Amphimixis and pseudogamy in fresh-water triclads: Experimental reconstitution of polyploid pseudogamic biotypes

In the fresh-water planarians Dugesia benazzii and D. lugubris, diploid amphimictic and polyploid pseudogamic biotypes occur, each of them with a peculiar chromosome cycle. In crosses between diploid biotype (acting as female) and polyploid ones, various degrees may be observed from amphimictic diploid to polyploid pseudogamic offspring. All cytological mechanisms that characterise the cycles of natural biotypes and of hybrids are inherited independently and only rarely can they be found together in a harmonic complex in such a way as to reconstruct the natural polyploid pseudogamic biotypes. The author analyses the genetical mechanisms which, in nature, may have led to the formation of these polyploid biotypes.Dedicated to Professor J. Seiler on the occasion of his 80th birthday.