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

Abstract

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. Liriodendron chinense 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 L. chinense and to explore the role of mountains in the conservation of L. chinense genetic resources.

Methods: Three chloroplast regions (psbJ-petA, rpl32-ndhF, and trnK5'-matK) were sequenced in 40 populations of L. chinense 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 L. chinense. SAMOVA revealed seven groups matching the major landscape features of the L. chinense 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 L. chinense 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 L. chinense. 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 L. chinense and other similar species in subtropical China.

Keywords: Chloroplast DNA; Glacial refugia; Liriodendron chinense; Mountains; Phylogeography; Subtropical China.

Figure 1. Sample locations and geographic distribution of the chloroplast (cp) DNA haplotypes detected in Liriodendron chinense.

Pie chart size is proportional to its numbers. Detailed informations of populations and haplotypes are found in Table 1.

Figure 2. Results of spatial analysis of molecular variance analysis (SAMOVA, K = 7) on Liriodendron chinense populations in subtropical China.

Figure 3. Median-joining network of the 20 haplotypes detected in Liriodendron chinense (clades A–C) and Liriodendron tulipifera (clade D).

Black dots indicated the number of mutational steps.

Figure 4. Ancestral area reconstructions based on the Bayesian binary Markov chain Monte Carlo (BBM) method implemented in RASP using the MP chronogram of Liriodendron chinense.

(A) The insert map shows major floristic divisions in Subtropical China. (B) Maximum parsimony (MP) tree of the 20 haplotypes detected in Liriodendron. Pie charts of each node illustrate the marginal probabilities for each alternative ancestral area derived from BBM with the maximum area number set to four. The number above the branches indicate bootstraps support value above 40. The colour key identifies possible ancestral ranges at different nodes. Possible dispersal events are indicated by red arrows.