Distribution and origin of chromosomal races in the freshwater planarian Dugesia polychroa (Turbellaria: Tricladida)

We present a karyotypic survey of the European freshwater planarian Dugesia polychroa, detailing frequencies of diploid and polyploid forms from 35 localities in seven countries. In this hermaphroditic species, diploids reproduce sexually and polyploids by pseudogamous parthenogenesis. Previous laboratory studies have shown that the two reproductive modes can interbreed, which may lead to new tri-, tetra-, and pentaploid lineages. We found four pure sexual, 25 pure parthenogenetic, and six mixed populations. Although some polyploid populations consisted entirely of triploids, most contained triploid and tetraploid individuals. Pentaploids were rare and reported for the first time from the field. In most populations, the higher ploidy levels were represented by fewer individuals. Our results indicate that occasional fertilisation of parthenogenetic eggs leading to genome addition is responsible for the maintainance of polyploid forms in natural populations of D. polychroa.