Low dispersing species : population genetic processes in space and time : the genus "Trochulus" (Gastropoda: Hygromiidae) as a case study

RÉSUMÉ Une espèce est rarement composée d'une population unique. Parce que les individus ont des capacités de dispersion limitées et que les paysages sont des mosaïques d'habitats, la plupart des espèces sont plutôt composées de sous-populations connectées par la migration. Cette variation spatiale influence directement la distribution de la variabilité génétique dans et entre les populations. Durant ce travail, nous avons abordé certains des processus populationnels qui ont joué un rôle supposé dans l'apparition de nouvelles espèces au sein du genre Trochulus. Plus précisément, nous avons tenté d'évaluer les impacts respectifs de l'isolement passé (facteurs historiques) et présent (facteurs locaux). Nous avons d'abord pu montrer que les faibles capacités de dispersion des escargots terrestres ont directement influencé leur histoire évolutive à toutes les échelles spatiales et temporelles. En réduisant l'effet homogénéisant de la migration, une faible dispersion maintient dans les populations les traces génétiques d'évènements passés. A l'échelle de la distribution globale de Trochulus villosus, ces traces ont permis de reconstruire une histoire faite d'isolements et d'expansions de populations. En combinant des données génétiques avec une modélisation de la niche climatique passée, il a été possible de proposer un scénario significativement meilleur que toutes les hypothèses alternatives que nous avons testées. A l'échelle locale par contre, l'héritage historique est difficile à distinguer de la dynamique actuelle. Ce fut le cas des lignées mitochondriales du complexe sericeus-hispidus : les deux principales lignées étaient phylogénétiquement éloignées, avaient eu des démographies passées différentes et corrélaient avec des différences morphologiques. D'un autre côté, le flux de gène nucléaire était fort, contredisant l'idée de deux espèces cryptiques isolées reproductivement. Pour pouvoir conclure à la présence ou non de deux espèces, il nous a manqué des informations locales sur la dynamique des populations et les conditions écologiques que l'on trouve dans la région d'étude. Enfin, nous avons pu souligner que la connectivité entre populations d'escargots est soumise à la qualité des habitats et à leur organisation spatiale. Les escargots sont dépendants d'un habitat et s'y adaptent, comme l'indiquent la présence de «poils » uniquement sur la coquille d'espèces vivant dans des habitats humides ou la corrélation entre morphologie et habitat au sein du complexe sericeus-hispidus. Logiquement donc, les escargots migrent préférentiellement au travers d'habitats favorables comme l'a montré la réduction de flux de gènes au travers des prairies chez T. villosus (une espèce forestière). De ces données, nous pouvons supposer que les populations d'escargots en particulier, et des espèces à faible dispersion en général, ont de fortes chances d'être affectées par les changements climatiques, avec de probables implications pour leurs histoires évolutives. SUMMARY : Species rarely consists in a single population. Because individuals have limited dispersal abilities, because landscapes are habitat patchworks, most species are made of several subpopulations connected by migration. This spatial variation has consequences on the distribution of genetic diversity within and between populations, creating a structure among the populations. During the present work, we investigated some of the population processes assumed to have played an important role on the speciation within the genus Trochulus. More specifically, we questioned the respective impacts of past (historical factors) or present (local factors) population isolations. We first could show that the poor dispersal abilities of land snails have had profound impacts on their evolutionary histories at all spatial and temporal scales. Low dispersal maintains a strong signature of past events in the populations by minimising the homogenising effects of geneflow. At the scale of Trochulus villosus global distribution, they allowed to retrieve the detailed history of this species population isolations and expansions. Combining a large genetic dataset with paleo-climatic niche modelling ended up with a historical scenario significantly better than all traditional alternatives we tested. At local scale on the contrary, past events become difficult to tease apart from ongoing processes. This was the case for the divergent mitochondria) lineages within the sericeus-hispidus complex: the two principal lineages appeared to be phylogenetically distant, to have experienced different demographic histories and to correlate with morphological differences. On the other hand, nuclear (present day) geneflow was high, contradicting the idea of two reproductively isolated cryptic species. Information on the local population dynamics and environmental conditions are lacking to be able to decide whether past isolation has indeed resulted here in new species. Finally, we emphasised the importance of the habitat types present in a landscape as well as their spatial organisation for the population connectivity of land snails. These species are tightly dependent on a habitat and adapt to it as shown by thé occurrence of hair-like structures only in species living in humid environments or by the correlation between shell morphology and habitat in the sericeus-hispidus complex. As a result, land snails preferentially migrate through favourable habitats: Trochulus villosus, a forest species, had its geneflow significantly reduced across meadows. From these data, we can hypothesise that the populations of land snails in particular and of low dispersing species in general are likely to be strongly affected by the ongoing climate changes, with potential major consequences on their evolutionary histories.

