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November 08, 2004

"Sperm competition"

We've talked about "sperm competition"1 on this blog before, well, here comes a study (abstract in the extended entry) in Nature Genetics which shows that "...that the evolution of SEMG2, the gene encoding semenogelin II, a main structural component of semen coagulum, is accelerated in polyandrous [multiple males] primates relative to monandrous [one male] primates. Our study showcases the intimate relationship between sexual selection and the molecular evolution of reproductive genes." The basic trend is transparent, species where females are likely to have copulations with multiple males during their fertile period are likely to be under selective pressures that alter the characteristics & quantity of sperm to maximize chances of insemination and minimize fertilization by other males (various popular articles highlight gleefully the fact that in some species semen can form a "solid plug" like glue). The proximate outcome of the ultimate evolutionary pressures is that you see a variation in the ratio of testicle size to body size that covaries with the type of mating regime practiced by a species, that is, while gorillas have relatively small testicles because males monopolize sexual access to females in their harems, chimpanzees have large testicles because female chimps tend to "shop around."2 Additionally, popular press articles listed below report that the nature of the semen itself varies from species to species (viscosity), features that are controlled by the gene mentioned above. Evidence of selection on this gene suggest an "arms race" as males engage in sperm competition. Another nail in the coffin of Desmond Morris' pair-bonding? The congruence of molecular genetic evidence with findings from ethology and primatology is a powerful testament to the fact that various scientific disciplines act like separate vectors that focus upon the same basic truth (though you might subscribe to the insane view that they are parallel social constructions).

The article highlights the fact that the evidence seems to point to an evolutionary history of human sexuality that can be characterized as a medium between the strategies of chimps & gorillas in terms of the polyandry/monandry dichotomy. I imagine the molecular data can be an avenue to test some of the hypotheses presented about variation in mating strategies between various human groups ("cads vs. dads").3 At this point, I'm not sure if a great amount of intergroup variation will be found on the molecular level because of the changes in social environment that disaparate populations have been through (though a few relatively stable cultures like China might offer possibilities), but it is an interesting question.

Related: An entry from me on polygyny vs. monogamy in humans. Another post on the relationship between pathogens & polygyny. Popular press pieces from the BBC and The Australian on the aforementioned research paper.

1 - Also known as "dipping your buddy's pot of glue."

2 - I have to say, I find the constant popular press descriptions of females "promiscuous" a little annoying. Perhaps I'm just PC about this, but males in polygynous circumstances are generally described not described such a fashion.

3 - An interesting case study might be to compare populations that share a common genetic history and live in close proximity, for example, the adivasi populations of India and the caste Hindu argrarian peoples who are their neighbors. If there are intergroup differences do manifest themselves because of microevolutionary forces one would imagine that in caste Hindus functional constraints might be released as the "arms race" abates under social enforced monogamy and strident sanctions against female infidelity.

Rate of molecular evolution of the seminal protein gene SEMG2 correlates with levels of female promiscuity

Steve Dorus1, 2, Patrick D Evans1, 2, Gerald J Wyckoff1, 3, Sun Shim Choi1 & Bruce T Lahn1

1 Howard Hughes Medical Institute, Department of Human Genetics, University of Chicago, Chicago, Illinois 60637, USA.

2 Committee on Genetics, University of Chicago, Chicago, Illinois 60637, USA.

3 Present address: Division of Biology and Biochemistry, University of Missouri-Kansas City, Kansas City, Missouri, 64108, USA.

Correspondence should be addressed to Bruce T Lahn blahn@bsd.uchicago.edu

Postcopulatory sperm competition is a key aspect of sexual selection and is believed to drive the rapid evolution of both reproductive physiology and reproduction-related genes1, 2, 3, 4. It is well-established that mating behavior determines the intensity of sperm competition, with polyandry (i.e., female promiscuity) leading to fiercer sperm competition than monandry1, 2, 3. Studies in mammals, particularly primates, showed that, owing to greater sperm competition, polyandrous taxa generally have physiological traits that make them better adapted for fertilization than monandrous species, including bigger testes, larger seminal vesicles, higher sperm counts, richer mitochondrial loading in sperm and more prominent semen coagulation2, 5, 6, 7, 8. Here, we show that the degree of polyandry can also impact the dynamics of molecular evolution. Specifically, we show that the evolution of SEMG2, the gene encoding semenogelin II, a main structural component of semen coagulum, is accelerated in polyandrous primates relative to monandrous primates. Our study showcases the intimate relationship between sexual selection and the molecular evolution of reproductive genes.

Posted by razib at 10:18 PM