Nicolas BIERNE Coadapted Genomes and the Genetic Architecture of Species Barriers
invité par Hélène MORLON - Section Ecologie et Biologie de l’Evolution
12h
Le séminaire de Nicolas BIERNE (Université de Montpellier) aura lieu dans la salle Favard, IBENS 46 rue d’Ulm 75005 Paris
Abstract : The genetics of speciation is often underpinned by relatively simple genetic determinisms, such as pairwise genetic incompatibilities, which form the conceptual basis for barrier loci. Barrier loci restrict gene flow between the genomes of diverging species, both genome-wide and more intensively within specific chromosomal regions surrounding them. Barrier loci are traditionally identified through three approaches : (i) mapping chromosomal islands of divergence across hybrid zones, (ii) mapping segregation distortions in hybrid crosses, and (iii) mapping ancestry outliers in admixed populations. In the marine mussels Mytilus edulis and M. galloprovincialis, we applied all three approaches. While islands of divergence were detected between these two species, which diverged approximately 2 million years ago, the latter two methods yielded inconclusive results. In order to reconcile these observations, we developed a simple multigenic fitness landscape model. The model predicts that, in a novel environment where parental species are maladapted, an admixed population can rapidly achieve higher fitness than either parent by combining advantageous parental mutations in recombined mosaic genomes. Notably, this result does not contradict the persistence of a strong barrier to gene flow between parental species or the resistance of genomic islands to secondary introgression. Our model therfore explains the absence of ancestry outliers in a newly admixed lineage of mussels adapted to a human-altered environment. Furthermore, the model’s high-dimensional epistasis predicts that strong selection against an introgressed mutation does not necessarily result in pronounced segregation distortions in a uniformly admixed F2 hybrid background. This approach introduces a genome-wide alternative to the ancestry outlier framework and provides insights into the genetic mechanisms underlying hybrid fitness in changing environments.