Distinct genetic structure in populations of Chrysoperla externa (Hagen) (Neuroptera, Chrysopidae) shown by genetic markers ISSR and COI gene

Distinct genetic structure in populations of Chrysoperla externa (Hagen) (Neuroptera, Chrysopidae) shown by genetic markers ISSR and COI gene. Green lacewings are generalist predators, and the species Chrysoperla externa presents a great potential for use in biological control of agricultural pests due to its high predation and reproduction capacities, as well as its easy mass rearing in the laboratory. The adaptive success of a species is related to genetic variability, so that population genetic studies are extremely important in order to maximize success of the biological control. Thus, the present study used nuclear (Inter Simple Sequence Repeat - ISSR) and mitochondrial (Cytochrome Oxidase I - COI) molecular markers to estimate the genetic variability of 12 populations in the São Paulo State, Brazil, as well as the genetic relationships between populations. High levels of genetic diversity were observed for both markers, and the highest values of genetic diversity appear associated with municipalities that have the greatest areas of native vegetation. There was high haplotype sharing, and there was no correlation between the markers and the geographic distribution of the populations. The AMOVA indicated absence of genetic structure for the COI gene, suggesting that the sampled areas formed a single population unit. However, the great genetic differentiation among populations showed by ISSR demonstrates that these have been under differentiation after their expansion or may also reflect distinct dispersal behavior between males and females.

Correlation of O6-methylguanine methyltransferase (MGMT) promoter methylation with clinical outcomes in glioblastoma and clinical strategies to modulate MGMT activity.

Resistance to alkylating agents via direct DNA repair by O(6)-methylguanine methyltransferase (MGMT) remains a significant barrier to the successful treatment of patients with malignant glioma. The relative expression of MGMT in the tumor may determine response to alkylating agents, and epigenetic silencing of the MGMT gene by promoter methylation plays an important role in regulating MGMT expression in gliomas. MGMT promoter methylation is correlated with improved progression-free and overall survival in patients treated with alkylating agents. Strategies to overcome MGMT-mediated chemoresistance are being actively investigated. These include treatment with nontoxic pseudosubstrate inhibitors of MGMT, such as O(6)-benzylguanine, or RNA interference-mediated gene silencing of MGMT. However, systemic application of MGMT inhibitors is limited by an increase in hematologic toxicity. Another strategy is to deplete MGMT activity in tumor tissue using a dose-dense temozolomide schedule. These alternative schedules are well tolerated; however, it remains unclear whether they are more effective than the standard dosing regimen or whether they effectively deplete MGMT activity in tumor tissue. Of note, not all patients with glioblastoma having MGMT promoter methylation respond to alkylating agents, and even those who respond will inevitably experience relapse. Herein we review the data supporting MGMT as a major mechanism of chemotherapy resistance in malignant gliomas and describe ongoing studies that are testing resistance-modulating strategies.

Schizophrenia connectivity: correlation between cognitive deficits and reduced thalamo-cortical fibers

