Supernumerary chromosomes on Southern European populations of the cockle Cerastoderma edule: Consequence of environmental pollution?

Cerastoderma edule (Cardiidae) has a diploid chromosome number of 2n ¼ 38, its karyotype consisting of 12 submetacentric, 4 subtelocentric and 3 telocentric chromosome pairs. Hyperdiploid cells had previously been observed in two populations of the Northern Galician coasts (northwest of Spain). The supernumerary chromosomes being easily distinguished by their reduced differentiated size and by their intra- and inter-individual variability. After the recent observation of 35% of cells with supernumerary chromosomes in a population of the Southern Galician coasts (Vigo) and 15% of cells with supernumerary chromosomes in a population of the south of Portugal (Ria Formosa, Algarve), we attempted, in this paper, an elucidation of the nature of these supernumerary chromosomes, by differential banding technique with restriction enzymes on these hyperdiploid cells. Analysis of the restriction enzyme banding of the 2n > 38 karyotypes led us to propose the occurrence of a chromosomal fission event involving the largest submetacentric chromosome pair. This study represents the first description of the occurrence of a possible chromosomal fission in marine bivalves. Different levels of environmental pollution are suggested as possible explanation for the differences observed on the proportion of hyperdiploid cells between the Southern Portugal population and the three Galician ones.