Genome-size variation in bivalve molluscs 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 (Chlamys opercularis and Pecten maximus), Ostreidae (Ostrea edulis), Mytilidae (Mytilus edulis and M. galloprovincialis), Mactridae (Spisula solidissima), Cardiidae (Cerastoderma edule), and Veneridae (Venerupis rhomboideus, V. pullastra, and Ruditapes decussatus) have been analyzed by measuring the fluorescent signal of ethidium bromide-stained gill cells using flow cytometry Mean genome sizes ranged from 2.23 pg per diploid nucleus in Chlamys opercularis to 3.84 pg in M. galloprovincialis. 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 species. 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 genome, on average similar or equal to 0.17 pg or 7%, is free to vary among individuals of the same species without phenotypic or biological consequence. The correlation between the 2 C value and chromosome number was low, suggesting some variation in the amount of DNA per chromosome. Within the subclass Pteriomorphia, the species of Pectinidae, Ostreidae and Mytilidae have shown high genome size-differences correlating with their evolutionary distance.