The dynamics of hybridisation between an avian island endemic and a recent coloniser
Dr. Sonya Clegg
University of Oxford
Hybridisation between species was once considered a relatively uncommon occurrence, being little more than an inconvenience for the biological species concept. It is now recognised to occur frequently across many different taxa. It can result in homogenisation of previously distinct forms - a potential conservation issue, but can also act as a catalyst for diversification through introgression and sharing of favourable genes. Repeated rounds of island colonisation followed by speciation results in secondary sympatry, with the potential for hybridisation between early and late arrivers. In the southwest Pacific, this situation has arisen in the avian family Zosteropidae (the white-eyes).
Here we use whole genome sequencing to characterise hybridisation between an ancient island endemic, Zosterops tenuirostris on Norfolk Island, and a recent colonist (1904), Zosterops lateralis, two species separated by over two million years divergence time. We recovered a signal of hybridisation between the two forms, with introgression occurring widely across the genome.
The best supported demographic model of timing and directionality of introgression based on the site frequency spectrum, indicated a scenario of higher levels of introgressive hybridisation immediately following secondary contact, that decreased but has not ceased in subsequent generations. This model also supported introgression in both directions initially, but marginally higher from the endemic to the colonist.
These initial dynamics are consistent with what is expected when hybridisation occurs in systems with large demographic imbalances (here, an initially rare colonist and a common endemic). Interestingly, the model also supports continued introgression from the endemic to the colonist over time, but rarely in the opposite direction. This is despite the fact that the colonist is now far more common than the endemic.
Future work will aim to incorporate WGS from museum specimens, some purported to be phenotypic hybrids, to understand how the combination of mate choice preferences, demography, and selection shapes the genomic landscape of hybridisation in this system, and the potential consequences for a near-threatened island endemic.
