Evolution

Sex Doesn't Pay for Females

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Science  30 Sep 2005:
Vol. 309, Issue 5744, pp. 2138
DOI: 10.1126/science.309.5744.2138c

In the battle of the sexes—also known as sexually antagonistic coevolution—it is the female who loses. For instance, in Drosophila, males harm females during both courtship and mating. But are there hidden benefits for females; that is, do they endure the injury of multiple mating to benefit their offspring? And could such benefits compensate for the direct costs of mating?

Stewart et al.address the latter question in Drosophila by creating an artificial selection system that protects females from the cost of injury by males, but also robs them of any indirect advantages. A population of red- and brown-eyed females were briefly mated, and the nonvirgin flies were separated, so that the red-eyed females were subsequently exposed to a low density of harassing males (1:8, male:female) and the brown-eyed flies were exposed to a high density of males (1:1). Progeny from these crosses were collected and counted for eye color, and the experiment was repeated for five generations. The frequency of the red-eye “male resistance” allele increased substantially, showing that the indirect benefits of multiple mating (being able to trade up for a better mate) fail, by a considerable margin, to outweigh the harm inflicted. So why hasn't a real male resistance allele appeared? The authors speculate that males stay ahead of females in the sexual arms race and that females cannot anticipate male adaptations. — GR

Proc. R. Soc. London Ser. B 10.1098/rspb.2005.3182 (2005).

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