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

Abstract

Chromosome sets of two North American sphaeriid species, Sphaerium rhomboideum Say, 1822 and S. occidentale Lewis, 1856, were studied using conventional Giemsa staining and karyometric analysis. Pronounced karyological divergence of congeners was revealed. The diploid number of 2n ¼ 44 was reported for S. rhomboideum and this is the first record of a diploid species in the highly polychromosomic Nearctic sphaeriid fauna. The karyotype was characterized by medium-sized and small chromosomes, which decreased in size gradually from 5.77 to 1.9 mm. Biarmed chromosomes with medially and submedially located centromeres predominated, but six pairs of subtelo-telocentric elements were also observed in the karyotype. The estimated mitotic chromosome number for S. occidentale ranges from 189 to 213, but most of the cells examined contained about 204– 209 chromosomes. A first attempt to karyotype a polyploid sphaeriid was made. It was revealed that the comparatively large and middle-sized chromosomes could be grouped in four, so the karyotype presumably evolved through tetraploidization. The small chromosomes formed the large fraction, about 137. Due to their similar and indistinct morphologies, it was impossible to arrange them into subgroups with confidence. Revealed karyological characteristics are discussed with reference to the existing phylogenetic interpretations of the evolutionary history of the Sphaeriinae.