New insight into the phylogeographic pattern of (Magnoliaceae) revealed by chloroplast DNA: east-west lineage split and genetic mixture within western subtropical China.

BACKGROUND

Subtropical China is a global center of biodiversity and one of the most important refugia worldwide. Mountains play an important role in conserving the genetic resources of species. is a Tertiary relict tree largely endemic to subtropical China. In this study, we aimed to achieve a better understanding of the phylogeographical pattern of and to explore the role of mountains in the conservation of genetic resources.

METHODS

Three chloroplast regions (J-A, 32-F, and K5'-K) were sequenced in 40 populations of for phylogeographical analyses. Relationships among chloroplast DNA (cpDNA) haplotypes were determined using median-joining networks, and genetic structure was examined by spatial analysis of molecular variance (SAMOVA). The ancestral area of the species was reconstructed using the Bayesian binary Markov Chain Monte Carlo (BBM) method according to its geographic distribution and a maximum parsimony (MP) tree based on Bayesian methods.

RESULTS

Obvious phylogeographic structure was found in . SAMOVA revealed seven groups matching the major landscape features of the distribution area. The haplotype network showed three clades distributed in the eastern, southwestern, and northwestern regions. Separate northern and southern refugia were found in the Wu Mountains and Yungui Plateau, with genetic admixture in the Dalou Mountains and Wuling Mountains. BBM revealed a more ancient origin of in the eastern region, with a west-east split most likely having occurred during the Mindel glacial stage.

DISCUSSION

The clear geographical distributions of haplotypes suggested multiple mountainous refugia of . The east-west lineage split was most likely a process of gradual genetic isolation and allopatric lineage divergence when the Nanling corridor was frequently occupied by evergreen or coniferous forest during Late Quaternary oscillations. Hotspots of haplotype diversity in the Dalou Mountains and Wuling Mountains likely benefited from gene flow from the Wu Mountains and Yungui Plateau. Collectively, these results indicate that mountain regions should be the main units for conserving and collecting genetic resources of and other similar species in subtropical China.