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

Abstract

AbstractEvolutionary 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 genusInga, which typifies rapid radiations of rainforest trees. We assess patterns of gene tree incongruence to ascertain whether hybridisation catalysed rapid radiation inInga.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 inInga.Our phylogenomic analyses of 189/288Ingaspecies using >1300 target capture loci showed widespread introgression inIngaand closely related genera. Specifically, we found widespread phylogenetic incongruence explained by introgression, with phylogenetic networks recovering multiple introgression events acrossIngaand 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 multipleIngaspecies, 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 ofInga’shistory, likely facilitated by extensive dispersal across Amazonia, and that in some cases introgression of chemical defence loci may facilitate adaptation inInga.