Molecular evidence for multiple paternity in a feral population of green swordtails

Genetic parentage analyses provide insights into mating systems and have revealed widespread evidence for polyandry in natural populations. Here, we use 5 microsatellite markers to estimate female mating rates in a feral population of green swordtails, Xiphophorus helleri, a live-bearing poeciliid fish that has become a model system in the study of precopulatory mate choice and mating competition. Although heralded as a potential model for investigating sperm competition as early as 1950, there has been no attempt to explore postcopulatory sexual selection in its mating system. We thus obtained information on the prevalence, and therefore biological relevance, of polyandry from a wild population. We genotyped the offspring from 14 wild-caught gravid females and determined the number of fathers in each brood using allele counting methods and the programs GERUD and PARENTAGE. Our analyses revealed that 57% (allele counts and GERUD) and 71% (PARENTAGE) of the sampled broods had at least 2 sires, with a global mean 1.74 fathers per brood. Paternity skew was generally high in mixed paternity broods so that our analyses almost certainly underestimate actual mating frequencies in the wild. Our data provide a solid underpinning for future studies of postcopulatory sexual selection in this species.