Breaking the reproductive barrier of divergent species to explore the genomic landscape.

Background: Climate change will have significant consequences for species. Species range shifts induce the emergence of new hybrid zones or the spatial displacement of pre-existing ones. These hybrid zones may become more porous as alleles are passed from one species to another. Currently, hybridization between highly divergent species living in sympatry seems extremely limited. Indeed, this phenomenon involves breaking two barriers. The first is the pre-mating barrier, related to the reproductive phenology of the two species. The second is the post-zygotic barrier, related to the genetic divergence between these species. Here, we were interested in identifying new hybridization patterns and potential implications, especially in the context of environmental modifications. Methods: We sampled Telestes souffia and Parachondrostoma toxostoma wild specimens from different locations across France and genotyped them for SNP markers. We identified discriminant loci using F1-hybrid specimens and parental species and performed principal component analysis and Bayesian model-based clustering to analyze phylogenetic information. Furthermore, we assessed deviation in allele frequency from F1 to F2 and for Hardy-Weinberg equilibrium for F2 and assessed gene function associated with two F2 cohorts. Results: We demonstrate that by breaking the ecological barrier, massive introgressive hybridization is possible between two endemic lineages of Cyprinidae belonging to two distinct genera. For both cohorts studied (=2 cm and >2 cm), a large majority of loci (>88%) presented no deviation in allele frequency and no departure from the Hardy-Weinberg equilibrium. For individuals beyond the 2 cm stage, two phenomena were observed. The first was an allelic imbalance in favor of P. toxostoma, for some genomic regions, with genes involved in developmental regulatory processes, cytoskeletal organization, and chromosome organization. The second was an excess of heterozygous loci coupled with an equilibrium of allelic frequencies for genes involved in immune response and kidney/liver development. Moreover, the 2 cm-sized specimens with high mortality yielded a particular genomic signature. Conclusion: Our study displayed important results for understanding the early stages of hybridization between divergent lineages and predicting the emergence of future hybrid zones in the wild. Moreover, this hybridization generates a wide spectrum of hybrids that are a potential source of important evolutionary novelties.

Heterogeneous colonization pattern of European Cyprinids, as highlighted by the dace complex (Teleostei: Cyprinidae).

The dace (Leuciscus leuciscus), with a very large geographic distribution all over Europe, represents an interesting species model for studies of the global mechanisms underlying aquatic system biodiversity. To assess the congruence with the past colonization process hypothesis of the freshwater fauna in Western Europe, we investigated the evolutionary history of this species, by integrating morphological variation (eight meristic characters), mitochondrial (cytochrome b, 16S rDNA and control region, over a total of 2169 bp) and nuclear (12 allozymes loci) phylogenetic relationships, and investigating population dynamics via expansion, migration, bottleneck, and divergence time analyses. We carried out nested clade phylogeographic analysis for a total of 663 specimens from 31 populations taken from all over the distribution area. Unlike previous studies, we found that L. leuciscus is currently constituted by five lineages belonging to two clades (yielding 6.3% of pairwise divergence). The relationships between these lineages were accounted for by complex biogeographical patterns due to Pliocene and Pleistocene paleoclimatic events, validating the identification of new glacial refuges for freshwater fish in Western Europe. Finally, we demonstrated hybridization between L. leuciscus and Leuciscus idus.

Genetic structure of Proclossiana eunomia populations at the regional scale (Lepidoptera, nymphalidae).

Populations of Proclossiana eunomia (Lepidoptera, Nymphalidae) occur in middle Europe in patchy habitats of hay meadows along valleys or peat bogs. Samples of P. eunomia populations from the Ardennes region (northern France and southern Belgium) were analysed by allozyme electrophoresis. Patches isolated by more than 2 km of mature forests proved genetically distinct from their neighbouring populations. Mantel tests and regression analysis showed that the degree of genetic differentiation between the 26 studied populations is related to the geographical distances between them. Autocorrelation analysis (Moran's I ) showed that allele frequencies are positively correlated for populations up to 13 km apart and that the genetic neighbourhood of individuals is in the range of 0.9 km, which is in accordance with movement studies in this species conducted in the same area. Analysis using Wright's F-statistics revealed that the highest differentiation occurs between populations of the same subregion, whereas the whole Ardennes region is not genetically partitioned into subregions. This is probably because the connectivity of the network of suitable habitats has significantly weakened only since the 1950s, and thus subregional differentiation has not yet occurred.

[Hybridization and introgression between "full-fledged species". The case of Parnassius apollo and P. phoebus]. / Hybridation et introgression entre "bonnes espèces". Le cas de Parnassius apollo et P. phoebus.

Two butterfly species living in the Alps, Parnassius apollo and P. phoebus, frequently hybridize in certain localities of this region. The features of this phenomenon have been previously studied by biometry and starch gel electrophoresis, but some points remained obscure. We present them in a study combining results from cellulose acetate electrophoresis and wing pattern biometry with a determination of the mitochondrial haplotype by a PCR-RFLP analysis in a sample of butterflies from the southern French Alps. It was already known that the male hybrids are fecund and thus that interspecific gene exchange could take place via backcrosses with the parent species. In the present case, combining the identification of mtDNA with the analysis of nuclear genotypes allows us to demonstrate that hybridization can involve both sexes of both species. Moreover, it suggests that at least some female hybrids are not sterile. The impact of Haldane's rule is therefore not very strong in the present case. However, although the prerequisites for introgression between the concerned species are fulfilled, at the level of both nuclear and mitochondrial genomes, no indication of such a phenomenon could be gathered in the studied sample.

[Study of the variability of Leuciscus soufia, cyprinidae fish, by analysis of enzyme polymorphism and meristic characteres]. / Etude de la variabilité de Leuciscus soufia, poisson cyprinidé, par analyse du polymorphisme enzymatique et de caractères méristiques.

An analysis of 30 presumptive enzyme loci and 7 meristic characters has been used to study different populations of the cyprinid Leuciscus (Telestes) soufia. This fish is considered autochthonous in south-eastern France, with 3 described sub-species. The estimates of gene diversity were similar to results from studies on Cyprinidae from North America. The sample from the upper reaches of the river Durance showed higher levels of gene diversity; heterozygosity and percentage of polymorphic loci than all other samples. Furthermore, the samples from the Durance, as a whole, seem to embrace a population with little or no subdivision. Six of the 7 measured meristic characters yielded no significant differences between samples. The population sample of the sub-species soufia from the river Var differs from the other samples only by allele frequencies, low level of heterozygosity and the number of gill rakers. It is argued that these characters are quite insufficient and may cast doubt as to the validity of the subspecies rank of this population.