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Tuesday "Noon" Seminar: Jan. 30, 2007Adam Bjork on "Postcopulatory Sexual Selection In Drosophila"
12:30-1:45 p.m. in Biosciences West (map of building location), Room 208 |
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Talk Abstract Postcopulatory sexual selection favors males that are strong offensive and defensive sperm competitors. Genetic variation for each of these strategies is high. I will present data showing that both offensive and defensive sperm competitive ability are significantly repeatable in Drosophila melanogaster only across matings involving the ejaculates of the same pair of males competing within the same female. These repeatabilities decrease when the rival male stays the same but the female changes, and they disappear when both the rival male and the female change. In addition, an experimental evolution approach was used to select independently for enhanced sperm offense and enhanced sperm defense. The results from these experiments underscore the complex nature of sperm precedence and the maintenance of genetic variation in ejaculate characteristics. Postcopulatory sexual selection on males can lead to decreased sperm numbers by favoring the production of larger sperm. However, a decline in sperm numbers is predicted to weaken selection on males and increase selection on females. As isogamy is approached (i.e. as investment per gamete by males approaches that by females), sperm become less abundant, ova become relatively less rare, and competition between males for fertilization success is predicted to weaken. Sexual selection for longer sperm, therefore, is expected to be self-limiting. I examined this paradox in Drosophila along the anisogamy-to-isogamy continuum using intraspecific experimental evolution techniques and interspecific comparative techniques. The results, which conflict with traditional theoretical predictions, show that the sex difference in sexual selection gradients decreases as sperm size increases. However, a resolution is provided when this finding is interpreted in concert with the opportunity for selection (I) and the opportunity for sexual selection (Is). Furthermore, the majority of the variation in measures of selection intensity is explained by sperm length and relative investment in sperm production. This work represents a portion of my graduate research at Syracuse University. |
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