Robertsonian polymorphism in the marine gastropod, Nucella lapillus: advances in karyology using rDNA loci and NORs

Previous studies of the Robertsonian polymorphism in the Atlantic dog-whelk, Nucella lapillus (2n = 26-36), have been limited by the inability to identify unequivocally individual chromosomes in the karyotype. This species, as with many other marine invertebrates, has proven largely refractory to the standard (mammalian) chromosome-banding techniques. In this study, fluorescence in situ hybridization (FISH) using a rDNA probe was applied to the metaphase chromosomes of the 2n = 26 and 2n = 36 forms of N. lapillus. The results were compared with silver-staining of the nucleolar organizer regions (NORs). The FISH technique was shown to be more sensitive and less intrinsically prone to variation than the silver-staining method. An additional NOR/rDNA locus was observed in the 2n = 36 form which, to date, has not been seen in any 2n = 26 population. The 2n = 36 karyotype is described for a south-west UK population that differs from that reported previously in the literature. After fission, Robertsonian metacentrics are shown to correspond to at least one subtelocentric product.