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There are only a few reports on the chromosomal location of DNA sequences in bivalve species, none of them using meiotic chromosomes. Mitotic chromosomes of the clam Dosinia exoleta were analysed by means of Giemsa, silver and fluorochrome staining and fluorescent in situ hybridization (FISH) with 18S + 28S rDNA and telomeric probes. A technique for surface spreading of synaptonemal complexes (SCs) of Dosinia exoleta was developed for the first time in a bivalve species. Silver and DAPI/PI staining and SC-FISH were also applied to the study of the meiotic chromosomes of this clam. The diploid chromosome number in this species is 38 and the karyotype is composed of 11 pairs of metacentric and eight pairs of submetacentric chromosomes. 18S + 28S rDNA clusters map to the subtelomeric region of the short arm of one metacentric chromosome pair whereas telomeric signals appear at both ends of every chromosome.

The chromosomal changes accompanying bivalve evolution are an area about which few reports have been published. To improve our understanding on chromosome evolution in Veneridae, ribosomal RNA gene clusters were mapped by fluorescent in situ hybridization (FISH) to chromosomes of five species of venerid clams (Venerupis corrugata, Ruditapes philippinarum, Ruditapes decussatus, Dosinia exoleta, and Venus verrucosa). The results were anchored to the most comprehensive molecular phylogenetic tree currently available for Veneridae. While a single major rDNA cluster was found in each of the five species, the number of 5S rDNA clusters showed high interspecies variation. Major rDNA was either subterminal to the short arms or intercalary to the long arms of metacentric or submetacentric chromosomes, whereas minor rDNA signals showed higher variability. Major and minor rDNAs map to different chromosome pairs in all species, but in R. decussatus one of the minor rDNA gene clusters and the major rDNA cluster were located in the same position on a single chromosome pair. This interspersion of both sequences was confirmed by fiber FISH. Telomeric signals appeared at both ends of every chromosome in all species. FISH mapping data are discussed in relation to the molecular phylogenetic trees currently available for Veneridae.

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