Allozyme variability in sexual and parthenogenetic freshwater planarians: Evidence for polyphyletic origin of parthenogenetic lineages through hybridization with coexisting sexuals

In Dugesia polychroa, a hermaphroditic planarian, diploid sexual and polyploid pseudogamous parthenogenetic forms exist in separate populations and in sympatry. Parthenogens produce viable, haploid sperm that are exchanged between two individuals during copulation and trigger sperm-dependent parthenogenetic egg development. In crossing experiments, parthenogens can fertilize the eggs of sexuals, occasionally initiating the origin of new polyploid parthenogenetic lineages. We present an allozyme study of genetic variation in both separate and mixed populations, in which multilocus genotypes defined by seven loci were used to characterize parthenogenetic lineages. Purely parthenogenetic populations were found to be composed of genotypically different lineages. In mixed populations, the allelic variation of parthenogens is similar to that of the coexisting sexuals. The observed genotypic diversity suggests that parthenogens from mixed populations are of polyphyletic origin. We propose that, in natural populations, hybridization between sympatric sexuals and parthenogens leads to the appearance of new, genetically diverse parthenogenetic lineages.