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

Abstract

Abstract The present work represents the first karyological and molecular characterisation of Sphaerium solidum, a rare European clam. Specimens of S. solidum were collected in Lithuania and Hungary. The modal diploid chromosome number found in both populations was 2n=30. Small, biarmed B chromosomes were found in 42.3% of cells studied in clams from Lithuania and in 11.8% of cells in clams from Hungary. Comparative analysis revealed no significant (P<0.05) interspecific differences in chromosome morphology of S. solidum and that of previously studied S. corneum. DNA sequence analyses of S. solidum showed no interpopulation differences in ITS1; moreover, only one site was different from ITS1 of S. corneum. However, differences in mitochondrial 16S sequence of S. solidum were revealed: two haplotypes in Lithuania and three in Hungary were identified. The genetic characteristics revealed in this study do not support ascription of S. solidum and S. corneum to different subgenus, Cyrenastrum and Sphaerium s. str., respectively. Comparative cytogenetic analysis disclosed that the chromosome morphology could be conserved in some sphaeriid species during speciation despite the fact that most other species in this family undergo radical karyotypic differentiation.