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 inabili ty to identify unequivocally individual chromosomes in the karyotype. This species, as with many other marine invertebrates, has proven larg ely refractory to the standard (mammalian) chromosome-banding techniqu es. In this study, fluorescence in situ hybridization (FISH) using a r DNA probe was applied to the metaphase chromosomes of the 2n = 26 and 2n = 36 forms of N. lapillus. The results were compared with silver-st aining of the nucleolar organizer regions (NORs). The FISH technique w as shown to be more sensitive and less intrinsically prone to variatio n than the silver-staining method, An additional NOR/rDNA locus was ob served 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 southwest U K population that differs from that reported previously in the literat ure. After fission, Robertsonian metacentrics are shown to correspond to at least one subtelocentric product.