Abstract
BackgroundNatural hybridization can influence the adaptive response to selection and accelerate species diversification. Understanding the composition and structure of hybrid zones may elucidate patterns of hybridization processes that are important to the formation and maintenance of species, especially for taxa that have experienced rapidly adaptive radiation. Here, we used morphological traits, ddRAD-seq and plastid DNA sequence data to investigate the structure of a Rhododendron hybrid zone and uncover the hybridization patterns among three sympatric and closely related species.ResultsOur results show that the hybrid zone is complex, where bi-directional hybridization takes place among the three sympatric parental species: R. spinuliferum, R. scabrifolium, and R. spiciferum. Hybrids between R. spinuliferum and R. spiciferum (R. ×duclouxii) comprise multiple hybrid classes and a high proportion of F1 generation hybrids, while a novel hybrid taxon between R. spinuliferum and R. scabrifolium dominated the F2 generation, but no backcross individuals were detected. The hybrid zone showed basically coincident patterns of population structure between genomic and morphological data.ConclusionsNatural hybridization exists among the three Rhododendron species in the hybrid zone, although patterns of hybrid formation vary between hybrid taxa, which may result in different evolutionary outcomes. This study represents a unique opportunity to dissect the ecological and evolutionary mechanisms associated with adaptive radiation of Rhododendron species in a biodiversity hotspot.
Highlights
Natural hybridization can influence the adaptive response to selection and accelerate species diver‐ sification
The flower width of R. spinuliferum (SN)×SA individuals were similar to that of R. scabrifolium individuals, while R. ×duclouxii individuals have a flower width that is more similar to individuals of R. spiciferum (Fig. 3d)
Our previous studies show that post-zygotic reproductive barriers between R. spinuliferum and R. spiciferum are weak [69], and given that R. scabrifolium is the sister taxon of R. spinuliferum [58], it seems reasonable that the three species can hybridize with each other, only two types of hybrid taxa were detected at the study site
Summary
Natural hybridization can influence the adaptive response to selection and accelerate species diver‐ sification. Understanding the composition and structure of hybrid zones may elucidate patterns of hybridization processes that are important to the formation and maintenance of species, especially for taxa that have experienced rapidly adaptive radiation. Hybrids have higher fitness than parental taxa across a patchwork of multiple local habitats where the parental species overlap in range (e.g., Aquilegia formosa × A. pubescens [29]; Senecio ovatus × S. hercynicus [30]). The evolutionary outcome of natural hybridization between plant taxa is likely to depend on the type of hybrid zone that is formed, where understanding the genetic composition and structure of hybrid zones is an important first step in revealing the evolutionary mechanisms (e.g., selection, dispersal, reproductive isolation) and outcomes associated with natural plant hybridization [17, 31,32,33]
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