deliver your sperm but is forced to accept the other guy’s sperm too.

The worst for you is not being able to deliver your sperm, as you wished. So 2 and 3 are the worst outcomes. 1 is the better outcome for you, but how will you convince your partner to be the loser? So, the best solution for everyone is 4. Both are neither fully happy nor fully frustrated.

Eartworms use the 69 position to exchange sperm. Photo by Beentree, Wikimedia user.*
Earthworms use the 69 position to exchange sperm. Photo by Beentree, Wikimedia user.*

But is this the end? Not necessarily. The most stable mating behavior in a population is indeed to agree to play both roles, but things can go on after you kiss your mate goodbye. Now you have to deal with post-copulatory selection.

You have had sex, you delivered your sperm, but received sperm in return. A low-quality sperm in your opinion. You won’t let that fertilize your eggs, will you? Of course not! So, as soon as your partner is out of sight, you simply spit the sperm out before it reaches your eggs! He will never know.

A pair of flatworms, Macrostomum sp., mating. See how the white one, at the end, bends over itself and sucks the other guy's sperm in order to get rid of them. Image extracted from Schärer et al. (2004) [see references].
A pair of flatworms, Macrostomum lignano, mating. See how the white one, in the end, bends over itself and sucks the other guy’s sperm out of the female pore in order to get rid of it. Image extracted from Schärer et al. (2004) [see references].

So you cheated your partner! You agreed to receive their sperm in exchange of your own, but then you discarded it as soon as your partner went away. You rule! Right? But… wait! What if they did the same? What if your sperm was discarded too?

You cannot risk that. That would be worse than not having get laid in the first place because you would have wasted energy and sperm for nothing! But how can you assure that the sperm remains where it is supposed to be?

One strategy is to include some stiff bristles on your sperm cells so that they stick on the inner wall of the female cavity and cannot be removed. The sperm cells function like thorns or spines that go in easily but are very hard to be pulled back. That’s what some flatworms do.

Two strategies used by species of Macrostomum to force the partner to have your sperm. (A) A species in which two individuals share sperm but later may try to get rid of the partners sperm have evoled sperm cells with bristles that hold the sperm in the female cavity. (B) Other species have evolved a more aggressive behavior, in which they inject sperm in the partner using a sytlet (penis) with a sharp end able to pierce the body. In this case there is no need to have bristled sperm cells. Image extracted from Shärer et al. (2011) [see references].
Two strategies used by species of Macrostomum to force the partner to have your sperm. (A) A species in which two individuals share sperm, but later may try to get rid of the partner’s sperm, have evolved sperm cells with bristles that hold the sperm in the female cavity. (B) Other species have evolved a more aggressive behavior, in which they inject sperm in the partner using a stylet (penis) with a sharp end able to pierce the body. In this case, there is no need to have bristled sperm cells. Image extracted from Shärer et al. (2011) [see references].

Other species evolved a more aggressive approach. They armed their penises with a sharp point that pierces the partner’s body, forcing it to take the sperm. The sperm is injected in the partner’s tissues and swims towards the eggs.

Both strategies may look like wonderful solutions for the male, but remember that they are hermaphrodites, so that everything can be used against themselves! And that’s the big hermaphrodite’s dilemma or the ultimate hermaphrodite’s paradox. They are constantly trying to outrun themselves.

Isn’t evolution amazing?

See also: Endosperm: the pivot of the sexual conflict in flowering plants.

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References and further reading:

Anthes, N., Putz, A., & Michiels, N. (2006). Hermaphrodite sex role preferences: the role of partner body size, mating history and female fitness in the sea slug Chelidonura sandrana. Behavioral Ecology and Sociobiology, 60 (3), 359-367 DOI: 10.1007/s00265-006-0173-5

Janicke, T., Marie-Orleach, L., De Mulder, K., Berezikov, E., Ladurner, P., Vizoso, D., & Schärer, L. (2013). SEX ALLOCATION ADJUSTMENT TO MATING GROUP SIZE IN A SIMULTANEOUS HERMAPHRODITE Evolution, 67 (11), 3233-3242 DOI: 10.1111/evo.12189

Leonard, J. (1990). The Hermaphrodite’s Dilemma Journal of Theoretical Biology, 147 (3), 361-371 DOI: 10.1016/S0022-5193(05)80493-X

