East China Normal University, Shanghai, China (People's Republic)
Abstract: The coexistence of closely related species is key to understanding the nature of biodiversity hotspots where in situ diversification has yielded rich communities of close relatives. Co-occurring species are predicted to be differentiated in response to limiting similarity, which might be further influenced by environmental gradients for the eco-evolutionary dynamics of community assembly. Flowering plants may be diverged along temporal or morphological niche axes that are selected by reproduction interference, varying among local conditions. We collected fine-scale data on abundance, morphology and phenology over a flowering season for 34 species of Rhododendron (Ericaceae) spanning a 2700 m elevation gradient in the eastern Hengduan Mountains, China. We used null models to test for patterns of clustering versus overdispersion in species' abundances, phylogenetic relatedness and functional traits across sites. In addition, we sequenced 15 populations of an alpine Rhododendron with a relatively large elevational range ( > 1000 m), to infer the adaption signature in its genomic structure. We found that species tended to be spatially aggregated locally, contrary to the expectation of competition exclusion. Environmental variables, including climate and topography, were strong predictors of species' ranges. At higher elevation sites, species tended to be vegetatively more similar and closely related (clustered). No evidence of niche differentiation was detected along spatial or morphological axes, but along the temporal axis, the phenology of co-occurring species showed significant divergence. Genomic data showed no significant geographic structure, while genotype environmental association tests showed a large number of single nucleotide polymorphisms (SNPs) being associated with fine-scale conditions along elevation. Among significant SNPs, the gene ontology enrichment analysis inferred a strong functional tendency associated with reproductive processes, such as pollination and signaling. Hence, evidence is apparent in the pervasive phenological divergence of co-occurring species, likely driven by reproductive interference from shared pollinators, as well as local environments. The evolutionary lability of flowering time appears to render it the quickest path to coexistence for recently diverged species that experience secondary contact in this biodiversity hotspot.
