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Population differentiation was investigated by confronting phenotypic and molecular variation in the highly selfing freshwater snail Biomphalaria pfeifferi, the intermediate host of Schistosoma mansoni. We sampled seven natural populations separated by a few kilometers, and characterized by different habitat regimes (permanent/temporary) and openness (open/closed). A genetic analysis based on five microsatellite markers confirms that B. pfeifferi is a selfer (s≈0.9) and exhibits limited variation within populations. Most pairwise FST were significant indicating marked population structure, though no isolation by distance was detected. Families from the seven populations were monitored under laboratory conditions over two generations (G1 and G2), allowing to record several life-history traits, including growth, fecundity and survival, over 25 weeks. Marked differences were detected among populations for traits expressed early in the life cycle (up to sexual maturity). Age and size at first reproduction had high heritability values, but such a trend was not found for early reproductive traits. In most populations, G1 snails matured later and at a larger size than G2 individuals. Individuals from permanent habitats matured at a smaller size and were more fecund than those from temporary habitats. The mean phenotypic differentiation over all populations (QST) was lower than the mean genetic differentiation (FST), suggesting stabilizing selection. However, no difference was detected between QST and FST for both habitat regime and habitat openness.

We review molecular methods for estimating selfing rates and inbreeding in populations. Two main approaches are available: the population structure approach (PSA) and progeny-array approach (PAA). The PSA approach relies on single-generation samples and produces estimates that integrate the inbreeding history over several generations, but is based on strong assumptions (for example, inbreeding equilibrium). The PSA has classically relied on single-locus inbreeding coefficients averaged over loci. Unfortunately PSA estimates are very sensitive to technical problems such as the occurrence of null alleles at one or more of the loci. Consequently inbreeding might be substantially overestimated, especially in outbred populations. However, the robustness of the PSA has recently been greatly improved by the development of multilocus methods free of such bias. The PAA, on the other hand, is based on the comparison between offspring and mother genotypes. As a consequence, PAA estimates do not reflect long-term inbreeding history but only recent mating events of the maternal individuals studied ('here and now' selfing). In addition to selfing rates, the PAA allows estimating other mating system parameters, including biparental inbreeding and the correlation of selfing among sibs. Although PAA estimates could also be biased by technical problems, incompatibilities between the mother's genotype and her offspring allow the identification and correction of such bias. For all methods, we provide guidelines on the required number of loci and sample sizes. We conclude that the PSA and PAA are equally robust, provided multilocus information is used. Although experimental constraints may make the PAA more demanding, especially in animals, the two methods provide complementary information, and can fruitfully be conducted together.

Biomphalaria glabrata is the main intermediate host of Schistosoma mansoni in America and one of the most intensely studied species of freshwater snail, yet very little is known about its population biology. Here, we used seven highly polymorphic microsatellite loci to analyse genetic diversity in populations from three regions (Lesser Antilles, Venezuela and southern Brazil). Considerable genetic variation was detected, with an average (s.d.) H(0) = 0.32 (0.24). More diversity per population was found in the Valencia lake basin in Central Venezuela, which suggests an influence of dispersal (via inter-population connectivity) on the restoring of genetic diversity after the demographic bottlenecks recurrently experienced by populations. A marked population structure was detected and there seems to be a relationship between mean differentiation and genetic diversity within regions. There is also a significant isolation-by-distance pattern. The Lesser Antilles populations appear clearly differentiated from the rest, which suggests a single colonisation event followed by local radiation within these islands or multiple colonisation events from the same source area. Our results indicate that B. glabrata essentially cross-fertilises, with little variation in selfing rates among populations. However, significant deficits in heterozygotes and linkage disequilibria were detected in two Venezuelan populations suggesting a mixture of at least two different genetic entities, probably with differences in their respective mating systems.

Bulinus globosus, one of the intermediate hosts of the genus Schistosoma, is an hermaphrodite freshwater snail species. This species occupies different kinds of environments in Africa, which are generally subjected to large variations in water availability. The mating system of this species is outcrossing, although selfing has been suggested for one population. Here we investigate the genetic structure of populations of B. globosus in natural and human-modified environments from Niger and Ivory Coast using protein electrophoresis. Our goal is to analyse the respective role of the environment (genetic drift and gene flow), the mating system and human activities. Our results indicate a low intrapopulation polymorphism. A large differentiation among populations occurs on the scale of the whole study area whereas neighbouring populations can show little differentiation. These results point to the role of genetic drift and gene flow as the main factors shaping the genetic structure, with human activities playing a role occasionally.

