Pronounced karyological divergence of the North American congeners Sphaerium rhomboideum and S. occidentale (Bivalvia: Veneroida: Sphaeriidae)

Chromosome sets of two North American sphaeriid species, Sphaeriumrhomboideum Say, 1822 and S. occidentale Lewis, 1856, were studiedusing conventional Giemsa staining and karyometric analysis.Pronounced karyological divergence of congeners was revealed.The diploid number of 2n = 44 was reported for S. rhomboideumand this is the first record of a diploid species in the highlypolychromosomic Nearctic sphaeriid fauna. The karyotype wascharacterized by medium-sized and small chromosomes, which decreasedin size gradually from 5.77 to 1.9 µm. Biarmed chromosomeswith medially and submedially located centromeres predominated,but six pairs of subtelo-telocentric elements were also observedin the karyotype. The estimated mitotic chromosome number forS. occidentale ranges from 189 to 213, but most of the cellsexamined contained about 204–209 chromosomes. A firstattempt to karyotype a polyploid sphaeriid was made. It wasrevealed that the comparatively large and middle-sized chromosomescould be grouped in four, so the karyotype presumably evolvedthrough tetraploidization. The small chromosomes formed thelarge fraction, about 137. Due to their similar and indistinctmorphologies, it was impossible to arrange them into subgroupswith confidence. Revealed karyological characteristics are discussedwith reference to the existing phylogenetic interpretationsof the evolutionary history of the Sphaeriinae. (Received 8 November 2006; accepted 25 June 2007)