Biomphalaria Glabrata
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Heredity (2002) 89, 258–265 2002 Nature Publishing Group All rights reserved 0018-067X/02 $25.00 www.nature.com/hdy Genetic differentiation, dispersal and mating system in the schistosome-transmitting freshwater snail Biomphalaria glabrata J Mava´rez1, J-P Pointier2, P David1, B Delay1 and P Jarne1 1Centre d’Ecologie Fonctionnelle et Evolutive, Centre National de la Recherche Scientifique, 1919 route de Mende, 34293 Montpellier cedex, France; 2Laboratoire de Biologie Marine et Malacologie, Centre de Biologie et d’Ecologie Tropicale et Me´diterrane´enne, EPHE UMR-5555 CNRS, 52 avenue de Villeneuve, 66860 Perpignan cedex, France Biomphalaria glabrata is the main intermediate host of Schi- mean differentiation and genetic diversity within regions. stosoma mansoni in America and one of the most intensely There is also a significant isolation-by-distance pattern. The studied species of freshwater snail, yet very little is known Lesser Antilles populations appear clearly differentiated from about its population biology. Here, we used seven highly the rest, which suggests a single colonisation event followed polymorphic microsatellite loci to analyse genetic diversity in by local radiation within these islands or multiple colonisation populations from three regions (Lesser Antilles, Venezuela events from the same source area. Our results indicate that and southern Brazil). Considerable genetic variation was B. glabrata essentially cross-fertilises, with little variation in = detected, with an average (s.d.) H0 0.32 (0.24). More diver- selfing rates among populations. However, significant defi- sity per population was found in the Valencia lake basin in cits in heterozygotes and linkage disequilibria were detected Central Venezuela, which suggests an influence of dispersal in two Venezuelan populations suggesting a mixture of at (via inter-population connectivity) on the restoring of genetic least two different genetic entities, probably with differences diversity after the demographic bottlenecks recurrently in their respective mating systems. experienced by populations. A marked population structure Heredity (2002) 89, 258–265. doi:10.1038/sj.hdy.6800127 was detected and there seems to be a relationship between Keywords: population structure; dispersal; microsatellites; selfing rate; Biomphalaria glabrata; Venezuela Introduction lack of adequate model organisms. In this respect, fresh- water snails, especially tropical pulmonate species, can A major aim of population genetics studies is to explain be excellent models (Sta¨dler and Jarne, 1997). (i) This the distribution of genetic diversity within and among group of organisms commonly live in well-defined habi- populations. This distribution may reflect a given tats fragmented in space, such as rivers and lakes (Jarne mutation-selection-drift regime at equilibrium. However, and Delay, 1991), with low connectivity among sites, recently, short-scale demographic variations and disper- which should limit dispersal. Water availability varies in sal events (eg, extinction and re-colonisation) have time because of seasonal climatic cycles, which can lead attracted increasing interest because they influence both to large fluctuations in population size and even to local within-population diversity and differentiation among extinction (Sturrock, 1973; Pointier et al, 1993; Brown, populations (Wade and McCauley, 1988; McCauley, 1991; 1994). Re-colonisation of empty habitats is generally fast, Rousset, 2001). Empirical evidence for these effects has and may sometimes be facilitated by human activities. been found in forked fungus beetles (Whitlock, 1992) and More generally, freshwater snails are extremely efficient in Silene dioica (Giles and Goudet, 1997). The mating sys- colonisers at a large geographical scale (Madsen and tem, especially self-fertilisation, also correlates with Frandsen, 1989; Pointier, 1999). Introduction into new diversity within and among populations (see Charles- habitats, such as islands, might be associated with a worth et al, 1993 for theoretical considerations; Jarne, 1995 reduction in genetic diversity as a consequence of genetic and Hamrick and Godt, 1989 for empirical examples on bottlenecks. (ii) Freshwater pulmonate snails are simul- animal and plant species, respectively). taneous hermaphrodites, and reproduction by both self- Surprisingly few studies have attempted to depict the and cross-fertilisation is possible (Jarne et al, 1993). How- joint influence of short-term demographic instability and ever, selfing rates seem to have a bimodal distribution in mating system in natural populations, probably due to a this group, with most species being preferentially selfers or outcrossers. We focus here on Biomphalaria