The reproductive success of the sneaky mating tactic is difficult to determine in fish with internal fertilization. We approached this problem by developing an assay that allows the recovery of sperm DNA from the reproductive tract of females. This assay was used to test whether sperm transfer occurs between sympatric populations of the guppy, Poecilia reticulata, and its putative sister species, Poecilia picta. Six hundred females of both species from two sympatric sites in Trinidad were collected, and then the contents of the females' reproductive tracts were analysed using reciprocally diagnostic microsatellite markers. Conspecific sperm occurred in approximately 86% of females, whereas heterospecific sperm were found in only 4% of females. Because females of either species do not mate willingly with heterospecific males, the results indicate that sneaky mating results in the transfer of sperm. The data are consistent with the idea that sexual conflict can result in differences in the strength of behavioural isolation between the sexes, and they suggest that behavioural isolation is unlikely to have driven speciation between the guppy and P. picta. (c) 2006 The Linnean Society of London.</p
