Population genetic structure of Iris ensata on sky-islands and its implications for assisted migration

Due to global warming since the Last Glacial Maximum, many plant populations have retreated to mountain tops, i.e., sky-islands, which are isolated by low-elevation barriers and inhospitable habitats. Under projected environmental changes, these populations may decline and face local extinction. Iris ensata Thunb. populations at the species’ most southern distribution range are in this position. We used eight polymorphic nuclear microsatellite markers and three fragments of chloroplast genome to analyze population structure of six extant sky-island populations of I. ensata. A total of 83 alleles were found across 192 individuals from six populations. High levels of intra-population genetic diversity (H_E = 0.578, H_O = 0.608) of I. ensata were detected. Moderate but significant levels of genetic differentiation were also found among the populations (F_ST = 0.133, P < 0.001). Mantel test showed no isolation-by-distance pattern (r = 0.339, P = 0.161). Assignment analysis classified all individuals into five groups, and four populations were dominated by only one group. Five chloroplast DNA (cpDNA) haplotypes were found, and one was shared by all populations. Two populations contained a private haplotype. Long-term fragmentation, but relatively large population sizes and restricted gene flow among populations, contributed to the above patterns. Under projected habitat changes, the studied populations are at risk of local extinction. We identified three populations that had a high priority of ex situ conservation to be the source populations for the future assisted migration. Number of individuals, how to select individuals, and a potential recipient site were discussed.