glabrata, a South-Amer- Correspondence: J Mava´a´rez, CEFE-CNRS, 1919 route de Mende, 34293 ican pulmonate freshwater snail. This species is interest- Montpellier cedex, France. E-mail: [email protected] ing not only as a model system (see above) but also Received 28 January 2002; accepted 23 June 2002 because it is the major vector of Schistosoma mansoni, the Population structure in B. glabrata J Mava´rez et al 259 causative agent of Schistosomiasis in the New World. The DNA extraction and microsatellite typing few available studies of genetic variation in natural popu- DNA was extracted from an individual’s foot using the lations were based on either allozymes (eg, Mulvey et al, QIAmp tissue kit (Qiagen). The variability was analysed 1988) or RAPD (eg, Sire et al, 1999) markers, and therefore at seven polymorphic microsatellite loci (Table 2) which suffer from either low polymorphism or technical limi- were cloned in B. glabrata, and showed non-ambiguous tations. They also included few populations sampled at amplification patterns. Detailed information about loci, limited geographic scale. Little is therefore known on the PCR amplification and scoring can be found in Mava´rez demography and mating system of B. glabrata, and it is et al (2000) for loci BgC7, BgC8, BgE2, BgE4 and BgE5, even possible that this taxon encompasses several geneti- and in Jones et al (1999) for loci BgU1 and BgU2. Briefly, cally heterogeneous entities (Woodruff and Mulvey, individual microsatellite genotypes were obtained using 1997; Mava´rez et al, 2002b). Microsatellite markers have either of two methods. In the first one, PCR amplification recently been described in this species (Jones et al, 1999; included ␥-33P-labelled primers, and was followed by Mava´rez et al, 2000) and proved to be useful for small- electrophoresis in acrylamide gels (Mava´rez et al, 2000). scale genetic analyses of population structure and mating The second method included PCR-amplification with system in central Venezuela (Mava´rez et al, 2002a). We fluorescent-labelled primers, and electrophoresis on a here use these markers to describe the genetic diversity ABI 310 automatic genetic analyser using the manufac- of B. glabrata at the scale of its distribution area, from turer’s protocols. Some individuals were analysed using Venezuela and the Lesser Antilles (north) to Brazil both methods in order to correct for potential differences (south). Note that a similar analysis has been performed in allele scoring. with nuclear and mitochondrial DNA sequences in a companion paper (Mava´rez et al, 2002b). These data are Diversity within populations used to answer three questions: (i) What does genetic Standard parameters, including the number of polymor- diversity within and among populations tell us about the phic loci, allelic richness (Petit et al, 1998), observed het- past and present demography and dispersal of snail erozygosity (Ho) and gene diversity (He corrected for populations? (ii) What is the mating system of B. glabrata? small sample size; Nei, 1987) were used to describe the (iii) Can B. glabrata be considered as a single homo- genetic polymorphism for each population/locus combi- geneous genetic entity across its distribution area? nation. Differences in mean allelic richness and mean He between geographic areas (ie, the Lesser Antilles, western Materials and methods Venezuela and central Venezuela) were tested using Mann-Whitney U tests (Sokal and Rohlf, 1995). Devi- Species studied ations from the genotypic proportions expected at Biomphalaria glabrata is a tropical species, widely distrib- Hardy-Weinberg equilibrium at each locus and from uted from south-eastern Brazil to the northern part of genotypic equilibrium between pairs of loci were tested Haiti (Paraense, 2001), where it is commonly found in each population using exact tests (Rousset and Ray- inhabiting various freshwater habitats (eg, ponds and dit- mond, 1995). Departures from Hardy-Weinberg equilib- ches; Pointier, 1976; Mulvey et al, 1988). These habitats rium over all loci were evaluated using Fisher’s method have a patchy distribution and dispersal of snails for combining probabilities (Sokal and Rohlf, 1995). All between localities can be very limited (Mava´rez et al, calculations and tests were performed using the genepop 2002a). Demographic instability, including large fluctu- version 3.1d package (Raymond and Rousset, 1995b), ations of population size and frequent local extinction/re- with the exception of allelic richness that was calculated colonisation phenomena, has been described in this spe- using fstat 2.9.3.1 package (Goudet, 2001). The unbiased ˆ cies (Sturrock, 1973; Pointier and Theron, 1979). This estimator of Wright’s inbreeding coefficient FIS, f, was cal- results in marked genetic differentiation (Mulvey and