The effects of self-fertilization and cross-fertilization on several fitness traits were examined in the freshwater hermaphrodite snail Lymnaea peregra. Laboratory strains were established from Lake Geneva populations. Comparisons of F2 snails and their offspring showed that there are no differences in hatching time, nor in the size of young snails monitored over one month. But there was a significant difference, when the distribution of the capsule weight against the number of eggs was compared, although the effects of this on fitness are probably small. There was also a significant difference for egg production and juvenile viability over one month; the selfing snails are 94 per cent less fit for these two traits than the outcrossing.

Hybridisation is a potent force in plant evolution, although there are few reported examples of stabilised species that have been created through homoploid hybridisation. We focus here on Quercus afares, an endemic North African species that combines morphological, physiological and ecological traits of both Q. suber and Q. canariensis, two phylogenetically distant species. These two species are sympatric with Q. afares over most of its distribution. We studied two Q. afares populations (one from Algeria and one from Tunisia), as well as several populations of both Q. suber and Q. canariensis sampled both within and outside areas where these species overlap with Q. afares. A genetic analysis was conducted using both nuclear (allozymes) and chloroplastic markers, which shows that Q. afares originates from a Q. suber x Q. canariensis hybridisation. At most loci, Q. afares predominantly possesses alleles from Q. suber, suggesting that the initial cross between Q. suber and Q. canariensis was followed by backcrossing with Q. suber. Other hypotheses that can account for this result, including genetic drift, gene silencing, gene conversion and selection, are discussed. A single Q. suber chlorotype was detected, and all Q. afares individuals displayed this chlorotype, indicating that Q. suber was the maternal parent. Q. afares is genetically, morphologically and ecologically differentiated from its parental species, and can therefore be considered as a stabilised hybrid species.

The phylogeography of the freshwater snail Biomphalaria glabrata remains poorly known, although this species is the major vector of schistosomiasis in the New World. It was here investigated in South America and the Lesser Antilles, based on partial mitochondrial large ribosomal subunit (16S rDNA) and nuclear internal transcribed spacer-2 (ITS-2) gene sequences. Sampling included 17 populations from a large part of the current geographic range of the species (Brazil, Venezuela and Lesser Antilles). Substantial variability was detected, as well as a high amount of phylogenetically informative signal. The molecular phylogeny inferred splits B. glabrata into Northern and Southern clades separated by the Amazon river, and may even suggest a supra-specific status for B. glabrata. Brazilian populations were the most diverse and appeared basal to the other populations. Venezuelan haplotypes formed a single clade, albeit not strongly supported. Two Venezuelan haplotypes appear rather similar to Brazilian haplotypes. Similarly, Lesser Antilles haplotypes clustered in the same monophyletic clade, which suggests that the recent colonisation of the Antilles has a northern South American origin. However, the estimated divergence time between Antilles and Venezuelan sequences is extremely large (conservatively higher than 10(5) years). These results are discussed in the light of (i) phylogeographic patterns at South American scale, and (ii) recurrent introduction of molluscs, especially in the Antilles, as a consequence of human activities.

The ontogenetic trajectory of plastic binary traits may provide valuable insights into their evolutionary rate of change. In this paper, the timing of the plastic response of a temperature-dependent sexual polymorphism, aphally, is investigated in the freshwater snail Bulinus truncatus. Aphally is defined as the loss of the male copulatory organ in otherwise hermaphroditic animals. Individuals from two inbred lines were switched at various times during their early development between 25 and 30 degrees C, and their phally status ascertained, in order to evaluate the parameters characterising the ontogenetic reaction norm of aphally to temperature. A series of nested models including parameters for the onset, offset, and the intensity of the response to temperature were fitted to the data, allowing for a wide range of reaction norms. One genotype did not show any variation in aphally ratio with switching temperature, while a switch-point model (onset and offset corresponding to the same developmental point in time) best fitted the second genotype. The results suggest that the plasticity of aphally is expressed before eggs hatch. Their consequences on the evolution of aphally are discussed. More generally, the methodology proposed here can be used to analyse variation in ontogenetic parameters of discrete traits.

Bulinus globosus, one of the intermediate hosts for Schistosoma, is a hermaphrodite freshwater snail. Multilocus DNA fingerprinting was applied in order to investigate the mating system of this facultatively self-fertilizing species. By analysing DNA fingerprints of eight broods, we have shown that selfed and outcrossed individuals could be unambiguously distinguished by comparison with their parent fingerprints. First, selfed offspring patterns shared a higher number of bands with their mother than did outcrossed ones. Secondly, at least three paternal bands could be detected per out-crossed offspring pattern.