Leonard, J., & Lukowiak, K. (1991). Sex and the simultaneous hermaphrodite: testing models of male-female conflict in a sea slug, Navanax intermis (Opisthobranchia) Animal Behaviour, 41 (2), 255-266 DOI: 10.1016/S0003-3472(05)80477-4

Marie-Orleach, L., Janicke, T., & Schärer, L. (2013). Effects of mating status on copulatory and postcopulatory behaviour in a simultaneous hermaphrodite Animal Behaviour, 85 (2), 453-461 DOI: 10.1016/j.anbehav.2012.12.007

Schärer, L., Joss, G., & Sandner, P. (2004). Mating behaviour of the marine turbellarian Macrostomum sp.: these worms suck Marine Biology, 145 (2) DOI: 10.1007/s00227-004-1314-x

Schärer, L., Littlewood, D., Waeschenbach, A., Yoshida, W., & Vizoso, D. (2011). Mating behavior and the evolution of sperm design Proceedings of the National Academy of Sciences, 108 (4), 1490-1495 DOI: 10.1073/pnas.1013892108

Schärer, L., Janicke, T., & Ramm, S. (2015). Sexual Conflict in Hermaphrodites Cold Spring Harbor Perspectives in Biology, 7 (1) DOI: 10.1101/cshperspect.a017673

Wethington, A., & Dillon, JR, R. (1996). Gender choice and gender conflict in a non-reciprocally mating simultaneous hermaphrodite, the freshwater snail,Physa Animal Behaviour, 51 (5), 1107-1118 DOI: 10.1006/anbe.1996.0112

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Endosperm: the pivot of the sexual conflict in flowering plants

ResearchBlogging.org by Piter Kehoma Boll

The theory of sexual selection, based on the idea that there are conflict of interests between males and females, is quite recognized, but almost entirely focused on animals, especially dioecious animals, i.e., animals in which males and females correspond to separate individuals. Meanwhile, hermaphroditic animals and other organisms, such as plants, are usually ignored, but does hermaphroditism or “non-animalism” prevent the occurrence of sexual selection?

The peacock is one of the most famous examples of how sexual selection can drive the evolution of dioecious species. Photo by Oliver Pohlmann.

The peacock is one of the most famous examples of how sexual selection can drive the evolution of dioecious species. Photo by Oliver Pohlmann.

In the last decades, hermaphroditic animals started to be investigated more deeply concerning sexual conflict as a considerable evolutionary force in these organisms. For example, some studies demonstrated that many hermaphrodites, during copulation, fight to play the role of male, or female, in something called “gender conflict” (which DOES NOT HAVE ANYTHING TO DO with any social aspect of the word “gender”. Here it refers to the sexual role that a hermaphroditic organisms plays during sex).

In plants, on the other hand, the subject is much less explored, especially due to the lack of direct interaction between the two mating organisms. Reproductive strategies in plants were seen, for a long time, as a mean to ensure the supposedly difficult task to unite male and female gametes when one is a sessile organism, i.e., an organism unable to move. After all, this disadvantage forces these organisms to develop special techniques that guarantee the transport of gametes through the environment. With such a relevant problem to assure that sex will happen, it seems absurd to think that plants could yet afford to choose with whom to get laid.


Plants need external agents, such as wind, water or animals, to carry their gametes. Photo by psyberartist (flickr.com/people/10175246@N08).*

So far, the most approached point about sexual selection in plants is related to mechanisms developed by the female part to avoid the ovule to be fertilized by pollen of the same individual (the so-called self-fertilization) or of incompatible individuals (such as pollen of another species or of a close relative, because yes, incest can be a taboo even for plants). Another studied mechanism is related to the prevention of future attempts of fertilization once the zygot has been formed, as an already fertilized flower is not interested in receiving more and more pollen grains.

The passive travel of pollen from the male part to the female one gives us the impression that the male part cannot carry out any intersexual selection. After all, once the pollen arrives at a flower, it cannot leave, so its only chance is to try fertilization in any case, even if it is on an incompatible organism. This also highlights the fact that competition between pollen grains may occur on the female part, on a real race to see who gets first to the ovule. This competition may be controlled by the female part by changings in pollen receptivity.


When a pollen grain reaches the female part of a flower, it has no option but to germinate, creating a pollen tube that grows towards the ovule. In this picture, three pollen tubes are running towards the ovule and one of them has a clear advantage over the others. It may be because it arrived first or because the female part changed its receptivity to accept this specific grain more eagerly than the others.

