Multispecies hybridization in birds

Hybridization is not always limited to two species;often multiple species are interbreeding. In birds, there are numerous examples of species that hybridize with multiple other species. The advent of genomic data provides the oppotunity to investigate the ecological and evolutionary consequences of multispecies hybridization. The interactions between several hybridizing species can be depicted as a network in which the interacting species are connected by edges. Such hybrid networks can be used to identify ‘hub-species’ that interbreed with multiple other species. Avian examples of such ‘hub-species’ are Common Pheasant (Phasianus colchicus), Mallard (Anas platyrhynchos) and European Herring Gull (Larus argentatus). These networks might lead to the formulation of hypotheses, such as which connections are most likely conducive to interspecific gene flow (i.e. introgression). Hybridization does not necessarily result in introgression. Numerous statistical tests are available to infer interspecific gene flow from genetic data and the majority of these tests can be applied in a multispecies setting. Specifically, model-based approaches and phylogenetic networks are promising in the detection and characterization of multispecies introgression. It remains to be determined how common multispecies introgression in birds is and how often this process fuels adaptive changes. Moreover, the impact of multispecies hybridization on the build-up of reproductive isolation and the architecture of genomic landscapes remains elusive. For example, introgression between certain species might contribute to increased divergence and reproductive isolation between those species and other related species. In the end, a multispecies perspective on hybridization in combination with network approaches will lead to important insights into the history of life on this planet.

Avian introgression in the genomic era

Introgression,the incorporation of genetic material from one(sub)species into the gene pool of another by means of hybridization and backcrossing,is a common phenomenon in birds and can provide important insights into the speciation process.In the last decade,the toolkit for studying introgression has expanded together with the development of molecular markers.In this review,we explore how genomic data,the most recent step in this methodological progress,impacts different aspects in the study of avian introgression.First,the detection of hybrids and backcrosses has improved dramatically.The most widely used software package is STRUCTURE.Phylogenetic discordance(i.e.different loci resulting in discordant gene trees) is another means for the detection of introgression,although it should be regarded as a starting point for further analyses,not as a definitive proof of introgression.Specifically,disentangling introgression from other biological processes,such as incomplete lineage sorting,remains a challenging endeavour,although new techniques,such as the D-statistic,are being developed.In addition,phylogenetics might require a shift from trees to networks.Second,the study of hybrid zones by means of geographical or genomic cline analysis has led to important insights into the complex interplay between hybridization and speciation.However,because each hybrid zone study is just a single snapshot of a complex and continuously changing interaction,hybrid zones should be studied across different temporal and/or spatial scales.A third powerful tool is the genome scan.The debate on which evolutionary processes underlie the genomic landscape is still ongoing,as is the question whether loci involved in reproductive isolation cluster together in ‘islands of speciation' or whether they are scattered throughout the genome.Exploring genomic landscapes across the avian tree of life will be an exciting field for further research.Finally,the findings from these different methods should be incorporated into specific speciation scenarios,which can consequently be tested using a modelling approach.All in all,this genomic perspective on avian hybridization and speciation will further our understanding in evolution in general.