Tag Archives: evolution

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.

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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.

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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.

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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?

maleintrofig3b

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.

drylakeclam

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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Xenoturbella, a growing group of weirdoes

by Piter Kehoma Boll

You may never have heard of Xenoturbella, and I wouldn’t blame you. Despite being a fascinating feature of evolution, little is known about it and its magic has been hidden from most of us.

The first Xenoturbella was described in 1949 and named Xenoturbella bocki. At the time, it was considered a strange flatworm, hence its name, from Greek xenos, strange + turbella, from Turbellaria, free-living flatworms. Xenoturbella bocki is a marine animal measuring up to 3 cm in length and looking like a flat worm… a flatworm! Well, actually more like a folded worm, because its body has a series of folds running londitudinally that make it have a W shape in cross section.

Found in the cold waters around northern Europe, its body lacks a centralized nervous system, having only a net of neurons inside the epidermis. There are also no reproductive organs, neither anything similar to a kidney or any other organ beside a mouth and a gut and some structures on its surface.

For decades, X. bocki was the only species of Xenoturbella known to us. A second species was described in 1999 as X. westbladi, but molecular analyses revealed that it was the same species as X. bocki, so we continued having only one species. Thanks to molecular studies, we also figured out that Xenoturbella is not a flatworm at all, but belongs to a group of very primitive bilaterian animals, being closely related to another group of former flatworms, the acoelomorphs. Together, Xenoturbella and the acoelomorphs (a good name for a rock band, right?) form the group called Xenacoelomorpha.

xenoturbella_churro_rouse

Xenoturbella churro, “head” to the right. Photo by Greg Rouse.*

Forming its own phylum (or perhaps class if it is grouped in a single phylum with the acoelomorphs) named Xenoturbellida, X. bocki recently discovered that it is not alone in the world. In 2016, four new species were described from the waters of the Pacific Ocean near the coasts of Mexico and the USA, being named Xenoturbella monstrosa, X. churro, X. profunda and X. hollandorum. Considering the small size of X. bocki, some of them were monsters, especially X. monstrosa, which reaches 20 cm in length!

Four new species was quite a finding. The phylum suddenly was five times bigger than before. As someone particularly interested in obscure animal groups, especially those that once were members of the lovely phylum Plathyelminthes, I was very excited by this discovery, but I wasn’t expecting at all what happened after that.

xenoturbella_japonica

Photo of the only known specimen of Xenoturbella japonica until now. “Head” to the left. Credits to Nakano et al. (2017).*

In December 2017, one more species was found, this time on the other side of the Pacific, near Japan. Named Xenoturbella japonica, the fifth member of the Xenoturbella genus is very welcome. The new species was based on two specimens, an adult “female” specimen (are they hermaphrodites? I don’t think we can be sure about it yet…) and a juvenile specimen. One more exciting thing is that the juvenile may actually be yet another species! But we need more material to be sure.

You can read the article describing Xenoturbella japonica here.

See also: Acoelomorpha, a phylogenetic headache

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References:

Nakano, H.; MIyazawa, H.; Maeno, A.; Shiroishi, T.; Kakui, K.; Koyanagi, R.; Kanda, M.; Satoh, N.; Omori, A.; Kohtsuka, H. (2017) A new species of Xenoturbella from the western Pacific Ocean and the evolution of XenoturbellaBMC Evolutionary Biology17: 245. https://doi.org/10.1186/s12862-017-1080-2

Rouse, G.W.; Wilson N.G.; Carvajal, J.I.; Vrijenhoek, R.C. (2016) New deep-sea species of Xenoturbella and the position of Xenacoelomorpha. Nature, 530:94–7. doi:10.1038/nature16545.

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How do new species form?

by Piter Kehoma Boll

A long, long time ago, I wrote two posts here about the definition of species, explaining briefly the most important horizontal and vertical species concepts. So we all agree that species exists, but how they emerge? How one species become two, or how one species become another?

