STRUCTURAL CHROMOSOMAL-POLYMORPHISM IN THE DOG-WHELK NUCELLA-LAPILLUS(MOLLUSCA, NEOGASTROPODA)

On the English and French Channel coasts, the dog-whelk Nucella lapill us (L.) exhibits variation in chromosome number which appears to corre late with the degree of wave action on the shore. The more common, 2n = 26 morph is typically found on exposed shores subjected to a high de gree of wave action, whereas those with higher chromosome numbers, up to the recorded maximum of 2n = 36, are restricted to more sheltered e nvironments. The polymorphism is thought to be Robertsonian in nature, involving centric (centromere) fission or fusion, but detailed analys is of the polymorphism has been restricted by lack of success in label ling individual chromosomes. Using a silver-staining technique for the nucleolar organiser regions (NORs), three pairs of chromosomes, in th e basic 2n = 26 karyotype, have been positively identified. A series o f structural chromosomal rearrangements (pericentric and paracentric i nversions) affecting one pair of chromosomes involved in the numerical polymorphism is described. Significant differences exist between popu lations with respect to this character. These chromosomal rearrangemen ts have the potential to reduce the level of interbreeding between the different types, and may act as isolating mechanisms between breeding groups. Structural chromosomal polymorphism is likely, therefore, to have greater significance in relation to adaptation than simple numeri cal variation. This finding raises important questions concerning the (cyto)taxonomic status of N. lapillus in different parts of its range.