Genetic variability in Littorina saxatilis from different habitats on an island in Galway Bay

Starch-gel electrophoretic techniques were applied to the investigation of molecular genetic variation in populations of the rough periwinkle Littorin a saxatilis. The investigation comprised two phases: (a) technique developm ent to resolve as many as possible of the allozyme loci reported in the lit erature as having been screened in the genus Littorina and in Melarhaphe ne ritoides; (b) the use of these loci to assess levels of genetic variation i n and patterns of genetic differentiation among populations of L. saxatilis from a relatively isolated group of populations from Galway Bay, Ireland. More than 43 allozyme loci (of which four were screened for the first rime here in this species), coding for 37 enzymes, were investigated and thirtee n of these loci (including two loci screened for the first time here) were found to be variable and reliably scorable. Samples from five pairs of tran sects were collected from Inismor, Aran Islands, from sites with known expo sure levels; one transect within each pair was collected from an exposed si te and the other from a nearby, but relatively sheltered site. UPGMA for el even loci, (ARK and PGDH were excluded from cluster and F-ST analysis as th ey were unscorable in a few samples:), showed that the samples cluster most ly by pair, reflecting their geographic origin and is indicative of little gene flow between populations. Levels of population differentiation were hi gh among samples from the top of the shore, but unusually so at AAT-1 which showed nearly three times the mean FST value for the eleven loci. There wa s also a significant regression of frequency of AAT-1(100) against level of exposure. In addition, among midshore samples, there was a consistently hi gher frequency of AAT-1(100) in sheltered habitats. These results support t he findings of others, indicating that this locus may be subject to natural selection.