Microsatellite and morphological analysis of population structure in the parthenogenetic freshwater snail Melanoides tuberculata: insights into the creation of clonal variability

The distribution of variability was studied at various geographical scales in the tropical freshwater snail Melanoides tuberculata, in order to analys e the role of factors shaping this distribution, including the mating syste m and population dynamics. This parthenogenetic polyploid species reproduce s mainly asexually, with males occurring at low frequency. About 800 indivi duals (38 sites) were sampled from Africa and the Middle East, where the sp ecies originated, and from recently colonized habitats in South and Central America, and especially the island of Martinique. We first described varia tion of general aspects and ornamentation of the shells. This analysis conf irms the existence of discrete morphs. Second, individuals were studied at three microsatellite loci, showing that each morph is a genetic clone with some minor variation compatible with models of microsatellite evolution. Th e genetic analysis also showed much more variation within than between clon es. However, two populations from Africa exhibited a large amount of variab ility, and a mixture of sexual and asexual reproduction might explain these genetic patterns. The worldwide distribution of variability is, therefore, compatible with the African origin of the species, and the introduction of a few clones in other parts of the world. These results also suggest that the distribution of variability in Martinique is influenced by flooding eve nts, and that two morphs from Martinique can be interpreted as hybrids betw een two pre-existing morphs, based on morphological, genetic and geographic al arguments.