Low genetic diversity and population differentiation in Thuja sutchuenensis Franch., an extremely endangered rediscovered conifer species in southwestern China

Thuja sutchuenensis Franch. is an extremely endangered conifer (Cupressaceae) species that is endemic to southwestern China. It was once considered extinct in the wild and was rediscovered after 100 years later in October 1999. Due to the habitat destruction from nature and humans, the population has decreased sharply and the distribution range is narrow. Thus there is an urgent need to evaluate genetic variation within this species in order to establish a feasible conservation strategy. In this study, we used six single-copy nuclear loci (4CL, LEAFY, NEEDLY, CC1147, GAPC and RABGGTB) to investigate the genetic diversity, population structure and dynamics of T. sutchuenensis based on a sampling of 109 individuals from 6 natural populations across the known extant geographical range. Our results indicated that the genetic diversity of T. sutchuenensis was relatively lower (pi(t) = 0.00219, theta(wt) = 0.00240) than that of other Cupressaceae species. Extensive gene flow (N-m >= 11.07) and low genetic differentiation among populations were detected. Extended Bayesian skyline plots revealed that the population size of T. sutchuenensis has remained constant through time. The low genetic diversity was believed to result primarily from genetic drift caused by habitat fragmentation and population dynamics caused by climate fluctuations. The low genetic differentiation among populations was largely attributed to high levels of gene flow and small geographic distribution. Based on the above analyses, we should first conserve all extant populations and their habitats to maintain its current genetic diversity. Besides these, artificial propagation efforts such as seed propagation, cuttage breeding or tissue culture methods should be made to rescue this rare and endangered tree species. (c) 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).