Introduction: Schizophrenia is associated with multiple neuropsychological dysfunctions, such as disturbances of attention, memory, perceptual functioning, concept formation and executive processes. These cognitive functions are reported to depend on the integrity of the prefrontal and thalamo-prefrontal circuits. Multiple lines of evidence suggest that schizophrenia is related to abnormalities in neural circuitry and impaired structural connectivity. Here, we report a preliminary case-control study that showed a correlation between thalamo-frontal connections and several cognitive functions known to be impaired in schizophrenia. Materials and Methods: We investigated 9 schizophrenic patients (DSM IV criteria, Diagnostic Interview for Genetic Studies) and 9 age and sex matched control subjects. We obtained from each volunteer a DT-MRI dataset (3 T, _ _ 1,000 s/mm2), and a high resolution anatomic T1. The thalamo- frontal tracts are simulated with DTI tractography on these dataset, a method allowing inference of the main neural fiber tracks from Diffusion MRI data. In order to see an eventual correlation with the thalamo-frontal connections, every subject performs a battery of neuropsychological tests including computerized tests of attention (sustained attention, selective attention and reaction time), working memory tests (Plane test and the working memory sub-tests of the Wechsler Adult Intelligence Scale), a executive functioning task (Tower of Hanoï) and a test of visual binding abilities. Results: In a pilot case-control study (patients: n _ 9; controls: n _ 9), we showed that this methodology is appropriate and giving results in the excepted range. Considering the relation of the connectivity density and the neuropsychological data, a correlation between the number of thalamo- frontal fibers and the performance in the Tower of Hanoï was observed in the patients (Pearson correlation, r _ 0.76, p _ 0.05) but not in control subjects. In the most difficult item of the test, the least number of fibers corresponds to the worst performance of the test (fig. 2, number of supplementary movements of the elements necessary to realize the right configuration). It's interesting to note here that in an independent study, we showed that schizophrenia patients (n _ 32) perform in the most difficult item of the Tower of Hanoï (Mann-Whitney, p _ 0.005) significantly worse than control subjects (n _ 29). This has been observed in several others neuropsychological studies. Discussion: This pilot study of schizophrenia patients shows a correlation between the number of thalam-frontal fibers and the performance in the Tower of Hanoï, which is a planning and goal oriented actions task known to be associated with frontal dysfonction. This observation is consistent with the proposed impaired connectivity in schizophrenia. We aim to pursue the study with a larger sample in order to determine if other neuropsychological tests may be associated with the connectivity density.

When genetic load does not correlate with phenotypic spectrum: lessons from the GnRH receptor (GNRHR).

CONTEXT: A broad spectrum of GnRH-deficient phenotypes has been identified in individuals with both mono- and biallelic GNRHR mutations. OBJECTIVE: The objective of the study was to determine the correlation between the severity of the reproductive phenotype(s) and the number and functional severity of rare sequence variants in GNRHR. SUBJECTS: Eight hundred sixty-three probands with different forms of GnRH deficiency, 46 family members and 422 controls were screened for GNRHR mutations. The 70 subjects (32 patients and 38 family members) harboring mutations were divided into four groups (G1-G4) based on the functional severity of the mutations (complete or partial loss of function) and the number of affected alleles (monoallelic or biallelic) with mutations, and these classes were mapped on their clinical phenotypes. RESULTS: The prevalence of heterozygous rare sequence variants in GNRHR was significantly higher in probands vs. controls (P < 0.01). Among the G1-G3 groups (homozygous subjects with successively decreasing severity and number of mutations), the hypogonadotropic phenotype related to their genetic load. In contrast, subjects in G4, with only monoallelic mutations, demonstrated a greater diversity of clinical phenotypes. CONCLUSIONS: In patients with GnRH deficiency and biallelic mutations in GNRHR, genetic burden defined by severity and dose is associated with clinical phenotype. In contrast, for patients with monoallelic GNRHR mutations this correlation does not hold. Taken together, these data indicate that as-yet-unidentified genetic and/or environmental factors may combine with singly mutated GNRHR alleles to produce reproductive phenotypes.

Gonadal alterations in male whitefish Coregonus fatioi: no evidence for genetic damage reducing viability in early life stages.

In recent years, numerous cases of morphological gonadal alterations in fish have been recorded throughout the world and across a wide range of species. In the whitefish Coregonus fatioi from the pre-alpine Lake Thun (Switzerland), the frequency of gonadal alterations is particularly high and the variety of alteration types large. Little is known about the proximal causes and the direct consequences of these morphological features on population persistence. In particular, the potential for the observed alterations to be the phenotypic expression of reduced genetic quality has not yet been addressed. In this study, we used offspring survival during embryogenesis as a proximate indicator of male genetic quality and tested whether the presence of gonadal alterations in males is an indicator of reduced quality. Embryos resulted from in vitro fertilizations of gametes from 126 males and females. We found no significant correlation between embryo survival and gonadal alteration in adults. Our findings suggest that in C. fatioi of Lake Thun, alterations in gonad morphology are not a phenotypic expression of variation in genetic quality.

