Genetic study uncovers gene flow patterns among three species of Ibis.
Six thousand kilometers. That is approximately the distance between Europe and North America. Quite a distance, but that didn’t stop the Glossy Ibis (Plegadis falcinellus) from crossing it. In the 1800s, this waterbird – which occurs in the Old World and Australia – managed to reach North America (either by flying there or by hitchhiking on a boat). The first record of this species was a specimen collected in New Jersey in May 1817. The Glossy Ibis is, however, not the first Plegadis species in North America. The White-faced Ibis (P. chihi) already roamed the American wetlands before the arrival of their close relative. But did the colonization of the Glossy Ibis result in hybridization and introgression with the native White-faced Ibis? A recent study in Molecular Ecology set out to answer this question.
Few Hybrid Reports
The first documentation of putative hybrids between Glossy and White-faced Ibis was reported in Oklahoma in 2002. Since then several other cases have been published (see here and here). The few and recent reports of hybrids are probably due to the difficulty of detecting hybrids between these morphologically similar species. Given the relative scarcity of hybrids, could there be genetic exchange between these species? To assess the possibility of introgression, Jessica Oswald, Michael Harvey and their colleagues sequenced ultraconserved elements (UCEs, read more about these markers here) of both species. In addition, they also included samples from the South American Puna Ibis (P. ridgwayi), the third member of this genus.
The genetic analyses revealed introgression between Glossy and White-faced Ibis. Genes were primarily flowing from White-faced into Glossy Ibis. This pattern conforms with the expectations of a colonization model described by Currat et al. (2008) in which gene flow occurs from the local to the invading species. I have described the rationale behind this scenario in my PhD thesis: “Initially, the expanding species is outnumbered and is thus more likely to engage in heterospecific matings. As the expansion proceeds, the resident species and previously produced hybrids are engulfed by the expanding species, thereby overturning the numerical imbalance. Consequently, hybrids have a higher chance of backcrossing into members of the expanding species, resulting in a genetic wake of introgressed genes following the wave front of the expanding species.”
Surprisingly, the analyses also pointed to introgression between Puna and White-faced Ibis. This signal is possibly due to recent contact or an older admixture event.
Although these three Ibis species look very similar, genetically they are clearly distinct. Genome scans revealed that divergence between the species is spread across the genome. This pattern contrasts with many other bird species where divergence is concentrated in specific genomic regions (see for example wood-warblers). However, this result could be due to the use of UCEs which only represent a small part of the genome. A whole genome analysis is needed to confirm these patterns of divergence.
Oswald, J.A., Harvey, M.G., Remsen, R.C., Foxworth, D.U., Dittmann, D.L. Cardiff, S.W. & Brumfield, R.T. (2019) Evolutionary dynamics of hybridization and introgression following the recent colonization of Glossy Ibis (Aves: Plegadis falcinellus) into the New World. Molecular Ecology
This paper has been added to the Pelecaniformes page.