EvoLunch Seminar: Nicolas Bierne (University of Montpellier)

Human transport of species across the Anthropocene ocean has increased hybridization in human-altered habitats. This hybridization blends divergent genomes into new genomic mosaics and provides valuable natural experiments for understanding selective processes in hybrid genomes, yet it remains understudied, especially in marine environments. I will present results from two marine systems: Ciona sea-squirts and Mytilus mussels. Introgression from the North Pacific Ciona robusta into the native North Atlantic C. intestinalis was proven to be adaptive, localized to a genomic region on chromosome 5. Using long-read and linked-read sequencing, we investigated the molecular variation conferring this advantage. Hybridization between the invasive Mytilus galloprovincialis and the native M. edulis led to a new ecotype, known as the “dock mussel”, in French commercial ports. Analysis of over 300 dock mussels with a 60K SNP array revealed two chromosomal islands of native parental ancestry, suggesting adaptive introgression. However, contrary to expectations for admixture between two species at the end of the speciation continuum, the dock mussel genome was found to be fully admixed. This finding is consistent with a multigenic fitness landscape model with parental maladaptation, which predicts selection for enhanced ancestry heterozygosity. Admixture and adaptive introgression may be common evolutionary pathways in novel human-altered habitats such as ports. Future work must integrate phenotypic and environmental data to better understand the evolutionary forces shaping hybrid mosaic genomes.