GENOME-SIZE VARIATION IN BIVALVE MOLLUSKS DETERMINED BY FLOW-CYTOMETRY

The nuclear DNA content (2 C value) of 200 individuals representing 10 species of bivalve molluscs belonging to the families Pectinidae (Chl amys opercularis and Pecten maximus), Ostreidae (Ostrea edulis), Mytil idae (Mytilus edulis and M. galloprovincialis), Mactridae (Spisula sol idissima), Cardiidae (Cerastoderma edule), and Veneridae (Venerupis rh omboideus, V. pullastra, and Ruditapes decussatus) have been analyzed by measuring the fluorescent signal of ethidium bromide-stained gill c ells using flow cytometry Mean genome sizes ranged from 2.23 pg per di ploid nucleus in Chlamys opercularis to 3.84 pg in M. galloprovinciali s. The distribution of DNA values among all species was continuous and overlapping. There was significant heterogeneity in mean genome size between individuals within species as well as among the different spec ies. A high per cent of the total genome size-variation was allocated at the species level (92%), and 7% at the level of individuals within species. The data suggest that a significant fraction of the bivalve g enome, on average similar or equal to 0.17 pg or 7%, is free to vary a mong individuals of the same species without phenotypic or biological consequence. The correlation between the 2 C value and chromosome numb er was low, suggesting some variation in the amount of DNA per chromos ome. Within the subclass Pteriomorphia, the species of Pectinidae, Ost reidae and Mytilidae have shown high genome size-differences correlati ng with their evolutionary distance.