MICROSATELLITE VARIATION IN GREY SEALS (HALICHOERUS-GRYPUS) SHOWS EVIDENCE OF GENETIC DIFFERENTIATION BETWEEN 2 BRITISH BREEDING COLONIES

Eight highly variable microsatellite loci were used to examine the gen etic variability and differentiation of grey seals (Halichoerus grypus ) at two widely spaced British breeding colonies. Samples were collect ed from adults and pups on the island of North Rona, off the north-wes t coast of Scotland, and on the Isle of May, situated at the mouth of the Firth of Forth on the east coast. Highly significant differences i n allele frequencies between these two sites were found for all eight loci, indicating considerable genetic differentiation. Thus, although grey seals are known to range over very large areas outside the breedi ng season, site fidelity of adults and philopatry of pups for these br eeding colonies must be sufficiently common to have effects, through g enetic drift, at the sub-population level. Migration rate was estimate d using Wright's fixation index (F-ST), Slatkin's private alleles mode l and the new statistic, R(ST), which is analogous to F-ST but which t akes into account the process of microsatellite mutation. An almost 8- fold discrepancy between the values we obtained provides cautionary ev idence that microsatellite loci may contravene one or more of the assu mptions on which these methods are based.