Long-distance dispersal or postglacial contraction? Insights into disjunction between Himalaya-Hengduan Mountains and Taiwan in a cold-adapted herbaceous genus, Triplostegia

Abstract

Current disjunct patterns can result from long-distance dispersal or postglacial contraction. We herein investigate the evolutionary history of Triplostegia to elucidate the disjunction between the Himalaya-Hengduan Mountain region (HHM) and Taiwan (TW). Genetic structure of Triplostegia was investigated for 48 populations using sequences from five chloroplast loci and the ribosomal nuclear internal transcribed spacer. Divergence time estimation, ancestral area reconstruction, and species distribution modeling (SDM) were employed to examine the biogeographic history of Triplostegia. Substantial genetic differentiation among populations from southwestern China (SW), Central China (CC), and TW was detected. Triplostegia was inferred to have originated in SW, and diversification began during the late Miocene; CC was colonized in the mid-Pliocene, and TW was finally colonized in the early Pleistocene. SDM suggested an expansion of climatically suitable areas during the Last Glacial Maximum and range contraction during the Last interglacial in Triplostegia. Disjunction between HHM and TW in Triplostegia is most likely the consequence of topographic isolation and postglacial contraction. The potential climatic suitability areas for Triplostegia by 2070s (2061-2080) are predicted to slightly shrink and move northward. With continued global warming and human-induced deforestation, extinction risk may increase for the cold-adapted species, and appropriate strategies should be employed for ecosystem conservation.

Keywords: Himalaya–Hengduan Mountain region; Taiwan; Triplostegia; conservation; phylogeography; species distribution modeling.

Figure 1

(a) Geographical distribution of the cpDNA haplotypes detected in Triplostegia (see Table S1 for population codes) and (b) network of genealogical relationships among 20 haplotypes. Each circle represents a single haplotype with sizes in proportion to frequency. Black dots in network represent missing haplotypes. Potential interglacial refugia recognized by this study are encircled by dashed lines

Figure 2

Analysis based on ITS dataset. (a) Geographical distribution of haplotypes in Triplostegia (see Table S1 for population codes); (b) network of genealogical relationships between 14 haplotypes. Each circle represents a single haplotype sized in proportion to its frequency. Black dots in the network represent missing haplotypes; (c) results of SAMOVA (the red point indicated the optimized number of groups K); and (d) neighbor‐joining tree without out‐group. Potential interglacial refugia recognized by this study are encircled by dashed lines

Figure 3

Number of pairwise nucleotide differences in Triplostegia based on chloroplast DNA sequences in all areas (a), southwestern China (b), Central China (c), and Taiwan (d). Black dots and dashed line show observed values (Obs.); solid lines indicate expected values (Exp.) under a model of sudden (stepwise) population expansion

Figure 4

Ancestral area reconstructions based on the Bayesian binary Markov chain Monte Carlo (BBM) method implemented in RASP using the BEAST‐derived chronogram of Triplostegia. The upper left map shows major distribution divisions of Triplostegia in China: southwestern (SW), Central (CC), and Taiwan (TW). Pie chart at each node illustrates the marginal probabilities for each alternative ancestral area derived from BBM with the maximum area number set to three. The legend denotes possible ancestral ranges at different nodes. Ages of key nodes are labeled above branches. The out‐group (Dipsacus asper, see Figure S5) was not shown here

Figure 5

Species distribution models showing climatic suitability for Triplostegia in East Asia: (a) the Last interglacial (LIG; 0.12–0.14 Ma); (b) the Last Glacial Maximum (LGM; 0.021 Ma); (c) current conditions (current, 1960–1990); and (d) 2070s (2061–2080) with the representative concentration pathways (RCP) of 4.5. Climatic suitability increases with color from blue to red. Resolution for the potential distribution map is 2.5 arc‐minutes