Recurrent Hybridisations During Diversification of One Rhododendron Species Complex

Hybridisation plays a significant role in plant diversification, yet successive hybrid speciation within species-rich groups remains undocumented. We focused on a monophyletic species complex comprising one subsection, which belongs to the highly diverse genus of Rhododendron centred in the mountains of southwest China. By assembling the genome of one individual and resequencing all populations found, we uncovered four well-delimitated lineages within this subsection. Further structure analyses and allele frequency spectrum tests revealed nearly stable genetic admixture and hybrid origin of two lineages. Coalescent modelling results tentatively suggested that hybridisation between the two early diverged lineages likely gave rise to the third lineage, potentially facilitating a shift in habitat preference from rocky substrates to trees. Subsequently, further hybridisation between this hybrid and one parental lineage gave rise to the fourth lineage. The alternating inheritance of parental alleles may have strengthened reproductive isolation between each hybrid lineage and their respective parent lineages during two potential successive hybrid speciation events. We further found that the second hybrid lineage exhibited similar phenotypes and habitat adaptations to the first, and most of the positively selected genes related to reproductive isolation in the second hybridisation event were derived from the first. Although we could not exclude other hybridisation-triggered scenarios during the diversification of this species complex, our findings highlight the crucial and recurrent hybridisations in promoting rhododendron species diversity across varied mountain niches.