The ‘Ward Line–Mekong–Salween Divide’ is an important floristic boundary between the eastern Himalaya and Hengduan Mountains: evidence from the phylogeographical structure of subnival herbs Marmoritis complanatum (Lamiaceae)

Abstract

The Ward Line-Mekong-Salween Divide (MSD), a classic geographical barrier, is considered the main driver for population differentiation of species in the Himalaya-Hengduan Mountains region. To identify the role of geological factors in shaping the population history of subnival plant species, we investigated the phylogeography of Marmoritis complanatum based on plastid and nuclear ribosomal DNA. The phylogeographical differentiation of M. complanatum was identified between two geographical regions (central-eastern Himalaya vs. Hengduan Mountains) across the MSD, and the differentiation of two lineages began in the early Pliocene (4.49 Mya), coinciding with the formation of the MSD. These results indicate the MSD was the main driver of phylogeographical differentiation in M. complanatum. High genetic diversity and inter-population differentiation were detected in M. complanatum. Mismatch distribution analyses and species distribution modelling indicated that the broad-scale species distributions remained relatively stable with signs of a slight shrinkage of populations after the Last Glacial Maximum, suggesting Quaternary glaciations had little impact on the large-scale distributional range shift of M. complanatum. This study provides the first evidence for the effects of the MSD on the phylogeographical structure of this subnival plant species, and verifies the importance of the floristic boundary represented by the MSD.