The strength of post-mating sexual selection in cactophilic Drosophila: How many flies are not born because of sperm competition?

J . HURTADO; P. P. IGLESIAS; E. HASSON

Simposio; VIII Symposium on Drosophila Ecology, Genetics and Evolution; 2013

SPERM COMPETITION (SC) is a major component of sexual selection that enhances sexual conflicts. Based on these conflicts, SC has been proposed as a selective agent responsible for the rapid and adaptive evolution of sexual characters, and as a trigger of the formation of reproductive isolating barriers between species. The actual role of SC on rapid and adaptive evolution, however, is poorly understood and the intensity of SC as a selective agent has not been fairly valued. Here, we assessed the strength of SC and the mating system factors that account for it in the South American Drosophila antonietae, D. buzzatii and D. koepferae. We observed stronger sperm displacement in D. antonietae and D. buzzatii than in D. koepferae. When sperm of two different males co-occurred inside a female, first-mating male fertility was reduced by 73% in D. antonietae, 71% in D. buzzatii and only 33% in D. koepferae. Additional experiments revealed that D. antonietae and D. buzzatii females retained sperm for longer periods and re-mated more frequently than D.koepferae females. Based on these results we estimated that, at the time of female remating, 100, 90 and 30% of the effective sperm load still remains stored by females in D. antonietae, D. buzzatii and D. koepferae, respectively. Multiple paternity analyses on wild-caught females resembled laboratory results showing that the incidence of sperm encounters is higher in D. antonietae and D. buzzatii than in D. koepferae. The projected probability that wild-inseminated females carry sperm from different males was 88% in D. antonietae, 87% in D. buzzatii and 58% in D. koepferae. In view of the estimated risk and cost of SC, we conclude that SC can exert very intense selective pressures, particularly in D. antonietae and D. buzzatii, in which we inferred that 64-73% of functional sperm do not reach fertilization because of SC. According to our results, SC has a great potential to generate adaptive  evolution.