Polyploidy and Meiosis in the Freshwater Clam Sphaerium striatinum (Lamarck) and Chromosome Numbers in the Sphaeriidae (Bivalvia, Veneroida).

Spermatogenetic meiosis as well as a very large number of chromosomes (n=76, 2n=ca. 152) were observed in a North American freshwater clam, Sphaerium striatinum. Sphaeriid species studied to date, except for one species, are all polyploids having high mitotic chromosome numbers, which range from ca. 150 to ca. 247. These results indicate that pronounced polyploidization may have played a major role in the evolution of the Sphaeriidae.This study also suggests that the basic chromosome number of the sphaeriid polyploids may be 19 and that a very significant variation in ploidy levels (2n to 13n) occur in the Sphaeriidae. Even though meiosis has been observed in a polyploid species, S. striatinum, and in the diploid S. corneum (n=18, 2n=36), it is not clear whether or not polyploidy in any of the other sphaeriid species is associated with asexuality.