On the evolutionary genomics and behavioural ecology of homoploid hybrid speciation in Passer sparrows

Natural hybridisation is currently receiving much attention in evolutionary biology. Aided by novel sequencing technologies, researchers are discovering that a great number of sexually reproducing taxa previously thought to be good species exhibit genetic signals of admixture, and that the levels of gene exchange in natural systems is much higher than previously appreciated. Even our own species has been shown to harbour a complex reticulate history involving several admixture events with archaic humans, including the Neanderthals whose genetic material some of us still carry with us today. Among the most spectacular outcomes of natural hybridisation is hybrid speciation. During this mode of speciation, a novel taxon arises through interbreeding between existing taxa, and develops reproductive barriers against them. Though recognised as a potentially vital contributor to biological diversification, questions regarding how reproductive isolation can develop during this process, and whether "special" and possibly unlikely processes are required, have made its evolutionary importance unclear. In a recent paper published in PLoS Genetics and highlighted by Science, CEES researchers show that the hybrid Italian sparrow is reproductively isolated from its parent species through sex-linked and mito-nuclear incompatibilities. This is the first time post-zygotic barriers in the form of genic incompatibilities have been shown to isolate a hybrid taxon. The study therefore shows that hybrid speciation may be driven by mechanisms similar to those involved in non-hybrid speciation, but with the formation of two geographically separated species boundaries instead of one. Hence, hybrid speciation can occur without the need of invoking "special" processes, and may therefore be more important for the diversification of life on earth than have previously been thought.

The Italian sparrow is a stabilized homoploid hybrid species that has originated from hybridization events between house sparrows and Spanish sparrows in the past. Using pyrosequencing of cDNA on the 454-platform at CEES we will characterize and compare t he transcriptomes of the hybrid species and its parental species to investigate the genomic changes that has occurred in association with this rare speciation event. The sequencing will also provide the necessary SNP-markers for the project. We will genot ype a selected sample of these SNP markers on a large number of birds to reconstruct the phylogeographic history of the species complex and to investigate pattern of introgression across transects of hybrid zones between the species. We will include marke rs from genes showing signals of being affected by selection in either species along with presumably neutrally evolving markers to investigate possible differential introgression and selection against hybrid genotypes in the contact zones. Potentially fav orable hybrid genotypes will also be looked for. The second part of the project is concerned with investigations of barriers against gene flow among sparrows bred in captivity. We will maintain six mixed-species populations in the lab and allow them to ma te and breed. All adults and nestlings will be genotyped to check paternity. Pattern of assortative pair formation and fertilization will be investigated, as well as potential developmental problems (including reduced fertility) of hybrid descendants. Gen otyping of captive birds with known pedigree (using a denser, genome-wide SNP-marker set) will be used to investigate the genetic architecture of traits of interest, that is, traits associated with reproductive isolation. Finally, we will investigate whet her species recognition is genetically inherited or caused by sexual imprinting on parental phenotype by cross-fostering experiments of nestlings in the lab-reared populations.