Unraveling enigmatic disjunctions: Population genetic analysis points to independent origins of rare rhododendrons in the Rhododendron keiskei complex (Ericaceae)

Unraveling species boundaries is pivotal for evolutionary biology and conservation endeavors. However, it proves challenging in instances where recent speciation is intertwined with complex demographic histories and natural selection processes. The Rhododendron keiskei complex, an evergreen rhododendron distributed in East Asia, consists of a widespread variety (R. keiskei var. keiskei) and a more restricted R. keiskei var. hypoglaucum. Intriguingly, the latter is exceptionally rare yet displays a disjunction that spans approximately 1100 km. This study aimed to elucidate the evolutionary backgrounds of the enigmatic disjunctions of R. keiskei var. hypoglaucum and to propose species delimitation within the species complex. An integrative approach, combining genomic data (MIG-seq and GBS-derived SNPs) with Scanning Electron Microscopy analysis of leaf microstructures was adopted in this study. Phylogenetic analyses revealed significant divergence among the studied rhododendrons. Genetic demographic analyses favored the population models that assumed non-monophyly of two disjunct populations of R. keiskei var. hypoglaucum indicating their independent origins. Recent gene flow between the widespread R. keiskei var. keiskei and “var. hypoglaucum” populations were limited due to geographic and habitat isolation factors, even in areas where their distributions overlap. Detailed morphological assessments detected distinctions between morphologically similar “var. hypoglaucum” populations based on leaf microstructures and flowering habits. Our study has shown that the apparent disjunctions of rare rhododendrons are more likely attributed to morphological convergence, possibly due to similar environmental selections in unrelated taxa. The finding highlights the importance of an integrative approach for resolving taxonomic challenges in plant species complexes.