Inferences of genetic structure and demographic history of Rhododendron protistum var. giganteum-The world's largest Rhododendron using microsatellite markers

•R. protistum var. giganteum is honored as “the king of Rhododendron” in China.•Genetic structure and demographic history of R. protistum var. giganteum were studied.•The remnant populations maintained a modrate level of genetic diversity.•The species have high historical and low contemporary gene flow.

Big tree rhododendron (Rhododendron protistum var. giganteum) is an extremely endangered plant with only two remnant populations distributed in the Gaoligong Mountains of northwestern Yunnan Province, China. However, the genetic structure, demographic history, and the population dynamics of this species have been rarely described. In the present study, data from 14 polymorphic nuclear microsatellite loci were used to assess the levels of genetic diversity, genetic structure, and demographic history of big tree rhododendron remnant populations. Results suggested that renmants maintained a moderate level of genetic diversity (NE = 2.86, AR = 5.726, Ho = 0.510, He = 0.602, I = 1.174) despite their small population. Bayesian clustering and principal coordinate analysis indicated that sampled individuals were clustered into two distinct genetic groups. Demographic history analysis revealed high historical and low contemporary gene flow between two remnant populations. BOTTLENECK analysis under a two-phase mutation model showed that the populations could have experienced a recent historical bottleneck. The effective population sizes of the two populations were smaller than 30. Therefore, in situ conservation of this species should be prioritized. Furtheremore, ex situ cultivation of field collected seeds and introduction of saplings into the source site might be considered for the future conservation and restoration of big tree rhododendron.