Gene flow and differences among local populations of the land snail Arianta arbustorum (Linnaeus, 1758) (Pulmonata: Helicidae)

Abstract

The genetic structure of nine populations of Arianta arbustorum (Linnaeus, 1758) in South Poland was studied by means of allozyme electrophoresis on cellulose acetate gel. The aim of the study was to answer the following questions: (1) is there (unlike in mtDNA haplotypes) any geographic variation in allozymes among local populations of A. arbustorum?; (2) what are the levels and pattern of gene flow?; (3) are the subpopulations really panmictic units? To answer the questions six enzyme systems, coded by eight loci (Hbdh, Iddh, Idh-1, Idh-2, Mpi, Pgdh, Pgm-1, Pgm-2), were assayed. Neither Fisher’s method (significant for four pairs of loci) nor Ohta’s D-statistics applied to study linkage disequilibrium indicated close linkage caused by population subdivision. Mean number of alleles per locus was 1.98, mean expected heterozygosity 0.337. Multilocus test showed a heterozygote deficiency for all populations but one; multipopulation test showed the same for all loci, though many loci were at Hardy-Weinberg equilibrium for individual populations. f values were high, values moderate. Inbreeding, Wahlund’s effect (mostly caused by the occurrence of several generations) and selection were considered the main sources of the observed heterozygote deficits. Mean value of was 0.270 and the resulting estimate of gene flow Nm=0.676. Pairwise Nm’s were low (almost 5 in one case, more than 2 in two cases, and more than 1 in four cases, out of 36 comparisons) fairly differentiated and not always consistent with biological reality. Neither pairwise values, nor Cavalli-Sforza and Edwards arc and unbiased Nei genetic distances were statistically significantly associated with geographic distances among populations. Multidimensional scaling on , as well as UPGMA clustering and neighbor joining trees computed on all the three parameters, showed no geographic pattern; correspondence analysis on allele frequencies did not show such a pattern, either. It is hypothesized that the present differentiation is a combined result of the following: (1) time (the snail has probably inhabited the studied area since before the latest glaciation); (2) the low levels of migration; (3) the immigrants’ poor chance of winning in the competition against the autochthons; (4) selection; (5) new mutations (possibly).