Do Reticulate Relationships Between Tropical Trees Drive Diversification? Insights from Inga (Fabaceae)

Evolutionary radiations underlie much of the species diversity of life on Earth, particularly within the world’s most species-rich tree flora – that of the Amazon rainforest. Hybridisation catalyses many radiations by generating genetic and phenotypic novelty that promote rapid speciation, but the influence of hybridisation on Amazonian tree radiations has been little studied. We address this using the ubiquitous, species-rich neotropical tree genus Inga , which typifies rapid radiations of rainforest trees. We assess patterns of gene tree incongruence to ascertain whether hybridisation catalysed rapid radiation in Inga. Given the importance of insect herbivory in structuring rainforest tree communities (and hence the potential for hybridisation to promote adaptation through admixture of defence traits), we also test whether introgression of loci underlying chemical defences against herbivory facilitated rapid speciation in Inga. Our phylogenomic analyses of 189/288 Inga species using >1300 target capture loci showed widespread introgression in Inga and closely related genera. Specifically, we found widespread phylogenetic incongruence explained by introgression, with phylogenetic networks recovering multiple introgression events across Inga and related genera. In addition, most defence chemistry loci showed evidence of positive selection and marginally higher levels of introgression. Interestingly, we recovered around 20% admixed variation between multiple Inga species, a similar proportion to that found in other radiations catalysed by ‘ancient’ hybridisation. Overall our results suggest that introgression has occurred widely over the course of Inga’s history, likely facilitated by extensive dispersal across Amazonia, and that in some cases introgression of chemical defence loci may facilitate adaptation in Inga . ### Competing Interest Statement The authors have declared no competing interest.