Genetic diversity of Rhododendron delavayi var. delavayi (C. B. Clarke) Ridley inferred from nuclear and chloroplast DNA: implications for the conservation of fragmented populations

Abstract

Rhododendron delavayi Franch. is an ecologically and horticulturally important tree species distributed in a global biodiversity hotspot in southwest China, an area with high level of human-induced habitat degradation and fragmentation. In an attempt to explore the level of genetic diversity and population differentiation in the remnant populations of R. delavayi var. delavayi (C. B. Clarke) Ridley, we assessed patterns of chloroplast (trnS-trnG and trnL-trnF) and nuclear (RPB2-i) DNA variation in 327 individuals representing 17 natural populations from southwest China. Analyses revealed two phylogeographic groups, with similar circumscription between the two markers. We found moderate levels of genetic differentiation (cpDNA: N (ST) = 0.125; G (ST) = 0.114 and nuclear DNA: N (ST) = 0.261; G (ST) = 0.152). Both genomes demonstrate significant correlation between genetic and geographic distances (cpDNA: r (2) = 0.248, p = 0.001 and nuclear DNA: r (2) = 0.250, p = 0.001). The genetic diversity was positively associated with an increase in longitude. Populations from the eastern region of the Yungui plateau, representing potential refugial area for R. delavayi var. delavayi, registered higher haplotype diversity and allelic richness. The mismatch distribution analysis rejected rapid population expansion. The overall population expansion time for R. delavayi var. delavayi was estimated to be 0.208-0.624 mya. The complex landscape of southwest China and the human-induced fragmentation of the natural habitat have led to significant pairwise population differentiation and moderate genetic diversity (cpDNA: 0.626 and nuclear DNA: 0.506) and haplotypic richness (cpDNA: 4.01 and nuclear DNA: 2.589) in R. delavayi var. delavayi. Based on these findings, we recommend strategies for the conservation and sustainable management of R. delavayi var. delavayi.