KARYOLOGICAL AND ALLOZYMIC CHARACTERIZATION OF RUDITAPES-PHILIPPINARUM, R-AUREUS AND R-DECUSSATUS (BIVALVIA, VENERIDAE)

Karyological and allozymic investigations in three species of Ruditapes, R. philippinarum, R. aureus and R. decussatus, were carried out in order to analyse their genetic relationships. The three species showed a diploid chromosome number of 2n=38, with 9 metacentric and 10 submetacentric chromosome pairs for R. philippinarum, 8 metacentric, 9 submetacentric, 1 subtelocentric and 1 telocentric for R. aureus, and 6 metacentric, 3 submetacentric and 10 subtelocentric for R. decussatus. Genetic variation was assayed at 9 loci which could be resolved in all three Ruditapes species. For the three species studied, 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. philippinarum and R. aureus, the proportion of polymorphic loci was 0.78, 0.89 and 1.00, 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 also observed in these genera. On the basis of electrophoretic data in the three Ruditapes species, which are equally genetically distant from each other, natural hybridization between Ruditapes species seems unlikely. The high number of subtelocentric chromosomes in R. decussatus could provide a postmating isolating mechanism. © 1990.