The phenomenon by which it occurs is called speciation. Well, sort of… It all depends on how you define a species, actually (so be certain to have read the posts I mentioned above).

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Model of a lineage splitting into two lineages that evolve independently and eventually become separated species. Extracted from Hawlitschek et al. (2012)*

Speciation is usually defined as the evolution of reproductive isolation, therefore it deals more with the concept of biological species, but also with the ecological concept and certainly needs some insights on the vertical concepts. If two populations are reproductively isolated, it means that the individuals of one of them are unable or unwilling to breed with those of the other. This usually arrives through genetic and ecological differences that lead to differences in behavior, morphology, physiology. And considering that, we can classify reproductive isolation into two groups: pre-zygotic and post-zygotic isolation.

In pre-zygotic isolation, the two species are reproductively isolated because they do not want or cannot mate and produce an zygote. This may happen simply because of different behaviors in which the two species occupy different places in the environment, mate at different times of the year or even because they are not sexually attracted to each other. There are several experiments using fruitflies that demonstrate how this may evolve pretty fast.

In the late 1980s, William R. Rice and George W. Salt separated individuals of Drosophila melanogaster depending on their preference for dark × light and wet × dry environments, allowing them to mate only with other specimens showing the same preferences. After several generations, the individuals of one group were unable to mate with those of other groups because of their strong habitat preferences, making them unlikely to interact. A similar experiment was performed by Diane Dodd using the species Drosophila pseudoobscura, in which one population was raised with starch as food and other with maltose as food. In this case, after several generations the flies showed a strong preference to mate with individuals of the same group and to reject those of the other group.

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Evolution of reproductive isolation in fruit flies of the species Drosophila pseudoobscura after several generations fed with different sugars.

Such speciation events are called ecological speciation and are also well-documented in the widl, especially regarding fish preferring different habitats, such as shallow × deep water or still × running water. Eventually the individuals will diverge into two groups that are ecologically isolated in the same environment and consequently become reproductively isolated as well.

Post-zygotic isolation is generally a more advanced form of isolation that indicates deep genetic divergences. This is more commonly associated with the notion of biological species and is based on the inability of the individuals of the two species to produce viable offspring. They may mate with each other and even produce a zygote, but this will be unable to developed into an embryo or the offspring will be sterile or otherwise unable to survive enough to breed. A classical example is the mule, the hybrid of a mare and a donkey that is usully sterile.

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A mare, Equus ferus caballus (left), a donkey, Equus africanus asinus (right) and a mule (center). Photos by ‘Little Miss Muffit’ (flickr.com/people/42562654@N00)(mare), Adrian Pingstone (donkey) and Dario Urruty (mule).

In both forms of speciation mentioned above, reproductive isolation usually arises from the accumulation of small differences due to natural selection. This may be enhanced by two phenomena, pleiotropy and genetic hitchhiking.

Pleiotropy is the phenomen by which a single gene have influence over more than one phenotypic trait. For example, a gene that influences the shape of a bird’s bill may also make it change its diet and its song. Several human genetic diseases, such as phenylketonuria (PKU), are examples of pleiotropy.

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The frizzled trait in chickens, which makes the feather curl outward, also leads to delayed sexual maturity and decreased metabolism rate. Photo by flickr user Just chaos.*

Genetic hitchhiking, on the other hand, is the phenomenon by which a gene that is naturally selected carries neighbours genes that are in the same DNA chain with it. In fruitflies, for example, a gene that is linked to courtship behavior may be drawn with the gene linked to a digestive enzyme, so that flies that specialize in one kind of sugar have a different courtship behavior than others specialized in another sugar.

That’s all for now. In a future post, I’ll talk about the geographic and genetic variables in species formation.

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References and further reading:

Bolnick, D. I., Snowberg, L. K., Patenia, C., Stutz, W. E., Ingram, T. & Lau, O. L. 2009. Phenotype-dependent native habitat preference
facilitates divergence between parapatric lake and stream stickleback. Evolution, 63(8): 2004-2016.

