Hello, Readers!

This week I am going to introduce you to a new kind of co-evolution, and it is much smaller than the wolves I discussed last time...

We are going into the sexual organs of mice, to learn about coevolution between gametes. While this sounds a little far-fetched, a study published in 2014 found that mice gametes co-evolve over generations, fighting a microscopic arms race with each other. Over 24 generations, researchers found that female mice that evolved in a polygamous program had "more defensive ova" than the females evolved under a monogamous system.

This study is interesting because it proves that sexual competition isn't just between males and females, or males and males, but between the sperm and egg. The female defense is driven by sperm competition, and acts by the use of a sperm barrier that can intercept or moderate sperm entrance. Sperm competition was seen after 8 generations of mice, when the male mice would output more sperm with a higher motility. Females eggs that were evolved under polygamous regimes were less likely to get fertilized than the eggs from monogamous regimes in a controlled test. While lower sperm defense can be a positive in monogamy, ova defenses can be beneficial in an environment with high sperm availability. This is because the female then is able to be fertilized by the mate that has the most competitive or fit sperm. This is an evolutionary advantage to the female, who can then have the fittest offspring. This  "postcopulatory sexual selection" may be able to explain asymmetry in fertilization rates when comparing different populations. Firman et al. also suggests that competition like this may also lead to genetic divergence in populations, eventually making the 2 populations impossible to breed due to reproductive isolation.

The researchers were able to study just the gamete integration using in vitro fertilization. 

This study is interesting to me, because it makes me wonder about all the other species that have gametes that are coevolving and competing with one another, especially mammals. Coevolution is an incredible thing to see among whole organisms, but between mere cells? That is something beyond cool. No matter how small, life finds a way to compete for the #1 spot. 

Thoughts? Please share below. I'd love to hear what you think of the smallest layer of sexual competition and coevolution!

Have a good week,

D

Reference:

Firman, R. C., Gomendio, M., Roldan, E. R., & Simmons, L. W. (2014). The coevolution of ova defensiveness with sperm competitiveness in house mice. The American Naturalist, 183(4), 565-572.