Abstract
Dramatic crustal deformation and river incision in Southwest China induced by the Indo-Asian collision have long been argued to contribute to the complicated landscapes, heterogeneous environment and abundant biodiversity in this region. However, biological impacts in promoting intraspecific phylogeographical subdivision and divergence along the Red River Fault zone (RRF) remain poorly understood. To investigate the possible biological effects of tectonic movements and environment variations within the RRF, the phylogeography of Cycas dolichophylla-an endemic but widely distributed Cycas in Southwest China and North Vietnam along the RRF were carried out based on four chloroplast DNA intergenic spacers (cpDNA), three nuclear DNA sequences (nDNA) and 16 simple sequence repeat variations (SSR). Two different phylogeographical patterns were detected: a Southwest-Northeast break across the RRF disclosed by chlorotypes and a China-Vietnam separation revealed by SSR. A Bayesian skyline plot from cpDNA data demonstrated a historical increasing, but a recent declining, dynamic in population size during the Pleistocene. Consequently, we infer it is the local environmental variation during Cenozoic that contributed to the complex landscape and microclimate mosaics, facilitating speciation and divergence of C. dolichophylla. Subsequently, the Quaternary climatic fluctuations coupled with human activities profoundly influenced the genetic structure and demographic history of this species.
Highlights
In recent years, accumulating evidence from tectonics, paleontology and climate simulations increasingly supported a view that the main body of the Tibetan Plateau (TP) reached its current height (4–5 km) since the mid-Eocene[1,2,3,4], that is, the uplift of TP and the evolution of Asian monsoon systems contribute little to the speciation and diversification in Southwest China[1,2,3,4,5]
The maximum clade credibility tree generated by BEAST uncovered the crown age of C. dolichophylla to be 1.94 Ma (95% HPD: 1.30–2.62 Ma) and retrieved the ‘northeastern’ and ‘southwestern’ chlorotypes as reciprocally monophyletic lineages, with coalescence time dated to about 1.84 Ma (95% HPD: 1.22–2.51 Ma) (Fig. 2)
A major finding of the present study was that C. dolichophylla comprised two geographically distinct lineages as inferred from the chlorotype network analysis, namely a southwestern clade distributed to the southwest of the River Fault zone (RRF) and a northeastern clade occupying the northeast region (Fig. 1a)
Summary
In recent years, accumulating evidence from tectonics, paleontology and climate simulations increasingly supported a view that the main body of the Tibetan Plateau (TP) reached its current height (4–5 km) since the mid-Eocene[1,2,3,4], that is, the uplift of TP and the evolution of Asian monsoon systems contribute little to the speciation and diversification in Southwest China[1,2,3,4,5] It might be the local ecological habitat diversity that resulted in the numerous young plant and animal clades endemic to such a biodiversity hotspot[1,6]. A phylogeographical study of C. dolichophylla was conducted by tracing variations from four chloroplast intergenic spacers (cpDNA), three nuclear sequences (nDNA) and 16 simple sequence repeats (SSR) in 13 populations across its entire range with an attempt to test the following hypotheses: (i) local environmental variation in its habitats influenced the lineage divergence of C. dolichophylla; (ii) climate variations, especially the glacial-interglacial cycles during the Quaternary re-sculptured this species’ genetic structure and accelerated its diversification
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