An intriguing aspect in angiosperm reproduction is the phenomenon of double fertilization. When a pollen grain falls onto the female organ, it germinates, originating a long tube that grows towards the ovule, the so-called pollen tube. The pollen tube carries with it two male gametes: one of them will fertilize the egg cell, giving rise to the zygote that will form the embryo, and the other fertilizes the central cell, an auxiliary cell that accompanies the egg, giving rise to a second zygot that forms the endosperm, a tissue that feeds the embryo during its development.


In the double fertilization of angiosperms, the pollen tube carries two male gametes to the ovule. One of them will fertilize the egg cell, leading to the embryo, and the other will fertilize the central cell, originating the endosperm.

Since the egg and the central cell, as well as both male gametes, are genetically identical, the endosperm is also identical to the embryo and may be seen as an altruist that sacrifices itself to assure the survival of its sibling. The evolutionary origin of the endosperm and its adaptive advantage remain subjects of much discussion and without much solution. The situation is yet more complicated because, in most angiosperms, the endosperm is triploid, having a duplicate maternal material because the central cell has two nuclei. In other words, the endosperm has two copies of the maternal genes and one copy of the paternal genes (configuration 2m/1p), while the embryo is an ordinary organism, having one copy of the maternal genes and one copy of the paternal genes (configuration 1m/1p).

Several hypothesis on the reason that led to the rising of this selfless triploid sibling have been raised and are usually based on different interpretations on the sequence of the events that happened during the evolution of the group. Functionally, the endosperm works are the female gametophyte of other plants, which is, in these, responsible for nourishing the developing embryo. The female gametophyte is the “mother” of the embryo, just like the pollen grain (male gametophyte) is the “father”. The plants with the flowers are, therefore, the embryo’s grandparents. Crazy, isn’t it? But that’s the rule for plants. One generation of large organisms (the sporophyte), gives rise to a generation of tiny organisms (the gametophyte), which in turn will “mate” to generate new large organisms.

Going back to the subject, the functional similarity between the endosperm and the female gametophyte seems to favor the hypothesis that the endosperm was initially a maternal tissue (having, therefore, an original configuration 1m/0p or 2m/0p) and the paternal intromission happened later. On the other hand, the phenomenon of double fertilization is also found in Gnetales (supposedly the closest group to angiosperms) and, in these, double fertilization originates two identical embryos. In addition, basal angiosperms also have diploid endosperms, with a single copy of chromosomes from each parent (1m/1p). This scenario points to a primitive situation of two embryos, in which one of them was deviated to the role of endosperm.

Here we need to include one more important concept in biology: genome imprinting. It is a phenomemon in which genes are differently expressed depending on the parent from which they came; and it is usually seen are a consequence of sexual conflict. What happens is that paternal cells may be silenced in some cells, so that the organism expresses, in those cells, only features inherited through the mothers. The opposite may also happen.

It is assumed that, in angiosperms, the paternal side benefits from the production of large endosperms that provide more nutrients to the embryo, so that there is interest both to express genes leading to a higher accumulation of resources coming from the mother and to silence genes that limit growth. In contrast, the maternal side would attempt to limit the nutrients destined to a single endosperm, as the excess of investment would compromise its future reproductive success. It is better for the mother to invest a little in each endosperm than to invest everything in a single one. Therefore, the maternal side would express genes that control the amount of resources invested in each embryo while inhibiting genes inducing an increased growth.

In such a scenario with genome imprinting, the increased expression of genes by duplication may be seen as a female strategy to counterattack a male attempt to express genes responsible for resource allocation. The paternal plant would express genes for resource collection, while the maternal plant, with two copies of its material in the endosperm, would express genes leading to a contrary response in higher intensity, trying to stop the paternal influence. Such a phenomenon has been attested in corn seeds, where 2m/0p endosperms are smaller than 2m/1p endosperms. As we can see, there is a fight between males and females even among plants!

In angiosperms, fertilization involves the direct interaction of five distinct organisms belonging to three generations: female sporophyte (maternal plant), masculine gametophyte (pollen grain), female gametophyte (ovule), embryo and endosperm. Each one of these organisms has an interest that may be contrary to one or more interests of the others, leading to a complex interaction still poorly defined and in which the endosperm certainly constitutes the most intriguing point and may be the consequence of certain strategies and, at the same time, lead to the emergence of new ones.

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