Homozygote excess and gene flow in the spring snail Bythinella (Gastropoda: Prosobranchia)

Abstract

Bvtltinella are minute prosobranch gastropods inhabiting springs. The northern border of the distribution area of the genus goes through South Poland. We studied departures from Hardy-Weinberg equilibrium (HWE) within and the level of gene flow between 20 Central European populations of Bythinda. From nearly all populations 30 or more specimens were assayed for eight enzyme systems coded by nine loci (AAT, ALP, GPI, IDDH-I, IDDH-2, MDH, MDHP, PGDH, PGM). Six polymorphic loci (ALP, GPI, IDDH-I, IDDH-2, PGDH, PGM) were analyzed for departures from HWE by means of exact probability tests and F-statistics. Two populations (9 and 20) were in HWE for all the loci, heterozygote excess was never observed. Heterozygote deficiency was found for two loci in two populations (population 2: IDDH-I and PGM; population 13: IDDH-2 and PGM), and for one locus in the remaining 16 populations, which in 13 cases was IDDH-1. F-statistics and other data suggest that not inbreeding but other disturbing forces appear responsible for the observed deficiencies. We assume differential temporal selection on juveniles, the Wahlund effect, the negative fitness of heterozygote juveniles due to their higher food demands as limiting factor, and/or the lower growth rate of homozygotes leading to a longer life span as possible causes. The presented estimates of gene flow were computed from B (Wright’s FST) and with the private allele technique; the former was found to reflect the reality better. Compared with similar estimates known from the literature for molluscs, the average value of B was high in our populations. Pairwise B values for each pair of populations were used to compute a minimum spanning tree projected on multidimensional scaling to visualize the pattern of interpopulation allozymic differences. Hierarchical F-statistics did not confirm the reality of the earlier distinguished species of the genus. 0 was, whereas N,,, was not, significantly correlated with geographic distance.