Selfing and outcrossing in hermaphrodite freshwater gastropods (Basommatophora): where, when and why

Hermaphrodite freshwater snails offer the opportunity to study the evolution of selling and outcrossing. Laboratory stuthes using genetic markers have shown that selling is possible in all species investigated, although outcrossing seems to be favoured in most. Predominance of outcrossing is promoted by sperm storing for up to several months. Stuthes using electrophoretic markers also suggest that outcrossing is the main breeding system in natural populations. However, heterozygote deficiencies found in some populations could indicate (partial) selfing. The evolution of self-fertilization and cross-fertilization is driven by numerous forces. Inbreeding depression is thought to select for outcrossing, and has been experimentally investigated in hermaphrodite freshwater snails. Other traits have evolved that prevent selfing. On the other hand, selfing could be selected under low density, or to preserve local adaptation, or when aphallic individuals occur within populations.