GENETIC-VARIATION IN COEXISTING SEXUAL DIPLOID AND PARTHENOGENETIC TRIPLOID FORMS OF FRIDERICIA-GALBA (ENCHYTRAEIDAE, OLIGOCHAETA) IN A HETEROGENEOUS ENVIRONMENT

The distribution of two sexual sibling species and a triploid parthenogenetic form of Fridericia galba, and the genetic variation within each biotype were studied within an area covering appr. 7 ha. The parthenogenetic form tends to reach its highest relative frequencies in stations at intermediate elevations and with generally favourable water conditions, whereas the two sexual species dominate in lower occasionally waterlogged resp. higher lying occasionally dry habitats. While the sexual sibling species show no intraspecific differentiation, they differ strongly in genetic heterogeneity. Based upon 5 polymorphic loci, 13 major clones were identified in the parthenogenetic form. The distribution of some clonal groups is nonrandom arid correlated with the relative frequencies of the two sexual sibling species. These findings are discussed in relation to a previous study on the likewise sexual/asexual isopod, Trichoniscus pusillus, within the same area. In both species the parthenogens successfully compete with their sexual congeners in parts of this heterogeneous environment, and they comprise a number of ecologically diverse clones. It is suggested, that parthenogenesis may result in a differential distribution of ecologically diverse clones, and thereby, perhaps, in an efficient exploitation of a spatially variable niche.