Genomics of Hybridization Among Sonoran Desert Selaginella

Hybridization - the crossing of two species - is an important evolutionary trigger by initiating gene flow processes, thus increasing species diversity, transferring adaptations and reinforcing or breaking down reproductive barriers. An additional by-product of hybridization is the emergence of new species. The process takes place either through allopolyploidy or homoploid speciation and occurs in a context of sympatric speciation. Indeed, hybrids arise where the parental species are already established and thus endure harsh ecological competition, along with recurrent gene flow, during the early stages of speciation. As a consequence, the emergence of successful hybrid species remains uncommon and often requires ecological and reproductive isolation mechanisms to escape from the parental constraints.

We first evaluate the evolutionary success of hybrid plant species, by adressing speciation versus extinction rates of a collection of homoploid and allopolyploid species, using a phylogenetic framework. We then focus on a hybrid zone in Arizona, between two North American Selaginella species (Lycophyte), where successful hybrid populations occur. The hybrids grow independently from their parental species and are fully fertile. We investigate what ecological and genetic features differentiate these successful hybrids from their parental species, using ecological niche modelling and next generation sequencing. Due to their small genome sizes (50 and 70Mb for the parental species, S. eremophila and S. arizonica), we expect to produce high quality assemblies and assess whether significant genome rearrangements have occurred in the hybrid lineage, a feature that is highly indicative of genetic isolation between hybrids and parent species.