Novel Cone Transducin Alpha Subunit Mutation In Tunisian Patients And Genotype-phenotype Correlation In Complete Achromatopsia

Purpose: Complete achromatopsia is a rare autosomal recessive disease due to CNGA3, CNGB3, GNAT2 and PDE6C mutations. We studied a large consanguineous Tunisian family including twelve individuals.Methods: Ophthalmic evaluation included a full clinical examination, color vision testing, optical coherence tomography and electroretinography. Linkage analysis using microsatellite markers flanking CNGA3, CNGB3, GNAT2 and PDE6C genes was performed. Mutations were screened by direct sequencing.Results: In all affected subjects, acuity ranged from 20/50 to 20/200. Fundus examination was normal except for two patients who had respectively 4 mm and 5 mm diameters of peripheral congenital hypertrophy. Likewise retinal layers exploration by OCT revealed no change in the thickness of the central retina. Color Vision with 100 Hue Farnsworth test described a profound color impairment along all three axes of color vision. The haplotype analysis of GNAT2 markers revealed that all affected offspring were homozygous by descent for the four polymorphic markers. The maximum lod score value, 4.33, confirmed the evidence for linkage to the GNAT2 gene.A homozygous novel nonsense mutation R313X was identified segregating with an identical GNAT2 haplotype in all affected subjects. This mutation could interrupt interaction with photoactivated rhodopsin, resulting in a failure of visual transduction. In fact, ERG showed a clearly abolished photopic b-wave and flicker responses with no residual cone function justifying the severe GNAT2 achromatopsia phenotype.Conclusions: This is the first report of the clinical and genetic investigation of complete achromatopsia in North Africa and of the largest family with recessive achromatopsia involving GNAT2, thus providing a unique opportunity for genotype phenotype correlation for this extremely rare condition.

Genetic structure and gene flow of Eugenia dysenterica natural populations

This study was carried out to assess the genetic variability of ten "cagaita" tree (Eugenia dysenterica) populations in Southeastern Goiás. Fifty-four randomly amplified polymorphic DNA (RAPD) loci were used to characterize the population genetic variability, using the analysis of molecular variance (AMOVA). A phiST value of 0.2703 was obtained, showing that 27.03% and 72.97% of the genetic variability is present among and within populations, respectively. The Pearson correlation coefficient (r) among the genetic distances matrix (1 - Jaccard similarity index) and the geographic distances were estimated, and a strong positive correlation was detected. Results suggest that these populations are differentiating through a stochastic process, with restricted and geographic distribution dependent gene flow.

Urotensin-II System in Genetic Control of Blood Pressure and Renal Function.

Urotensin-II controls ion/water homeostasis in fish and vascular tone in rodents. We hypothesised that common genetic variants in urotensin-II pathway genes are associated with human blood pressure or renal function. We performed family-based analysis of association between blood pressure, glomerular filtration and genes of the urotensin-II pathway (urotensin-II, urotensin-II related peptide, urotensin-II receptor) saturated with 28 tagging single nucleotide polymorphisms in 2024 individuals from 520 families; followed by an independent replication in 420 families and 7545 unrelated subjects. The expression studies of the urotensin-II pathway were carried out in 97 human kidneys. Phylogenetic evolutionary analysis was conducted in 17 vertebrate species. One single nucleotide polymorphism (rs531485 in urotensin-II gene) was associated with adjusted estimated glomerular filtration rate in the discovery cohort (p = 0.0005). It showed no association with estimated glomerular filtration rate in the combined replication resource of 8724 subjects from 6 populations. Expression of urotensin-II and its receptor showed strong linear correlation (r = 0.86, p<0.0001). There was no difference in renal expression of urotensin-II system between hypertensive and normotensive subjects. Evolutionary analysis revealed accumulation of mutations in urotensin-II since the divergence of primates and weaker conservation of urotensin-II receptor in primates than in lower vertebrates. Our data suggest that urotensin-II system genes are unlikely to play a major role in genetic control of human blood pressure or renal function. The signatures of evolutionary forces acting on urotensin-II system indicate that it may have evolved towards loss of function since the divergence of primates.

Genetic distance estimated by RAPD markers and its relationship with hybrid performance in maize

The objective of this work was to evaluate the genetic diversity of 16 maize inbred lines, and to determine the correlation between genetic distance and hybrid performance, using random amplified polymorphic DNA (RAPD) molecular markers. Twenty-two different random primers were used, which resulted in the amplification of 265 fragments, 237 (84.44%) of them being polymorphic. A genetic similarity matrix was created from the RAPD data, using Jaccard coefficient, and a dendrogram was constructed. Hybrid analyses were carried out using random block design and Griffing method VI for diallel crossings. The genetic associations showed five distinct heterotic groups. Correlations between genetic divergences detected by RAPD, as well as the means observed in the diallel crossings were positive and significant for plant height, ear height, prolificacy, and grain weight. The correlation of genetic divergences, detected by RAPD, and the specific combining ability between heterotic group associations, showed significance in all characteristics under study, except prolificacy. A direct relationship between genetic divergence and productivity was found in 79.2% of the 120 hybrids confirming the hypothesis that genetic divergence is directly related to the performance of hybrids and is efficient in predicting it.

