Rp. Weinzierl et al., Spatial and ecological overlap between coexisting sexual and parthenogenetic Schmidtea polychroa (Tricladida; Platyhelminthes), HYDROBIOL, 392(2), 1999, pp. 179-185
Theoretical models on the costs and benefits of sexual reproduction usually
assume that sexual and parthenogenetic individuals coexist and are identic
al, except for their mode of reproduction. Empirical studies, however, show
that conspecific sexuals and parthenogens can differ in ecological prefere
nces 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 sexu
als and parthenogens. Nineteen samples contained both biotypes. All but one
of the 13 ecological parameters that we measured, could not explain a sign
ificant part of the variance in relative abundance of each type. Only leech
abundance had a significant, negative effect on the presence of sexual ind
ividuals. 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 bet
ween sites, suggesting isolation of local populations by migration barriers
. There were smaller differences between sexuals and parthenogens within si
tes, suggesting that genetic exchange between biotypes may be limited. We c
onclude that there appears to be weak niche differentiation between sexuals
and parthenogens in Lago di Caldonazzo in late summer. Fluctuations in rel
ative frequency appears to be a consequence of low dispersal between local
populations and stochastic effects within them.