Quantitative genetic correlation between trait and preference supports a sexually selected sperm process

Abstract

Sperm show patterns of rapid and divergent evolution that are characteristic of sexual selection. Sperm competition has been proposed as an important selective agent in the evolution of sperm morphology. However, several comparative analyses have revealed evolutionary associations between sperm length and female reproductive tract morphology that suggest patterns of male-female coevolution. In the dung beetle Onthophagus taurus, males with short sperm have a fertilization advantage that depends on the size of the female's sperm storage organ, the spermatheca; large spermathecae select for short sperm. Sperm length is heritable and is genetically correlated with male condition. Here we report significant additive genetic variation and heritability for spermatheca size and genetic covariance between spermatheca size and sperm length predicted by both the "good-sperm" and "sexy-sperm" models of postcopulatory female preference. Our data thus provide quantitative genetic support for the role of a sexually selected sperm process in the evolutionary divergence of sperm morphology, in much the same manner as precopulatory female preferences drive the evolutionary divergence of male secondary sexual traits.

Fig. 1.

Image of the spermatheca of O. taurus with 32 landmarks placed around its periphery. For geometric morphometric analysis, landmarks 1, 13, and 32 were assigned as fixed, and the remaining were assigned as sliding semilandmarks.

Fig. 2.

The consensus shape (Center) from geometric morphometric analysis is shown with partial warps shown as vectors with their origin at the consensus position of each landmark. Thin-plate splines show variation in shape along the first relative warp, with extreme negative scores shown in Left and extreme positive scores shown in Right.

Fig. 3.

Plot of the sire family mean (±SE) sperm lengths against sire family mean (±SE) spermatheca centroid size. The data show a genetic correlation across the sexes for these sperm and sperm storage traits.