Polyploid Chromosome Numbers in the Torquis Group of the Freshwater Snail Genus Gyraulus (Mollusca: Pulmonata: Planorbidae)

Abstract

The first case of polyploidy in pulmonate snalis was found in the planorbid species Gyraulus (Torquis) circumstriatus (n=36, 2n=72). We now report the chromosome numbers of the other two members of the subgenus in North America, G. (T.) parvus and G. (T.) huronensis. The latter two species have the same chromosome number, i.e., they are also tetraploid. The basic chromosome number for the family Planorbidae is x=18. The tetraploid condition found in the three polyploid species was probably originally the result of one single event. Once the tetraploid condition was reached, the stability of this number has been maintained in each of the three species.