Spatial and ecological overlap between coexisting sexual and parthenogenetic Schmidtea polychroa (Tricladida; Platyhelminthes)

Theoretical models on the costs and benefits of sexual reproduction usually assume that sexual and parthenogenetic individuals coexist and are identical, except for their mode of reproduction. Empirical studies, however, show that conspecific sexuals and parthenogens can differ in ecological preferences and geographical distribution, which complicates the investigation of the costs and benefits of sex. The freshwater planarian Schmidtea polychroa exists in a sexual and a sperm-dependent, parthenogenetic form. The latter produce fertile sperm and mate, but received sperm is used only to induce parthenogenetic embryo development. We compared the spatial and ecological distribution between forms within a lake from which both had been reported. Forty samples showed large differences in the relative frequencies of sexuals and parthenogens. Nineteen samples contained both biotypes. All but one of the 13 ecological parameters that we measured, could not explain a significant part of the variance in relative abundance of each type. Only leech abundance had a significant, negative effect on the presence of sexual individuals. The causes of this effect remained unclear. We also estimated the amount of genetic isolation between sites and between reproductive modes, using body coloration as a genetic marker. Large differences were found between sites, suggesting isolation of local populations by migration barriers. There were smaller differences between sexuals and parthenogens within sites, suggesting that genetic exchange between biotypes may be limited. We conclude that there appears to be weak niche differentiation between sexuals and parthenogens in Lago di Caldonazzo in late summer. Fluctuations in relative frequency appears to be a consequence of low dispersal between local populations and stochastic effects within them. This revised version was published online in July 2006 with corrections to the Cover Date.