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.

Does polyploidy occur in central European species of the family Sphaeriidae (Mollusca: Bivalvia)?

“…All the 11 species investigated exhibit high chromosome numbers (140 in P. nitidum up to 240 in S. rivicola), which fall within the range of previously reported chromosome counts in sphaeriids (see [8] for an overview).…”

“…However, even Sphaerium species with strikingly different chromosome numbers can be closely related, provided that polyploidization was involved in speciation within the genus. Petkeviciute et al [8] hypothesize on the basis of karyotype and molecular data that S. rhomboideum (2n=44) represents a diploid ancestor of polyploid S. occidentale (2n=209-213). The Sphaerium species investigated in this study, despite having different chromosome numbers, showed very similar DNA contents.…”

“…The length of chromosomes in mitotic metaphase ranges from 3 μm to almost 13 μm in S. corneum [6] and 2 to 6 μm in S. rhomboideum [8], whereas in the species with high chromosome numbers it was usually about 1 μm. The predominance of small uniarmed chromosomes in karyotypes of species with high 2n points out to multiple chromosome fissions.…”

“…), S. rhomboideum (2n=44) [8] and S. nitidum (2n=30) [7], The remaining 11 species exhibited chromosome counts from 100 to 247 [7]). Lee and Ó Foighil [9] considered the pattern of chromosome numbers within the Sphaerium-Musculium clade to be a consequence of ancient polyploidization, whereas within the genus Pisidium they found molecular evidence for recent autopolyploidization, based on the sequences of the multiple alleles of the single-copy gene for 6-phosphogluconate dehydrogenase.…”

Phylogeny of Sphaerium solidum (Bivalvia) based on karyotype and sequences of 16S and ITS1 rDNA

“…Karyological studies of sphaeriid species, although scarce, revealed the exceptionally high variability of mitotic chromosome numbers, from 30 to 247 [17]. Clearly, significant changes in chromosome number and structure occurred during the evolution of Sphaeriidae.…”