Evidence for Polyploidy in the Hermaphrodite Groups of Animals

Abstract

IT is now well known that a large number of species of higher plants are polyploids, and it is reasonable to infer that polyploidy has been one of the main evolutionary methods of species formation in the angiosperms. In animals a few instances of polyploidy are known in parthenogenetic forms (for example, in the Crustaceans Artemia¹ and Trichoniscus², the moth Solenobia³ and some of the weevils⁴). Apart from these cases where polyploidy is associated with a complete abolition of the sexual process, there is little evidence for its existence in animals. It was pointed out by Muller⁵ that polyploidy could not be expected to occur in groups of bisexual animals, since it would upset the sex-chromosome mechanism. In groups of hermaphrodite animals, however, there seems no a priori reason why polyploidy should not be one of the methods of species formation, just as it is in plants. In order to determine whether this is so, I have collected all the available chromosome numbers for the four main groups of hermaphrodite animals (Platyhelminthes, Oligochgæta, Hirudinea and Pulmonate Mollusca). After eliminating early determinations which must be regarded as unreliable, the remaining data were plotted in the form of the accompanying histograms. In the case of the Platyhelminthes only the data for the Rhabdocœla were utilized, since very few chromosome numbers are known for the other classes of that phylum.