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Postmating Sexual Selection Favors Males That Sire Offspring with Low Fitness

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Science  26 Jun 2009:
Vol. 324, Issue 5935, pp. 1705-1706
DOI: 10.1126/science.1171675

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Two's a Crowd

The process by which males and females compete to maximize their individual fitness also affects the fitness of their offspring. Sexual selection largely results from polyandry (multiple mating by females), and several competing hypotheses attempt to explain the evolution of polyandry. Bilde et al. (p. 1705) staged double mating experiments in seed beetles to distinguish between the theories underlying cryptic female choice and sexual antagonism. Contrary to expectation, males of high genetic quality, as measured on the basis of the number of offspring sired when singly mated to a female, consistently produced fewer offspring when females were doubly mated to males of both high and low genetic quality. Thus, postmating sexual selection can favor male genotypes with low fitness, and females risk genetic costs when mating with multiple males.

Abstract

Despite the costs of mating, females of most taxa mate with multiple males. Polyandrous females are hypothesized to gain genetic benefits for their offspring, but this assumes paternity bias favoring male genotypes that enhance offspring viability. We determined net male genetic effects on female and offspring fitness in a seed beetle and then tested whether fertilization success was biased in favor of high-quality male genotypes in double mating experiments. Contrary to expectations, high-quality male genotypes consistently had a lower postmating fertilization success in two independent assays. Our results imply that sexually antagonistic adaptations have a major and unappreciated influence on male postmating fertilization success. Such genetic variation renders indirect genetic benefits an unlikely driver of the evolution of polyandry.

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