Polymorphism of theSphaerium corneum(Bivalvia, Veneroida, Sphaeriidae) revealed by cytogenetic and sequence comparison

Abstract

Two populations of Sphaerium corneum were sampled from River Vilnel and small pond in Vilnius, Lithuania. The chromosomes were studied using conventional Giemsa staining and karyometric analysis. Inter-and intra-individual variation in the diploid chromosome numbers was revealed and two different sources of chromosome variability were identified: B chromosomes and the structural changes of chromosomes of the basic (A) set. The chromosome set of the more common karyotypic form, 2 n = 30, found in both populations, consists of all biarmed metacentric and meta-submetacentric chromosomes of gradually decreasing size. Small, biarmed, mitotically unstable B chromosomes were found in the cells of this karyotypic form. Specimens with 2 n = 36 were found only in pond. No B chromosomes were detected in their cells. The karyotype is characterized by presence of two pairs of medium telocentrics and four pairs of small subtelocentrics. The remaining chromosomes are biarmed. Robertsonian fusions appear to be involved in formation of two karyotypic forms of S. corneum . DNA sequence analyses showed that ITS1 is identical in both karyotypic forms. On the other hand, differences in 16S sequence were revealed and two haplotypes, corresponding to two karyotypic forms, were identified. The present study opens new perspectives in establishing speciesspecific characters for confident identification of Sphaerium species and provides insights to the genetic intraspecific variability and possible mechanisms of speciation.

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

“…The striking difference in chromosome numbers between the S. corneum group (S. corneum and S. nucleus 2n=30 [6]) and representatives with high 2n (more than 200 chromosomes) raises the question of monophyly of the genus. Molecular data corroborate its subdivision into 4 groups [16] and place S. rivicola into the proximity of North American species of the subgenus Amesoda, rather than in a close relationship with the S. corneum group.…”

“…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.…”

“…Sphaerium corneum (2n=30 and 2n=36, respectively) [5][6][7]Kořínková,unpublished], S. nucleus (2n=30) (Kořínková, unpublished. ), S. rhomboideum (2n=44) [8] and S. nitidum (2n=30) [7], The remaining 11 species exhibited chromosome counts from 100 to 247 [7]).…”

Supernumerary chromosomes on Southern European populations of the cockle Cerastoderma edule: Consequence of environmental pollution?

“…O'Foighil and Thiriot-Quié vreux, 1991) where odd ploidy numbers were also observed and in Sphaerium corneum (Petkeviciute et al, 2006). In the last case, B chromosomes were one of the causes of observed inter-and intra-individual variation in the diploid chromosome number.…”

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

“…The first species studied cytogenetically was S. corneum (Linnaeus, 1758), for which Keyl [18] demonstrated the chromosome number n=18 and 2n=36. More recent analyses of Lithuanian populations of S. corneum have revealed intrapopulation and intraindividual variation in chromosome numbers, associated with the existence of two different karyomorphs, 2n=30 and 2n=36, and with the variable number of B chromosomes in the cells of karyomorph 2n=30 [19]. During the recent studies of chromosome sets of North American sphaeriid species, the diploid number of 2n=44 was reported for S. rhomboideum (Say, 1822) and this is the first record of a diploid species in the highly polychromosomic Nearctic sphaeriid fauna [17].…”