Hybridization, a pivotal but neglected contributor to marine biodiversity dynamics – HYSEA

Hybridization processes are complex in nature: they are influenced by environmental effects, species life-history traits and genetic background of the two species. A back and forth process between field and laboratory studies is carried out in addition to a combination of experiments and theoretical modeling. Many laboratory work is based on molecular analyses to depict the genome architecture of the study species. To carry out simultaneously these different approaches, HySea is gathering a consortium of four partners, all marine evolutionary biologists, plus two associated partners (one private company and one academic foreign laboratory which will collaborate on the development of NGG and theoretical approaches respectively).

The first step of teh HySea project is to build-up tools and obtain biological specimens. This was started during the first semester with:
- Lab-crosses experiments between hybridizing species to demonstrate the viability of their offspring and obtain biological material for buildgin-up genetic maps
- Samplings in the field were carried out to determine the relative abundance of the two species
- Development of protocols to use next-generation sequencing tecnhology on the 4 selected pairs of species studied in HySea.
- Theoretical models have been built-up to explore the consequences of matings between species that have been isolated for a long time.

HySea is expected to provide with a clear picture about hybridization processes in marine coastal systems, a major challenge in our understanding of the outcome of biological invasions and resources management in the sea. On a more fundamental perspective, partners of the HySea project would like to contribute to set-up an analytical framework to study hybridization in the sea by integrating ecological and evolutionary approaches.

Camille Roux and colleagues compared sequences of the gene expression products of two ascidian species which came recently into contact in the English Channel. Their bioinformatics analyses showed that the two species are lilkely to have been into contact also in a recent evolutionary past (4000-15000 yrs BP). They also identified hot-spot of introgression, i.e. genomic regions where the gene flows between the two species is particularly high. Results are detailed in their paper Roux, C., Tsagkogeorga, G., Bierne, N. &Galtier, N. (accepted). “Crossing the species barrier: genomic hotspots of introgression between two highly divergent Ciona intestinalis species”. To be published in “Molecular Biolog and Evolution”.

Marine biodiversity is changing continuously as a response to environmental and anthropogenic drivers. This is exemplified with the reshuffling of species distribution at a global scale through –accidental or deliberate– introductions of new species, i.e. biological invasions, and with species displacement due to climate changes. One consequence of such global and rapid changes is an increase in the number of secondary contacts between previously allopatric taxa, promoting hybridization between incompletely isolated (sub)species. Hybridization is a pivotal mechanism for the evolutionary trajectories and the ecological responses of species. Hybridization is also modifying the genetic architecture in a complex way, creating a mosaic of “islands of differentiation” and “valleys of introgression” at the genome level. HySea specifically aims at depicting these complex hybridization processes resulting from human-mediated dispersal and environmental changes in marine coastal species, at genome and population levels.
Four model systems will be investigated, each of them composed by two or more (sub)species partially isolated and for which secondary contacts have been documented in different regions and/or several times. A first challenge is to establish a tight and formal link between ecology and genomics in marine coastal species. Population genomics surveys of marine non-model invertebrates are still scarce. A back and forth process between field and laboratory studies will be carried out to build this eco-genomics framework. A second key point addressed is to deliver accurate expectations of the genomic footprints left by hybridization: a combination of experiments and theoretical modelling will be used.
To address these challenges, in addition to a Management Task and a Dissemination Task, the work is organized with four Scientific Tasks aiming at: (1) characterizing “islands of differentiation” vs. “valleys of introgression” using lab-crosses experiments between hybridizing taxa and analyses based on Next-Generation Sequencing technologies ; Rad SEQ will be used for the rapid screening of tens of hundreds of loci on a large set of individuals, (2) describing the fine-scale genomic architecture of these regions through target-sequencing of region of interest thanks to BAC libraries or genome data (already available for 3 model systems), (3) surveying natural populations with Next-Generation Genotyping (NGG) tools needed to get an accurate description of the proportion of admixture and the dynamics of hybridization (4) providing robust theoretical predictions to guide the interpretation of empirical results and understand the synergy between different evolutionary drivers.
HySea is gathering a consortium of four partners, all marine evolutionary biologists, plus two collaborators (one private company and one academic foreign laboratory which will collaborate on the development of NGG and theoretical approaches respectively). The partners will bring to HySea (i) their knowledge about the model systems (ecology, lab-crosses etc.), (ii) molecular tools that will complement the tools developed in the course of the project and (iii) their expertise in key issues addressed (e.g. biological invasions, hybrid zones). HySea is expected to provide with a clear picture about hybridization processes in marine coastal systems, a major challenge in our understanding of the outcome of biological invasions and resources management in the sea.