Partial selfing, ecological disturbance and reproductive assurance in an invasive freshwater snail.

Although reproductive assurance (RA) might play a central role in the evolution of the selfing rate, this hypothesis has never been seriously investigated in an hermaphroditic animal. We studied the mating system of the freshwater snail Physa acuta in which the availability of mating partners might be highly variable, because this species is an efficient colonizer occupying unstable habitats. A total of 11 populations differing in ecological disturbance regime (water level, openness) and snail densities were monitored over 2 years. The outcrossing rate was estimated in ca 10 families per population using microsatellite markers and the progeny-array approach. Components of fecundity and survival were recorded for each progeny. Predominant outcrossing (t(m)=0.94) was detected, with a few individuals (4%) purely selfing. The outcrossing rate did not explain among-family variation in fitness components. None of the predictions formulated under the RA hypothesis were verified: (i) selfing was related neither to disturbed habitats, nor to temporal density fluctuations, (ii) it was positively related to population density, (iii) it co-occurred with multiple paternity, and (iv) it did not induce delayed reproduction. Explanations for these negative results are discussed in light of other arguments supporting the RA hypothesis in P. acuta, as well as alternative theories explaining the occurrence of partial selfing, as either a genetically fixed or plastic trait.

Population biology of the invasive freshwater snail Physa acuta approached through genetic markers, ecological characterization and demography.

Abstract The respective role of factors acting on population functioning can be inferred from a variety of approaches, including population genetics and demography. We here investigated the role of four of these factors (mating systems, population size, bottlenecks and migration) in the hermaphroditic freshwater snail Physa acuta. Twenty-four populations were sampled either around Montpellier (local scale), or at the scale of France (global scale). At local scale, eight populations were sampled twice, before and after summer drying out. The genetic structure of these populations was studied using microsatellite loci. Populations were classified according to openness (ponds vs. rivers) and water regime (permanent vs. temporary) allowing predictions on genetic patterns (e.g. diversity within populations and differentiation). At local scale, progeny-arrays analysis of the selfing rate was conducted, and size distributions of individuals were followed over two years. Results with regard to the four factors mentioned above were: (i) Estimates of population selfing rates derived from inbreeding coefficients were only slightly higher than those from progeny-arrays. (ii) More variation was detected in rivers than in ponds, but no influence of water regime was detected. One reason might be that permanent populations are not going less often through low densities than those from temporary habitats at the time scale studied. (iii) There was limited evidence for genetic bottlenecks which is compatible with the fact that even marked reduction in water availability was not necessarily associated with demographic bottlenecks. More generally, bottlenecks reducing genetic variation probably occur at population foundation. (iv) Lower genetic differentiation was detected among rivers than among ponds which might be related to limitations on gene flow. Demographic and temporal genetic data further indicates that flooding in rivers is unlikely to induce marked gene flow explaining the strong genetic differentiation at short geographical scale in such habitats. Finally, the demographic data suggest that some populations are transitory and subject to recurrent recolonization, a pattern that was also detected through genetic data.

Population genetics and dynamics at short spatial scale in Bulinus truncatus, the intermediate host of Schistosoma haematobium, in Morocco.

The population biology of the schistosome-vector snail Bulinus truncatus was studied in an irrigation system near Marrakech, Morocco using both genetic and demographic approaches. The population genetic survey was conducted in 4 sites, 2 sites being sampled on 2 separate occasions. Individuals were genotyped at 6 microsatellite loci. No variability was found at 4 loci, and the 2 other loci had less than 4 alleles. The differentiation, both spatial and temporal, among populations was extremely weak. The demographic survey was conducted using 2 capture-mark-recapture analyses in 2 separate sites, the first in 1999 and the second in 2000. The second analysis permitted the estimation of parameters based on recent methodological developments (multisite models). Although these studies provided information on several traits, we report here on dispersal only. Both analyses showed that individual dispersal is of the order of a few hundreds of metres per reproductive life, that is the scale of the whole irrigation area. Both the genetic and demographic studies indicated that this area harbours a single--or no more than a few--populations of B. truncatus. This has implications for our understanding of the coevolutionary process between snails and flukes.

Molecular characterization and mitochondrial sequence variation in two sympatric species of Proechimys (Rodentia: Echimyidae) in French Guiana.

Variations in mitochondrial DNA characters were used to characterize two morphologically similar and sympatric species of Neotropical terrestrial rodents of the genus Proechimys (Mammalia: Echimyidae). We sequenced both cytochrome b (1140pb) and part of the control region (445pb) from four individuals of P. cuvieri and five of P. cayennensis from French Guiana, which allowed us to depict intra- and inter-specific patterns of variation. The phylogenetic relationships between the nine sequences evidence the monophyly of each species, and illustrate that more polymorphism might exist within P. cuvieri than within P. cayennensis. By developing species-specific primers to amplify a fragment of the cytochrome b gene, we were able to identify 50 individuals of Proechimys spp. caught in two localities of French Guiana. In both sites of primary rainforests, we showed that the two species live in syntopy, and this observation emphasizes the need to document ecological differences which should exist in order to diminish inter-specific competition.

