Karyological and allozymic characterization of Ruditapes philippinarum, R. aureus and R. decussatus (Bivalvia, Veneridae).
The 3 species showed a diploid chromosome number of 2n = 38, with 9 metacentric and 10 submetacentric chromosome pairs for Ruditapes philippinarum, 8 metacentric, 9 submetacentric, 1 subtelocentric pairs and 1 telocentric pair for R. aureus, and 6 metacentric, 3 submetacentric and 10 subtelocentric pairs for R. decussatus. Genetic variation was studied at 9 loci. For the 3 species, one locus, Idh-2, was completely diagnostic, and the values of absolute genetic distances, calculated over 9 loci, were about 0.8. In R. decussatus, R. philipppinarum and R. aureus, the proportion of polymorphic loci was 0.78, 0.89 and 1.00 resp., the mean observed heterozygosity was 0.216, 0.331 and 0.389, and the mean genetic diversity was 0.259, 0.343 and 0.525. These levels of genetic variation are among the highest observed in bivalve species. Each species exhibited an overall heterozygote deficiency, but in the hatchery population of R. philippinarum the heterozygote deficiency was about 10 times lower than in the wild populations of R. aureus and R. decussatus. It has been suggested that the high karyological variation recorded within Pinctada and Ruditapes species may be associated with the high genetic diversity observed in these genera. On the basis of electrophoretic data in the 3 Ruditapes species, which are genetically equidistant from each other, natural hybridization between Ruditapes species seems unlikely. The high number of subtelocentric chromsomes in R. decussatus could provide a postmating isolating mechanism.