Chromosomal polymorphism in the Atlantic dog-whelk, Nucella lapillus (Gastropoda: Muricidae): nomenclature, variation and biogeography

Nucella lapillus appears to be unique within the Muricidae, and indeed the Mollusca, in exhibiting a Robertsonian chromosomal polymorphism. The recorded diploid values range from 2 n  = 26 to 2 n  = 36, putatively brought about by fusion of smaller chromosomes to form five pairs of large metacentrics in the 2 n  = 26 form. In this study, the karyotypes of the numerically extreme forms (2 n  = 26 and 2 n  = 36) have been illustrated and described in detail, and a new scheme of nomenclature is proposed for the chromosomal rearrangements observed in N. lapillus . Chromosome numbers and karyotypes were recorded and analysed from 70 sites throughout its wide geographical range, mainly from around the UK coasts but also from sites in Norway, France, Spain, Portugal and the USA, showing remarkable variation in both diploid number and karyotypic configuration. The results provide new evidence that seven chromosomes in the 2 n  = 26 form can be involved in the Robertsonian rearrangements, but the maximum recorded diploid number remains 2 n  = 36. Inversions were confirmed in three chromosomes and one population was found to exhibit aneuploidy. Possible explanations for the geographical trends in karyotypic variation are discussed, but despite the advances in the karyology of N. lapillus , a simple solution to this enigmatic phenomenon remains elusive.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 87 , 195–210.