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Journal Molluscan Studies Advance Access originally published online on October 23, 2007
Journal of Molluscan Studies 2007 73(4):315-321; doi:10.1093/mollus/eym025
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© The Author 2007. Published by Oxford University Press on behalf of The Malacological Society of London, all rights reserved

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

Romualda Petkeviciute1,, Grazina Staneviciute1, Virmantas Stunzenas1, Taehwan Lee2 and Diarmaid Ó Foighil2

1 Institute of Ecology, Vilnius University, Akademijos 2, LT-08412, Vilnius 21, Lithuania; and 2Museum of Zoology and Department of Ecology and Evolutionary Biology, University of Michigan, 1109 Geddes Avenue, Ann Arbor, MI 48109-1079, USA

Correspondence: R. Petkeviciute; e-mail: Romualda{at}ekoi.lt


  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 = 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 µm. 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.

(Received 8 November 2006; accepted 25 June 2007)


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