Hendry, A. P.2009. Ecological speciation! Or the lack thereof? Canadian Journal of Fisheries and Aquatic Sciences, 66: 1383-1398.

Hoskin, C. J. & Higgie, M. 2010. Speciation via species interactions: the divergence of mating traits within species. Ecology Letters, 13: 409-420.

Maan, M. E., Hofker, K. D., van Alphen, J. J. M. & Seehausen, O. 2006. Sensory drive in cichlid speciation. The American Naturalist, 167(6):
947-954.

Nosil, P. 2008. A century of evolution: Ernst Mayr (1904-2005). Ernst Mayr and the integration of geographic and ecological factors in
speciation. Biological Journal of the Linnean Society, 95: 26-46.

Turelli, M., Barton, N. H. & Coyne, J. A. 2001. Theory and speciation. TRENDS in Ecology and Evolution, 16(7): 330-343.

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Friday Fellow: Gold-and-Brown Rove Beetle

ResearchBlogging.orgby Piter Kehoma Boll

It’s time for our next beetle. Today the fellow I chose is Ontholestes cingulatus or gold-and-brown rove beetle. Rove beetles are the second most numerous family of beetles after weevils. Their more remarkable feature is that their elythra are short, not covering the abdomen most of the time. I always say that they look like if they were wearing a little jacket. So if you find an elongate beetle with short jacket-like elythra, it is most likely a rove beetle.

The gold-and-brown rove beetle is found throughout North America and is a predator as most rove beetles. It is usually found near carrion and dung, but it is not a scavenger. What it does there is too prey on fly larvae feeding on the rotten material.

An adult showing the nice golden "tail". Photo by Bruce Marlin.*

An adult showing the nice golden “tail”. Photo by Bruce Marlin.*

The gold-and-brown rove beetle is 13–20 mm long and mostly brown, but the last abdominal segments, as well as the underside of the thorax, have a beautiful and shiny gold color.

The mating behavior of the gold-and-brown rove beetle is interesting. Usually the male stays around the female after copulating with her in order to guard her from other males. This behavior usually ends soon after the female has laid the eggs, since at this point the male can be sure that he is the father of the children. To perform this guarding behavior is costly for the male, as he could be using this time to copulate with another female. But as receptive females are kind of rare, it is more advantageous to assure the paternity of the offspring of at least one female than to risk losing everything.

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References:

Alcock, J. (1991). Adaptive mate-guarding by males of Ontholestes cingulatus (Coleoptera: Staphylinidae) Journal of Insect Behavior, 4 (6), 763-771 DOI: 10.1007/BF01052230

BugGuide. Species Ontholestes cingulatus – Gold-and-Brown Rove Beetle. Available at: < http://bugguide.net/node/view/9548 >. Access on August 1, 2016.

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

ResearchBlogging.orgby Piter Kehoma Boll

Everyone with some sort of knowledge on evolution have 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 classical 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 a unusual natural environment which may increase male investment. But all of this stuff refers to dioeicious 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 everytime they get laid?

Well, there is a lot of diversity in these organism, but all the principles of sexual conflict are still valid. Even if you are male and female at the same time, you still has 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 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 whan 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 successully 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.*

Eartworms 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 sp., mating. See how the white one, at 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 at 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 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 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 partners 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 futher 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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Biological fight: kites, mites, quite bright plights

ResearchBlogging.orgby Piter Kehoma Boll

A recently described fossil from the Silurian Herefordshire Lagerstätte in the United Kingdom has called much attention.

A photo of the fossil itself. Image by Briggs et al., extracted from news.nationalgeographic.com

A photo of the fossil itself. Image by Briggs et al., extracted from news.nationalgeographic.com

The appearance of the creature was build by scanning the rock and creating a 3D reconstruction of the fossil. It revealed that the animal, obviously and arthropod, had several smaller creatures attached by long threads, like kites. The species was named Aquilonifer spinosus, meaning “spiny kite-bearer”.

