Allozyme differentiation of populations of the dogwhelk Nucella lapillus, (L.): the relative effects of geographic distance and variation in chromosome number

Abstract

Geographic patterns of genic differentiation were compared with differentiation between karyotypes in the intertidal snail Nucella lapillus. Samples from 24 sites covering the species range in Europe and North America were analysed for allozyme variation at 16 soluble enzyme loci. Two homokaryotypes have been identified with diploid numbers 2n = 26 and 2n = 36 (variation is Robertsonian and hybrids have intermediate chromosome numbers) and samples were classified (on the basis of published data) according to karyotype. Group 1 consisted of samples from three English Channel populations of higher chromosome number (on average 2n > 32) and Group 2 consisted of the remaining 21 samples (presumed to be 2n = 26). Karyotype variation accounts for roughly the same amount of the absolute allozyme variance as geographic variation (46.3% and 53.7% respectively). Yet the patterns of differentiation seen between karyotypes and with geographic separation are very different. In samples classified as 2n = 26 (Group 2), while there is a significant amount of heterogeneity (FST per locus averaged 0.128 for 10 polymorphic loci), allozyme variation occurs independently at different loci so mean genetic identity (Nei) is high: 0.972. There is only a slight decline in genetic identity with distance (genetic identity averaged 0.965 for amphi-atlantic comparisons) indicating that passive transport of juveniles or adults may contribute significantly to gene flow. Conversely, allozyme variation between karyotypes was concordant. High chromosome number populations possessed a suite of alleles at four allozyme loci (Est-3, Lap-2, Mdh-1 and Pep-2) which were absent or rare in Group 2 samples resulting in high FST values for these loci (from 0.294 to 0.472) when karyotypic classes were combined. Consequently the mean genetic identity between these Robertsonian rares is low, 0.856, and falls within the range more usually associated with congeneric comparisons than with con-specific comparisons. The mechanisms maintaining this genic difference are unclear. However the distribution of the karyotypes and physiological and morphological differences (in shell shape) between them strongly suggest that karyotypic variation in Nucella is adaptive.