Dec 13, 2006

and now a mega sperm...

A recent study by co-authors -Bjork and SU biology associate professor Scott Pitnick, titled “Intensity of sexual selection along the anisogamy-isogamy continuum,” will be published in the June 8 issue of Nature. In the article, Bjork and Pitnick discuss a paradox they have discovered within the foundations of sexual selection theory as it relates to the sperm and egg production of Drosophila (fruit flies).
Previous work in the Pitnick lab has shown that post-copulatory sexual selection on males (the sperm competition that takes place after females mate with more than one male) can lead to decreased sperm quantities by favoring the production of larger sperm. In other words, their production moves toward a “quality, not quantity” mode, because female fruit flies have evolved so that longer sperm have a greater chance of successfully fertilizing eggs.
Successful fertilization is important, since individuals that fail to pass their genes on to the next generation are at an evolutionary dead end. This leads to the “big sperm paradox” because the idea that postcopulatory sexual selection could favor the evolution of giant sperm clashes with traditional sexual selection theory, which predicts that the most successful sperm competitors will be the males that produce many, tiny sperm. As males evolve to produce larger—and therefore fewer—sperm, eggs become less rare, and sexual selection should weaken, according to theory.

The term “isogamy” refers to the state at which males and females have equal investment per gamete (sex cell) when producing sperm and eggs. In a truly isogamous population, each sperm and each egg would have a chance to participate in a successful fertilization. In such a population, sexual selection would be extremely weak, as there would be little or no competition among males to fertilize eggs. more on this confusing story here

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