Microgeographic variation in allozyme and shell characters in Littorina saxatilis Olivi (Prosobranchia: Littorinidae)

Abstract

Twenty-three enzymes and five shell parameters were screened in 11 subpopulations of Littorina saxatilis Olivi ( = L. rudis Maton) occupying different habitats over a 1 km stretch of coastline. Shell morphology varied considerably and consistently with respect to degree of exposure, and since there is evidence that such morphology is at least partly under genetic control, it is likely that natural selection selects particular genotypes at particular locations. There was significant allozyme heterogeneity between neighbouring subpopulations, sometimes only metres apart, but little of the allozyme variability could be related directly to environmental pressures. Thus, with the exception of the Odh locus, the considerable morphological differentiation between snails from exposed and sheltered sites was not reflected in differentiation of those genes coding for electrophoretically assayed enzymes. At the Odh locus, virtually all the genetic differentiation between subpopulations was-attributable to differentiation between habitat types. Two loci, Sod-l and Ant-1, showed highly significant genetic disequilibrium, and possible reasons for this are explored. The population structure as assessed electrophoretically accords well with the stepping-stone model which permits greater differentiation of neighbouring populations than the island model, and which seems realistic in the ovoviviparously reproducing L. saxatilis, where the greater part of gene flow is likely to occur through the occasional migration of adults between contiguous populations.

Genetic drift in small and recently founded populations of the marine snail Littorina Saxatilis

“…Populations of L. saxatilis differ genetically in allele frequencies on a large geographic scale, as well as over microgeographic distances (<10 m) (Ward and Warwick, 1980;Janson and Ward, 1984). In accordance with this, a gene diversity analysis (Nei, 1973) of the 18 populations from the Koster archipelago reveals differentiation of all five polymorphic loci over this area (8 km2), between 4 and 21 per cent being the fraction of between-population variation (table 2).…”

“…The level of genetic variation were estimated by means of starch gel electrophoresis of 13 allozyme loci. (Methods and allele designations as in Warwick, 1980, andJanson andWard, 1984. ) Five of the loci (Aat-l, Pgm-1, Pgm-2, Mpi, and Pgi) were polymorphic when applying the 0.95 criterion of polymorphism, while the remaining eight (Mdh-1, Idh-l, Odh, Xdh, Sod, Est-1, Lap-i, and Ap-2) were monomorphic.…”

“…The intertidal zones of several of these skerries are populated by L. saxatilis and, as I will argue, small founder groups of perhaps only one fertilised female, are the likely origin of each population. I have assayed the effects of genetic drift during the founder event of these populations from allozyme patterns at a number of enzyme loci which seem to be largely neutral (Janson and Ward, 1984). MATERIALS …”

Neighbourhood size and the importance of barriers to gene flow in an intertidal snail

“…That the contrast is not simply between intertidal and terrestrial species is suggested by a comparison with Littorina saxatilis, another littorine snail with direct development. Janson & Ward (1984) presented allozyme frequencies in 11 samples from a 1 km transect in Norway, the distances between samples ranging from 4 to 1054 m. We calculated pairwise values of GST from these data to test for an effect of distance. For comparisons of samples less than 50 m apart, the mean GST was 0.059, more than five times as great as for B. vittatum at distances as great as 250 m. For samples of L. saxatilis separated by more than 500 m, the mean GST was 0.078, similar to that for B.…”

“…It is not clear from Janson & Ward (1984) whether L. saxatilis was distributed continuously along their transect, but the high degree of subdivision over even very short distances emphasizes that the spatial scale of the neighbourhood found for B. vittatum is relatively large for a semiterrestrial snail that disperses primarily by crawling.…”