Are recession populations of the desert locust (Schistocerca gregaria) remnants of past swarms?

The desert locust (Schistocerca gregaria) undergoes crowding-induced phase transformation from solitary to gregarious, which involves changes in behaviour, colour, development, morphometry, fecundity and endocrine physiology. During recession, solitary locusts persist in the central, drier part of the species' range in small pocket populations that are prone to extinction. During the intermittent upsurges and the subsequent plagues, gregarious swarms attain huge population size and invade a vast area causing major damage to agriculture. A highly variable nuclear DNA marker, a noncoding 3' end fragment of an antennapedia-class homeobox gene, was screened in locust samples from Eritrea. Despite the homogenizing potential of plague swarms, the last of which was in 1986-89 and originated in this region, the population genetic structure of solitary phase locusts along the Red Sea coast of Eritrea revealed significant divergence. The pattern of divergence indicated that the invasion of the western and northern plains in the summer of 1995 may not, as reported then, have originated in eastern Chad or western Sudan. A number of interrelated hypotheses have been presented to explain the observed genetic heterogeneity between the sampled populations. We conclude, with caution due to the limited sample sizes, that: (i) geographical isolation between breeding sites during plagues and recession; (ii) the marked differences in the flight behaviour of plague swarms and recession populations; (iii) possible failure of gregarious locusts to solitarize and re-establish in recession areas; and (iv) the effect of repeated extinction and recolonization in the meta-population contribute to the maintenance of the genetic structure of recession populations. Potentially productive future research has been identified.

Evidence for a 90 kDa heat-shock protein gene expression in the amphibian oocyte.

In order to study expression of a 90-kDa heat-shock protein during amphibian oogenesis at physiological temperature, we isolated a Pleurodeles waltl hsc90 cDNA by screening an ovarian cDNA library with a chicken hsp90 cDNA probe. The cDNA thus obtained--named Pw90--shows a high homology level with the hsp90 gene in other species. RNase protection analysis led us to conclude that this sequence is part of the cognate gene hsc90 and is constitutively expressed in oocytes. Furthermore, results of quantitative Northern blot analysis, as well as in situ hybridizations on oocyte sections or lampbrush chromosome spreads, provide evidence for expression of hsc90 transcripts at every stage of oogenesis. Moreover, they point to the fact that an accumulation of transcripts occurs very early in oogenesis. Simultaneously, the expression of HSC90-related protein was analyzed on Western blots using a monoclonal antibody (AC88) and a polyclonal antibody (AP90Ct) raised against the Pleurodeles C-terminal part of HSC90. We provide evidence for a net accumulation of HSC90-related protein in oocytes. Immunolocalization shows that a nuclear transfer occurs in the course of oogenesis and leads to a concentration equilibrium between cytoplasm and nucleus in stage VI oocytes.

Microdissection and cloning of DNA from landmark loops of amphibian lampbrush chromosomes.

Microdissection of the "globular" and "granular" landmark loops of Pleurodeles lampbrush chromosomes and subsequent cloning of their DNA yielded several recombinant clones. The 6.6-kb insert of one of them was subcloned and the 600 bp of one subclone was characterized by Southern and slot hybridizations as well as by sequencing. This sequence, designated p130B, was shown to belong to a class of moderately repetitive DNA. RNA expression of this sequence was investigated by in situ hybridization of p130B to the nascent transcripts of lateral loops. Results showed that: (1) the same transcripts were not always found in matrices of landmarks exhibiting the same morphological features; (2) the same transcripts were expressed in loops of different morphological types. Based on these results we suggest that even if there is a morphological similarity of landmark loops, this does not reflect total similarity of their transcripts.

Chromosomes of amphibian oocytes as a model for gene expression: significance of lampbrush loops.

Amphibian lampbrush chromosome loops exhibit morphological variability in their RNP matrix. The biological significance of such variability remains unknown. In order to approach this problem, the structural organization of each RNP matrix type was analyzed in relation to transcriptional and post-transcriptional processes. First, autoradiographic and transcription inhibition studies in conjunction with macromolecular spread analysis revealed a particular transcription pattern in the most typical loops, i.e. the globular loops. This pattern was characterized by asynchronous variations in RNA synthesis in the different transcription units present in a given loop. Second, morphological and experimental studies provided evidence that the typical morphologies of different RNP matrices were interconvertible and that the differences between the different RNP matrices resulted from various degrees of tightness in packaging of transcription products. In particular, analysis of thermic-shock-induced changes in the structure of lampbrush chromosomes enabled us to visualize the progressive disorganization of dense RNP matrices into globular, granular and normal matrices. Furthermore, these studies suggested that changes in post-transcriptional processes might play a determining role in the specific morphology of the loops. In particular, the kinetics of each of these different processes, related to one another and/or proteins specific to one or another of these processes, might determine the morphological appearance of the loops. The immunological approach revealed that specific nuclear proteins might therefore interfere with each of these processes. Third, the problem of a possible relationship between the specific morphologies of lateral loops and the expression of particular DNA sequences was approached at the molecular level.