Introgression across evolutionary scales suggests reticulation contributes to Amazonian tree diversity

Hybridisation has the potential to generate or homogenize biodiversity and is a particularly common phenomenon in plants, with an estimated 25% of plant species undergoing inter-specific gene flow. However, hybridisation in Amazonia’s megadiverse tree flora was assumed to be extremely rare despite extensive sympatry between closely related species, and its role in diversification remains enigmatic because it has not yet been examined empirically. Using members of a dominant Amazonian tree family (Brownea, Fabaceae) as a model to address this knowledge gap, our study recovered extensive evidence of hybridisation among multiple lineages across phylogenetic scales. More specifically, using targeted sequence capture our results uncovered several historical introgression events between Brownea lineages and indicated that gene tree incongruence in Brownea is best explained by reticulation, rather than solely by incomplete lineage sorting. Furthermore, investigation of recent hybridisation using ~19,000 ddRAD loci recovered a high degree of shared variation between two Brownea species that co-occur in the Ecuadorian Amazon.

Our analyses also showed that these sympatric lineages exhibit homogeneous rates of introgression among loci relative to the genome-wide average, implying a lack of selection against hybrid genotypes and persistent hybridisation. Our results demonstrate that gene flow between multiple Amazonian tree species has occurred across temporal scales, and contrasts with the prevailing view of hybridisation’s rarity in Amazonia. Overall, our results provide novel evidence that reticulate evolution influenced diversification in part of the Amazonian tree flora, which is the most diverse on Earth.