The population structure of three endemic land snail species of the genus Discula (Lowe,1831) on the archipelago of Madeira were studied. Ten allozyme loci were screened in 13 populations (11 from Madeira Island and two from the adjacent Desertas Islands) of D. polymorpha (Lowe, 1831), two populations of D. attrita (Lowe, 1831) and one population of D. calcigena (Lowe, 1831) from Porto Santo Island. All populations, except those from the Desertas, showed high levels of genetic variability. Deviation from Hardy- Weinberg equilibrium due to heterozygote deficiency was detected in all D. polymorpha populations from Madeira and populations of D. attrita. This deficiency was mainly attributed to inbreeding within the populations. No heterozygote deficiency was found in D. calcigena, probably due to the larger population size of this species in the recent past. The FST values were significant in many pairwise comparisons between the populations of D. polymorpha, with the highest values occurring between populations from Deserta Grande and the peninsula of Madeira. All interspecific FST values were highly significant. FST values between populations of D. polymorpha from Deserta Grande and the Madeiran peninsula were comparable to observed interspecific values. Based on the factorial analysis all Madeiran populations grouped together, the populations from Deserta Grande formed a separate group, while the two populations of D. attrita show clear structuring.
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