A 3D reconstruction of what Aquilonifer and its kites would have looked like. Image by Briggs et al. extracted from sci-news.com

A 3D reconstruction of what Aquilonifer and its kites would have looked like. Image by Briggs et al. extracted from sci-news.com

The authors (Briggs et al., 2016) thought about three possibilities to explain the unusual “kites”. They could be parasites, phoronts (i.e., hitchhikers), or babies. The idea of parasites was discarded because such long threads separating them from the host would have made it difficult to feed properly. They also considered it unlikely to be a case of phoronts, i.e., a species that uses the host as a mean to move from one site to another, because there were too many of them and the host most likely would have removed them by using the long antennae.

Artistic impression of Aquilonifer spinosus by Andrey Atuchin.

Artistic impression of Aquilonifer spinosus by Andrey Atuchin.

The remaining option is that the kites were offspring. The mother (or father) would have attached them to itself in order do carry them around in a unique mode of brood care. The authors compare it to several other arthropod groups in which some species carry their babies around during their first days. They also consider that the animal could have delayed its molting process to avoid discarding the babies with the exoskeleton.

But can we be sure that this is the case? The entomologist Ross Piper thinks differently. He compares the kites to uropodine mites, in which the juveniles (deutonymphs) attatch themselves to beetles by long stalks in order to be transported from one food source to another. As there are marine mites, that could be the case. He also points out that the kites are scattered through the body, which would make them unlikely to be offspring, as such a distribution would only hinder the parent’s mobility.

Briggs at al. responded to Piper’s critique arguing that marine mites have only recently evolved and that Aquilonifer is very different from a terrestrial beetle. It was most likely a bentonic species, crawling on the ocean’s floor, and not a swimmer, so that it would not be a very good dispersal agent.

What do you think of it? I find it difficult to choose one side. Piper’s comparison with mites is interesting, but only as a way to suggest a convergent evolution. I cannot see how the kites would have been really mites or even arachnids. Now the argument on the kites’ position on the body is a good point. No other group of animals carries their young attached to long stalks spread all over the body. Furthermore, how would the parent properly place the juveniles there? I can only see it as a plausible way if the host were the father and the mother crawled over him to stick the eggs in place. Additionally, couldn’t they be true phoronts  that were benefitial to the host? The little fellows could benefit by moving around on the big pal and reaching new food sources while giving protection or other advantage in return. And regarding the delay in molting, I cannot see any evidence that there was any delay. We don’t know how long the kites remained there and perhaps after molting they could simply leave their little houses and build new ones on the host’s new skeleton.

We may never know the truth, but we can keep exchanging ideas.

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References:

Briggs, D., Siveter, D., Siveter, D., Sutton, M., & Legg, D. (2016). Tiny individuals attached to a new Silurian arthropod suggest a unique mode of brood care Proceedings of the National Academy of Sciences, 113 (16), 4410-4415 DOI: 10.1073/pnas.1600489113

Briggs, D., Siveter, D., Siveter, D., Sutton, M., & Legg, D. (2016). Reply to Piper: Aquilonifer’s kites are not mitesProceedings of the National Academy of Sciences, 113 (24) DOI: 10.1073/pnas.1606265113

Piper, R. (2016). Offspring or phoronts? An alternative interpretation of the “kite-runner” fossil Proceedings of the National Academy of Sciences, 113 (24) DOI: 10.1073/pnas.1605909113

Switek, B. 2016. This bizarre creature flew its babies like kites. National Geographic News. Available at < http://news.nationalgeographic.com/2016/04/160404-bizarre-creature-flew-babies-kites-arthropod-fossils-science/ >. Access on July 07, 2016.

