Small-bodied males invest in larger testes when highly ornamented

Sperm competition and male mating rate are two non-mutually exclusive key evolutionary pressures selecting for larger testes within and across animal taxa. A few studies have tried to test the role of mating rate in the absence of sperm competition. Under the mating rate hypothesis, particular phenotypes of a given population that are expected to gain more mates (e.g., more ornamented males) are expected to make higher investments in testes size (a proxy for sperm production). We test this prediction in Polistes simillimus, a neotropical paper wasp in which females are single mated (no sperm competition) and males can mate with multiple partners. Testes size was predicted by body size (positive association), sexual ornamentation (negative association), and their interaction (among small males, testes size was positively related to ornamentation, but the opposite pattern was observed among large males). We propose that small-bodied well-ornamented males may face the highest risk of sperm depletion. Small-bodied males make relatively higher investment in testes size when highly ornamented. This strategy might be less profitable to large males, as they have overall larger testes. Our results provide strong evidence for the mating rate hypothesis.

Positive genetic covariance between male sexual ornamentation and fertilizing capacity

Postcopulatory sexual selection results from variation in competitive fertilization success among males and comprises powerful evolutionary forces that operate after the onset of mating.^1,2 Theoretical advances in the field of sexual selection addressing the buildup and coevolutionary consequences of genetic coupling^3-5 motivate the hypothesis that indirect postcopulatory sexual selection may promote evolution of male secondary sexual traits-those traits traditionally ascribed to mate choice and male fighting.^6,7 A crucial prediction of this hypothesis is genetic covariance between trait expression and competitive fertilization success, which has been predicted to arise, for example, when traits subject to pre- and postcopulatory sexual selection are under positive correlational selection.⁸ We imposed bidirectional artificial selection on male ornament (sex comb) size in Drosophila bipectinata and demonstrated increased competitive fertilization success as a correlated evolutionary response to increasing ornament size. Transcriptional analyses revealed that levels of specific seminal fluid proteins repeatedly shifted in response to this selection, suggesting that properties of the ejaculate, rather than the enlarged sex comb itself, contributed fertilizing capacity. We used ultraprecise laser surgery to reduce ornament size of high-line males and found that their fertilizing superiority persisted despite the size reduction, reinforcing the transcriptional results. The data support the existence of positive genetic covariance between a male secondary sexual trait and competitive fertilization success, and suggest the possibility that indirect postcopulatory sexual selection may, under certain conditions, magnify net selection on ornamental trait expression.