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In this research note we simulate the effect of two mating systems (outcrossing vs. self-fertilisation) on the distribution and loss of rare alleles in populations exposed to a severe bottleneck. For example, with b = 0.05 (frequency of the rare allele in the initial population) and 50 surviving individuals the probability of losing the rare allele is 0.5% in an outcrossing population. In the self-fertilising population the probability is fifteen times higher (7.5%). As the breeding system in connection with bottlenecks has a crucial impact on the maintenance of genetic variability in a population we conclude that endangered land snails reproducing by self-fertilisation should receive more attention for conservation issues. Two co-occurring land snails of the Red List of Germany, Xerocrassa geyeri (an outcrossing species) and Cochlicopa lubricella (a self-fertilising species), are used to exemplify the effects. The findings are discussed with respect to the land snails of Vertigo. Four Vertigo species are considered in an European Council Directive to have a special conservation status, but their mating system and levels of genetic variability have not yet been investigated in depth. It is suggested that research on reproductive behaviour and genetic variability in Vertigo snails is urgently needed.