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Filed under Behavior, Evolution, Paleontology, Zoology

Male dragonflies are not as violent as thought

ResearchBlogging.orgby Piter Kehoma Boll

Males and females are defined by their gametes. Males have tiny, usually mobile gametes, while females have very large gametes that usually do not move. This means that females produce less gametes, but put a lot of resources in each one, i.e., female gametes are expensive. On the other hand, male gametes are very cheap, small and produced in large quantities. As a result of these differences, males and females have different interests during sex.

As females produce more expensive and less numerous gametes, they tend to be very selective on who they let fertilize them. But males benefit from fertilizing every female gamete they find in their way. In other words, females want quality and males want quantity. This difference in interests is called sexual conflict and is a strong evolutionary force.

One evolutionary adaptation that has been seen as resulting from sexual conflict is the mating system in odonates (dragonflies and damselfies). During sex, the male dragonfly grasps the female neck using a grapsing apparatus at the end of its abdomen. The female is then induced to connect the tip of its abdomen to the second and third segments of the male’s abdomen, where sperm is stored. The couple than flies together in a heart-like formation.

Two dragonflies of the species Rhionaescna multicolor copulation. The male is the blue one, which is grasping the female's neck and making her touch the tip of her abdomen to his second and third abdominal segments, where sperm is stored. Photo by Eugene Zelenko.*

Two dragonflies of the species Rhionaeschna multicolor copulating. The male is the blue one, which is grasping the female’s neck and making her touch the tip of her abdomen to his second and third abdominal segments, where sperm is stored. Photo by Eugene Zelenko.

It was thought that the male grasping apparatus forced an unwilling female to copulate with him, suggesting that the organ evolved through sexual conflict. The fact that males usually grab females way before they accept to mate and continue to hold them for a long time after the mating has finished (preventing her from mating with other males) seem to be good evidence for this theory. If this is true, than the female would try to get rid of the male, selecting stronger and bigger grasping apparatuses in males, as those would be more efficient in holding the female and, as a result, would lead to more descendants.

A study published last year tested this hypothesis. Córdoba-Aguilar et al. (2015) evaluated the allometry (the proportional size of a structure with respect to body size) of the male grasping apparatus in several dragonfly species. If males forced females to copulate, a hyperallometric relationship should be expected.

What does that mean? Well, let’s try to explain it the simplest way. When you plot data on the size of a structure according to the size of the body as a whole on a graph, using values that lead to a linear relationship, you may have different results. The structure may increase in size in the same way as the body, in a 1:1 relationship. In this case, the line in the graph is said to have a slope equal to 1 and there is an isometric relationship of the structure to the body. If the slope is greater than one, this means that the structure grows faster than the body, having a hyperallometric relationship. If the slope is smaller than one (but greater than zero), the relationship is hypoallometric and the structure grows slower than the body.

allometry

The measurements of the grasping apparatus in dragonflies in general showed an isometric relationship. So, according to this approach, the structure did not evolve as a “weapon” to subdue females. But which other explanations may exist then? It could be used as a courtship tool, a way for the male to convince the female to mate with him. It could also be a way to avoid interspecific mating, as the grasping apparatus has a strong specificity in shape to the female neck of the same species. A male dragonfly cannnot grasp a female of other species because the grasping apparatus simply does not fit in the female’s neck.

Both alternative hypotheses for the evolution of the apparatus are possible, but further studies are needed to test them.

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References:

Chapman, T., Arnqvist, G., Bangham, J., & Rowe, L. (2003). Sexual conflict Trends in Ecology & Evolution, 18 (1), 41-47 DOI: 10.1016/S0169-5347(02)00004-6

Córdoba-Aguilar, A., Vrech, D., Rivas, M., Nava-Bolaños, A., González-Tokman, D., & González-Soriano, E. (2014). Allometry of Male Grasping Apparatus in Odonates Does Not Suggest Physical Coercion of Females Journal of Insect Behavior, 28 (1), 15-25 DOI: 10.1007/s10905-014-9477-x

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Filed under Behavior, Entomology, Evolution