Genetic diversity in soybean germplasm identified by SSR and EST-SSR markers

The objectives of this work were to investigate the genetic variation in 79 soybean (Glycine max) accessions from different regions of the world, to cluster the accessions based on their similarity, and to test the correlation between the two types of markers used. Simple sequence repeat markers present in genomic (SSR) and in expressed regions (EST-SSR) were used. Thirty SSR primer-pairs were selected (20 genomic and 10 EST-SSR) based on their distribution on the 20 genetic linkage groups of soybean, on their trinucleotide repetition unit and on their polymorphism information content. All analyzed loci were polymorphic, and 259 alleles were found. The number of alleles per locus varied from 2-21, with an average of 8.63. The accessions exhibit a significant number of rare alleles, with genotypes 19, 35, 63 and 65 carrying the greater number of exclusive alleles. Accessions 75 and 79 were the most similar and accessions 31 and 35, and 40 and 78, were the most divergent ones. A low correlation between SSR and EST-SSR data was observed, thus genomic and expressed microsatellite markers are required for an appropriate analysis of genetic diversity in soybean. The genetic diversity observed was high and allowed the formation of five groups and several subgroups. A moderate relationship between genetic divergence and geographic origin of accessions was observed.

Bayesian genetic parameters for body weight and survival of Nile tilapia farmed in Brazil

The objective of this study was to estimate genetic parameters for survival and weight of Nile tilapia (Oreochromis niloticus), farmed in cages and ponds in Brazil, and to predict genetic gain under different scenarios. Survival was recorded as a binary response (dead or alive), during harvest time in the 2008 grow-out period. Genetic parameters were estimated using a Bayesian mixed linear-threshold animal model via Gibbs sampling. The breeding population consisted of 2,912 individual fish, which were analyzed together with the pedigree of 5,394 fish. The heritabilities estimates, with 95% posterior credible intervals, for tagging weight, harvest weight and survival were 0.17 (0.09-0.27), 0.21 (0.12-0.32) and 0.32 (0.22-0.44), respectively. Credible intervals show a 95% probability that the true genetic correlations were in a favourable direction. The selection for weight has a positive impact on survival. Estimated genetic gain was high when selecting for harvest weight (5.07%), and indirect gain for tagging weight (2.17%) and survival (2.03%) were also considerable.

Comparative population genetic structure in a plant-pollinator/seed predator system.

Comparative analyses of spatial genetic structure of populations of plants and the insects they interact with provide an indication of how gene flow, natural selection and genetic drift may jointly influence the distribution of genetic variation and potential for local co-adaptation for interacting species. Here, we analysed the spatial scale of genetic structure within and among nine populations of an interacting species pair, the white campion Silene latifolia and the moth Hadena bicruris, along a latitudinal gradient across Northern/Central Europe. This dioecious, short-lived perennial plant inhabits patchy, often disturbed environments. The moth H. bicruris acts both as its pollinator and specialist seed predator that reproduces by laying eggs in S. latifolia flowers. We used nine microsatellite markers for S. latifolia and eight newly developed markers for H. bicruris. We found high levels of inbreeding in most populations of both plant and pollinator/seed predator. Among populations, significant genetic structure was observed for S. latifolia but not for its pollinator/seed predator, suggesting that despite migration among populations of H. bicruris, pollen is not, or only rarely, carried over between populations, thus maintaining genetic structure among plant populations. There was a weak positive correlation between genetic distances of S. latifolia and H. bicruris. These results indicate that while significant structure of S. latifolia populations creates the potential for differentiation at traits relevant for the interaction with the pollinator/seed predator, substantial gene flow in H. bicruris may counteract this process in at least some populations.

Genetic parameters and simultaneous selection for root yield, adaptability and stability of cassava genotypes

The objective of this work was to estimate genetic parameters and to evaluate simultaneous selection for root yield and for adaptability and stability of cassava genotypes. The effects of genotypes were assumed as fixed and random, and the mixed model methodology (REML/Blup) was used to estimate genetic parameters and the harmonic mean of the relative performance of genotypic values (HMRPGV), for simultaneous selection purposes. Ten genotypes were analyzed in a complete randomized block design, with four replicates. The experiment was carried out in the municipalities of Altamira, Santarém, and Santa Luzia do Pará in the state of Pará, Brazil, in the growing seasons of 2009/2010, 2010/2011, and 2011/2012. Roots were harvested 12 months after planting, in all tested locations. Root yield had low coefficients of genotypic variation (4.25%) and broad-sense heritability of individual plots (0.0424), which resulted in low genetic gain. Due to the low genotypic correlation (0.15), genotype classification as to root yield varied according to the environment. Genotypes CPATU 060, CPATU 229, and CPATU 404 stood out as to their yield, adaptability, and stability.

