MICROGEOGRAPHIC GENETIC DIFFERENTIATION IN THE INTERTIDAL ISOPOD JAERA ALBIFRONS LEACH .1. SPATIAL-DISTRIBUTION OF ALLOZYME VARIATION

Populations of the intertidal isopod Jaera albifrons from three sites in South Wales (U.K.), each containing animals from ten 'rock populati ons' separated by an average of 3 m, were examined by using starch gel electrophoresis to determine their degree of genetic divergence. Nine enzyme-coding loci were found to be polymorphic (P < 0.95) and displa yed high levels of variability (proportion of polymorphic loci, P = 0. 34; mean observed heterozygosity, H-o = 0.131). Allozyme frequencies r evealed remarkable genetic divergence between sites separated by less than 5 m (absolute differentiation between populations, D-m = 0.066; N ei's mean genetic identity, I (26 loci) = 0.990). Gene diversity analy sis showed that most diversity was attributable to between rock popula tions within sites rather than due to site differences. Populations de viated significantly from Hardy-Weinberg expectations for panmixia, in all cases due to a deficiency of heterozygotes. A rare allele, Mpi(75 ), found exclusively in sewage-polluted regions, was used as a genetic marker to determine dispersal capacity and estimate gene how. Restric ted vagility was indicated, which, when taken together with direct dev elopment in J. albifrons and the patchy distribution of suitable habit ats, predict the observed levels of microgeographic genetic differenti ation.