Population genetic studies are efficient for inferring the invasion history based on a comparison of native and invasive populations, especially when conducted at species scale. An expected outcome in invasive populations is variability loss, and this is especially true in self-fertilizing species. We here focus on the self-fertilizing Pseudosuccinea columella, an invasive hermaphroditic freshwater snail that has greatly expanded its geographic distribution and that acts as intermediate host of Fasciola hepatica, the causative agent of human and veterinary fasciolosis. We evaluated the distribution of genetic diversity at the largest geographic scale analysed to date in this species by surveying 80 populations collected during 16 years from 14 countries, using eight nuclear microsatellites and two mitochondrial genes. As expected, populations from North America, the putative origin area, were strongly structured by selfing and history and harboured much more genetic variability than invasive populations. We found high selfing rates (when it was possible to infer it), none-to-low genetic variability and strong population structure in most invasive populations. Strikingly, we found a unique genotype/haplotype in populations from eight invaded regions sampled all over the world. Moreover, snail populations resistant to infection by the parasite are genetically distinct from susceptible populations. Our results are compatible with repeated introductions in South America and flash worldwide invasion by this unique genotype/haplotype. Our study illustrates the population genetic consequences of biological invasion in a highly selfing species at very large geographic scale. We discuss how such a large-scale flash invasion may affect the spread of fasciolosis.

A wild population of the lymnaeid snail Pseudosuccinea columella infected by larval stages of Fasciola hepatica was discovered in the Pinar del Río Province, Cuba. One of 100 snails was infected in a rice culture field. This is the first time this species has been found acting as intermediate host of F. hepatica under natural conditions, not only for Cuba but also for the Caribbean area.

Several anatomical parameters of the reproductive system have been used to distinguish Key words: Neotropical lymnaeids -sibling species -Lymnaea cubensis -Lymnaea viatrixEleven species of lymnaeid snails have been described in the Caribbean area and South America. However, only six species are considered as valid by Hubendick (1951) and Paraense (1976Paraense ( , 1982aParaense ( , 1983Paraense ( , 1984Paraense ( , 1986Paraense ( , 1994Paraense ( , 1995: Paraense, 1982 and L. columella Say, 1817. This last species has now a very large distribution in all tropical countries particularly in the neotropical area (see Malek & Chrosciechowski 1964, Paraense 1982b, Gomez et al. 1986). Two other species may be added to that list (i) L. truncatula (Müller, 1774) from Europe which recently invaded several Andean countries (Oviedo et al. 1995, Jabbour-Zahab et al. 1997, Durand et al. 2002; (ii) L. plicata Hylton-Scott, 1953 from Chubut Province, Argentina which validity has not yet been confirmed.The morphology of the shell and reproductive system of five of those species has been well studied and allows a clear identification (Paraense 1982a, 1984, 1986, 1995, Samadi et al. 2000, Pointier et al. 2004 Moreover, an analysis of the genetic variability based on 12 enzyme loci revealed different fixed alleles at nine loci between two syntopic samples of L. viatrix var. elongata at the type locality in Lima, Peru (Durand et al. 2002). The absence of heterozygotes within this locality thus indicated the presence of isolated populations or cryptic species within the variety elongata. In the same study, Durand et al. (2002) also found significant genetic differences between the two taxa from Lima and another population sampled in Bahia Blanca, Argentina, located about 350 km to the north of the type locality for L. viatrix var. ventricosa. More recently, the use of several molecular markers (PCR-RLFP analysis of the ITS1 and ITS2 rDNA and 16S rDNAmt using 12 restriction enzymes) in a study of several populations morphologically identified as L. viatrix from Brazil, Uruguay, and Argentina showed a high level of genetic polymorphism between populations which was interpreted as an intra-specific variation (Carvalho et al. 2004).A morphological comparison of several populations of L. cubensis from the Caribbean area and several populations of the introduced L. truncatula from Bolivia was carried out by Samadi et al. (2000) using several conchological and anatomical characters of the reproductive system. The conchological study showed the presence of a large amount of variability within and between populations that did not allow distinction between taxa. The analysis of anatomical parameters of the reproductive system also showed a high variability, but all these characters clearly separated L. cubensis from L. truncatula independently from their geographical origin. In the present paper, we use these anatomical characters in the same way to see if it is possible to distinguish morphologically L. cubensis from L. viatrix. The materia...