Mating systems and sexual selection in male-pregnant pipefishes and seahorses: Insights from microsatellite-based studies of maternity

In pipefishes and seahorses (family Syngnathidae), the males provide all po stzygotic care of offspring by brooding embryos on their ventral surfaces. In some species, this phenomenon of male "pregnancy" results in a reversal of the usual direction of sexual selection, such that females compete more than males for access to mates, and secondary sexual characteristics evolve in females. Thus the syngnathids can provide critical tests of theories re lated to the evolution of sex differences and sexual selection. Microsatell ite-based studies of the genetic mating systems of several species of pipef ishes and seahorses have provided insights into important aspects of the na tural history and evolution of these fishes. First, males of species with c ompletely enclosed pouches have complete confidence of paternity, as might be predicted from parental investment theory for species in which males inv est so heavily in offspring. Second, a wide range of genetic mating systems have been documented in nature, including genetic monogamy in a seahorse, polygynandry in two species of pipefish, and polyandry in a third pipefish species. The genetic mating systems appear to be causally related to the in tensity of sexual selection, with secondary sex characters evolving most of ten in females of the more polyandrous species. Third, genetic studies of c aptive-breeding pipefish suggest that the sexual selection gradient (or Bat eman gradient) may be a substantially better method for characterizing the mating system than previously available techniques. Finally, these genetic studies of syngnathid mating systems have led to some general insights into the occurrence of clustered mutations at microsatellite loci, the utility of linked loci in studies of parentage, and the use of parentage data for d irect estimation of adult population size.