Genetic diversity and structure of English yew (Taxus baccata L.) as a tertiary relict and endangered tree in the Hyrcanian forests

Maintenance of standing genetic diversity and effective population size (N-e) in trees is essential for sustainability, adaptation, and evolution, especially for dioecious tree species under changed climatic conditions. We examined the gene pool of Taxus baccata along latitudinal gradients in the Hyrcanian forest from west to east using 15 simple sequence repeat markers (SSR). A high level of genetic diversity and a significant level of fixation index was observed, and the populations showed spatial genetic structure. The fixation index ranged from 0.027 to 0.197. In six out of eleven populations inbreeding was the significant factor influencing deviation from the Hardy-Weinberg equilibrium. There was no bottleneck effect in any of the analysed populations and the global Fst with ENA correction was 0.044. The average total number of migrants per generation ranged from 4.93 to 2.14. Under the six tested scenarios, the DIYABC analysis showed that the eastern and western parts of the Hyrcanian yew forests split about 134 generations ago but these two pools meet in the central part of these forests about 49 generations ago. Although the genetic diversity of T. baccata in the Hyrcanian forest is relatively high, it is expected that the inbreeding depression will increase over time, causing the accumulation of destructive alleles and intensifying the extinction process. This process is caused by anthropogenic activities, habitat fragmentations, the age of the trees, low regenerations, the existence of high geographical barriers, and a significant reduction in gene flow among the habitats.