Evolution fun facts 3/3 – Sexual Conflict and genomic imprinting

Another conflict hiding under everybody’s eyes is something called sexual conflict. What we mean with this is the difference in strategy that mothers or fathers should take in sexual behaviours. This time, relatedness doesn’t have a big importance: both parents usually have r=0.5 with every offspring, but the conflict can arise because males get more benefits behaving differently than females, leading to interesting conflicts when having sex and caring for children are concerned.

I will get quickly over some basic ideas that have been mostly agreed upon. Typical sex roles in the animal kingdom are eager males and choosy females. Which means large, aggressive and competitive males who try to reproduce as much as possible and more selective, careful and somewhat loyal females. Bateman and Trivers (who came up with this theory) explained typical sex roles based on usual differences in investment in offspring by each sex.

Let me explain: males can produce A LOT of super tiny sperm for a super cheap price. In comparison females have to produce expensive big eggs, in many cases go through gestation for getting a baby and generally spend a lot of time for the offspring. So, overall, based on these, and other differences, there’s a huge difference of the costs that each sex has to invest on reproductive behaviour. Males are better off reproducing as much as possible with as many females as possible most of the time, thus increasing a lot their reproductive success. Instead females are usually obliged to raise childrens, trying to be pregnant at least at every breeding season to maximize her reproductive success. Such trends can be partly observed even in us humans.

In the end what we’re left with is that females are generally the rare and scarce resource in nature and many ways for finding them and for having sex with them are being and have been promoted by natural selection. In other words, if you’re a male, do whatever you can to get the next female. Males have to compete between each other becoming typically very large and can showcase their skill or traits to the female. On the other hand, females should be very choosy and at least try to guarantee themselves the best partner looking from the available ones.

Examples of conflicts between these two approach are evident and plentiful. Things such as coercive sex (abuse), traumatic insemination (harmful sexual act), toxic sperm (harmful sperm that reduce lifespan) are all extreme manifestations of the need of the male to copulate with a female and not let her copulate with others. Stuff like infanticide (killing children from previous fathers) makes sense because males want females to focus on their new children without wasting resources on old ones (not related to the new father). Things like extreme behavious or striking showcases (like dances, singing or colorful patterns), nuptial gifts (in general food or shelter) and even sexual cannibalism (where males get eaten by females) are all explained by the need of the male for being chosen by the female (without letting her reproduce again with others).

traumatic insemination
Yep, you didn’t guess it, they’re all penises. Insect penises: Bean beetle, Damselflies and Bedbugs (starting from the left one proceeding counter-clockwise). I guess now it’s easier to understand why it’s called traumatic insemination.


Finally this conflict reflects itself in parental care as well. Males mostly don’t care for their own offspring while females obviously do. A lot of strategies can arise from males or females to guarantee cheatability or loyalty. But these and many more examples have been described and told by many and they’re always cited when such phenomenons are taught.

What I want to talk about is something, books or people don’t talk about enough, I feel. And it’s a a very subtle form of Sexual Conflict. It takes place at the molecular level, just after the fertilization of the egg by the sperm. Even before the new children is born.

We’re talking about genomic imprinting. What we need to know about this amazing epigenetic phenomenon (epigenetic means that is not directly determined by the DNA sequence but by other mechanisms but we won’t go into details) is that when the zigote (fertilized egg, the starting of the new organism) is conceived, it acquire genes from both parents. However this can sometimes happen in a parent-specific way: contributions from two sexes are not identical and each of the sex pushes for what they want. And what do they want? Well, just as we said, males want as many offspring as possible, so parental genes will increase growth and maturation of the new children as much as possible even at cost of the mother (who has also to care about resources she needs for her own survival). Females, want an optimal equilibrium between offspring quality and her own life quality (just like the egg size conflict we’ve seen last time) so maternal contribution of genomic imprinting will be to inhibit paternal stimulus to eccessive growth and promote and normal maturation rate.

When I heard of this, it almost sounded like science finction. Do we actually know an example of this form of sexual conflict or is it everything just speculation? Apparently we do! The gene for IGF-II (a hormone with similar functions to insulin) has two different versions (alleles) that are differently expressed by the two sexes. Paternal allele is always highly express (it produces a lot of this protein) while the maternal allele is normally inactive. IGF-II stimulates deeply cell division and growth, thus having the exact role we just examined for paternal contribution of the sexual conflict but what is the females doing? Well, she’s not expressing that gene for one, hence not stimulating even more growth but so far she’s not contrasting the father’s willing of rapid growth. It turns out that mothers express another gene: CI-MPR (which is absent in the paternal contribution). This gene has nothing to do with growth but it can bind to IGF-II preventing its normal function! Males need for fast offspring maturation is, at least in part, counteracted by females effect of CI-MPR! How amazing is that?

In summary, sexual conflicts can take many different shapes and forms. You should remember that the natural world is full of crazy, weird but amazing stuff, even when we talk about sex. We still have a lot of stuff to say on this topic but I really wanted to communicate how secret conflicts, such as sexual conflicts, are well present in the world, from the macro to the micro level, and they can shape and determine entire individuals relationships. Phenomenons on this scale can have huge effects on what we do even if most people don’t know about them.

This series is finally over, I hope you got something out of it; while you think on it, let me tell you another story..



One Comment Add yours

  1. Minu says:

    Thats so cool! Very informative 🙂

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