Linking genetic and species diversities in a community of freshwater gastropods

Summary Biodiversity is usually studied through species or genetic diversities. To date, these two levels of diversity have remained the independent .fields of investigations of community ecologists and population geneticists. However, recent joint analyses of species and genetic diversities have suggested that common processes may underlie the two levels. Positive correlations between species diversity and genetic diversity may arise when the effects of drift and migration overwhelm selective effects. The first goal of this thesis was to make a joint investigation of the patterns of species and genetic diversity in a community of freshwater gastropods living in a floodplain habitat. The second goal was to determine, as far as possible, the relative influences of the processes underlying the patterns observed at each level. In chapter 2 we investigate the relative influences of different evolutionary forces in shaping the genetic structure of Radix balthica populations. Results revealed that the structure inferred using quantitative traits was lower or equal to the one inferred using neutral molecular markers. Consequently, the pattern of structure observed could be only due to random drift, possibly to uniform selection, but definitely not to selection for local optima. In chapter 3, we analyze the temporal variation of species and genetic diversities in five localities. An extended period of drought occurred at the end of the study period leading to decay of both species and genetic diversities. This parallel loss of diversity following a natural perturbation highlighted the role sometimes predominant of random drift over selection on patterns of biodiversity in a floodplain habitat. In chapter 4, we compare the spatial genetic structures of two sympatric species: Radix balthica and Planorbis carinatus. We found that R. balthica populations are weakly structured and have moderate to high values of gene diversity. In contrast, P. carinatus populations are highly structured and poorly diverse. Then we measured correlations between various indices of species and genetic diversity using genetic data .from the two species. We found only one significant correlation: between species richness and gene diversity of P. carinatus. This result highlights the .need to use genetic date from more than one species to infer correlations between species and genetic diversities. Overall, this thesis provided new insights into the common processes underlying patterns of species and genetic diversity. Résumé La biodiversité est généralement étudiée au niveau de la diversité génétique ou spécifique. Ces deux niveaux sont restés jusqu'à maintenant les domaines d'investigation séparés des généticiens des populations et des écologistes des communautés. Cependant, des analyses conjointes des diversités génétique et spécifique ont récemment suggéré que des processus similaires pouvaient influencer ces deux niveaux. Des corrélations positives entre les diversités génétique et spécifique pourraient être dues aux effets de migration et de dérive qui dominent les effets sélectifs. Le premier but de cette thèse était de faire une étude conjointe des diversités génétique et spécifique dans une communauté de gastéropodes d'eau douce. Le second objectif était de déterminer les influences relatives des différents processus liés à chaque niveau de diversité. Dans le chapitre 2 nous cherchons à déterminer quelles forces évolutives influencent la structure génétique de quatre populations de Radix balthica. La structure mesurée sur des traits quantitatifs s'est révélée être plus faible ou égale à celle mesurée avec des marqueurs moléculaires neutres. La structure observée pourrait ainsi être due uniquement à la dérive génétique, potentiellement à la sélection uniforme, mais en aucun cas à la sélection locale pour différents optima. Dans le chapitre 3 nous analysons la variation temporelle des diversités génétique et spécifique dans cinq localités. Une récente période de sécheresse a causé une diminution parallèle des deux niveaux de diversité. Cette perturbation à mis en évidence le rôle parfois prépondérant de la dérive par rapport à celui de la sélection dans le déterminisme de la biodiversité dans un écosytème alluvial. Dans le chapitre 4, nous comparons la structure génétique spatiale de deux espèces vivant en sympatrie : Radix balthica et Planorbis carinatus. Les populations de R. balthica sont peu structurées et présentent un niveau de diversité relativement élevé alors que celles de P. carinatus sont fortement structurées et peu diversifiées. Nous avons ensuite mesuré différentes corrélations entre les diversités génétique et spécifique, mais la seule relation significative a été trouvée entre la richesse spécifique et la diversité génétique de P. carinatus. Ainsi, cette thèse a permis de découvrir de nouveaux aspects des processus qui influencent en parallèle la diversité aux niveaux génétique et spécifique.