Large scale patterns of genetic differentiation at enzyme loci in the land snails Cepaea nemoralis and Cepaea hortensis

Abstract

Samples of 231 populations of the land snails C'epaea nemoralis and C. hortensis from Britain, France, Switzerland and Spain were analysed for genetic polymorphism in six enzyme systems. These sibling species show similar levels of variation detected by electrophoresis, and have a generally similar degree of local divergence over homologous enzyme loci. As well as extensive local and regional differentiation, both species show large-scale changes in allele frequency across Europe. In C. hortensis there is a continuous gradient in allele frequency from northern Britain to northern Spain, while in C. nemoralis north-south dines on the continent of Europe are reversed in direction in Britain. There are few obvious correlations of allele frequency change at single loci with components of the environment. Patterns of allele frequency variation in 24 sympatric populations of the two species are generally independent of each other, although there is a positive association of the frequencies of alleles at a leucine aminopeptidase locus in the two species. The statistical significance of this association depends on a single locality, and there is little indication of shared patterns of allele frequency change which might reflect a common response to natural selection. In addition, there is no evidence that the extensive geographical change in C. nemoralis and C. hortensis is a precursor of speciation.

A two–stage model for Cepaea polymorphism

“…The degree of variability at enzyme loci which has come to light in European surveys (Johnson 1976;Jones et al 1980;Caugant et al 1982;Ochman et al 1983Ochman et al , 1987Valdez et al 1988;Guiller & Madec 1991Vicario et al 1989), however, limits the use of enzyme data in this way. Although there are largescale clines in allele frequencies (Ochman et al 1987;Guiller & Madec 1991) few are correlated with the environment. Associations of frequency and habitat occur at a smaller geographical scale than the surveys already mentioned, but are not consistent (Wilson 1996).…”

“…On the same basis, the mean Fvalue for the Valle de Aran (Jones et al 1980), a river system in an area roughly 35 Â25 km 2 , suggests about 220 generations since the ¢rst founders arrived. Ochman et al (1987) note that their F-value for C. nemoralis within regions in Britain and Europe is close to that found in recently introduced H. aspersa in California (Selander & Whittam 1983). The latter species has a high level of passive dispersal by man.…”

“…Thomaz and co-workers (1996) show mtDNA sequences in the species to be di¡erent. The possibility and nature of interaction and competition between the two has been examined (see, for example, Arthur 1978Arthur , 1982Tilling 1985a;Cowie & Jones 1987;Mazon et al 1989;Arthur et al 1993), as have correspondence in variation of homologous isoenzyme systems (Ochman et al 1987;Guiller & Madec 1991) and chiasma frequencies (Price & McBride 1981).…”

Genetic differentiation reflects geological history in the Azorean land snail, Leptaxis azorica

“…The genetic distances between the four OTUs are typical for congeneric (allopatric) species in several families of pulmonate land snails (eg Boato, 1988;Chiba, 1993;Johnson et al, 2000). Nevertheless, genetic distances do not provide a yardstick for discriminating between taxonomic levels, and considerable differentiation between conspecific populations is not uncommon in land snails (eg Selander and Kaufman, 1975;Ochman et al, 1987). Additional evidence for (ongoing) speciation is the reproductive isolation that may be inferred from the morphometric differences in the genital tract.…”

North/South differentiation in the distribution of Cepaea nemoralis in Spain

“…Large scale studies of visible (Jones eta!., 1977) and biochemical (Ochman et al, 1987) polymorphism have demonstrated the existence of gradients in the frequencies. If the area effects were an early stage of speciation, a coincidence in variation could be expected between the loci of biochemical polymorphism and the morphological characters of the shells.…”