Diversification of Plant Species in Arid Northwest China: Species-level Phylogeographical History of Lagochilus Bunge ex Bentham (Lamiaceae)

Phylogeography is a relatively new discipline that examines the spatial arrangements of genetic lineages, and infers phylogeographical patterns, as a powerful tool for investigating processes that determine genetic composition, especially within and among related species. This can untangle historical changes at various spatial and temporal scales in the patterns of gene flow, isolation, demographic expansion, and secondary contact among divergent populations.

Currently, most plant phylogeographical studies in China have focused on the Sino-Japanese Floristic Region of East Asia. Comparatively, few studies have been undertaken concerning the evolutionary history of arid-adapted plant species in arid Northwest China.

The genus Lagochilus is highly drought-tolerant, and is also considered as a typical montane plant, found across arid Northwest China. It provides an excellent system for examining the potential influence of geologic and climatic effects on range fluctuations and diversification of species across the temperate steppe and desert regions. The genus (or perhaps its progenitors) is thought to have evolved from ancient seaside habitats, and most likely underwent rapid speciation during the Quaternary. Therefore, understanding the phylogeographical history of Lagochilus will provide insights into the diversification of desert plants triggered by the past geologic and/or climatic changes in arid Northwest China.

To understand the evolutionary history of Lagochilus and the divergence related to these past shifts of habitats among these regions, MENG Honghu and Prof. ZHANG Mingli sequenced the plastid intergenic spacers, psbA-trnH and trnS-trnG from populations throughout the known distributions of ten species of the genus. They investigated species level phylogeographical patterns within Lagochilus. Phylogenetic trees were constructed using Neighbor-joining and Bayesian inference. The divergence times of major lineages were estimated with BEAST and IMa. Genetic structure and demographic history were inferred by AMOVA, neutrality tests, mismatch distribution, and Bayesian skyline plot analyses.

The results showed that most chloroplast haplotypes were species-specific, and that the phylogeny of Lagochilus is geographically structured. The estimated Bayesian chronology and IMa suggested that the main divergence events for species between major eastern and western portions of the Chinese desert occurred at the Plio-/Pleistocene boundary (ca. 2.1–2.8 Ma ago), and likely coinciding with the formation of these deserts in Northwest China. The regional demographic expansions, in the western region at ca. 0.39 Ma, and in the eastern at ca. 0.06 Ma, or across all regions at ca. 0.26 Ma, showed the response to aridification accompanied by cooling of the Pleistocene sharply increased aridity in the Chinese deserts, which reflects a major influence of geologic and climatic events on the evolution of species of Lagochilus. The results suggest that diversification is most likely the result of the past fragmentation due to aridification; the expansion of the range of species along with the deserts was an adaptation to dry and cold environments during the Quaternary. The result was published in Molecular Phylogenetics and Evolution, 68(3): 398-409.