Diploid Amazon mollies (Poecilia formosa) show a higher fitness than triploids in clonal competition experiments

Abstract

The gynogenetic livebearing Amazon molly (Poecilia formosa) is a sexual parasite that exploits males of closely related species for sperm. This is needed as physiological stimulus for embryo development; however, none of the male’s genes are normally incorporated into the genome of the gynogenetic offspring. Mostly diploid individuals were reported from the natural habitats in North-Eastern Mexico and South-Eastern Texas but stable populations of triploids have been reported from the Rio Soto la Marina drainage and in the Rio Guayalejo in North-Eastern Mexico. Triploidy is the result of defects in the mechanisms that normally clear the host sperm from the ameiotic diploid egg. Triploids also reproduce gynogenetically and their frequencies fluctuate markedly between years, seasons, and localities. To understand the dynamics of this mating system, it is important to understand the relative reproductive success of triploids and diploids. We hypothesize that triploids should have a selective advantage over diploids due to heterosis and/or gene redundancy based on the additional genetic material from the sexual host. However, clonal competition experiments revealed a clear reproductive advantage of diploids competing with triploids. This result contradicts not only our hypothesis but also the stable co-existence of diploids and triploids in natural habitats. Frequency dependent selection, niche partitioning and environmental heterogeneity are discussed as possible explanations.