Ancient vicariance followed by long-term northern microrefugia in Lindera obtusiloba (Lauraceae), which show disjunct distribution in East Asia
Vicariance and Long-Distance dispersal (LDD) are two hypotheses explaining disjunct distributions. Different hypotheses have different genetic predictions. Firstly, during vicariance scenario, lineages from the disjunct areas should be reciprocally monophyletic, while during LDD hypothesis, lineages from colonized area populations should nested with ancestral area populations. Secondly, there should be great differentiation between disjunct populations in vicariance scenario while the opposite in the alternative LDD scenario. Thirdly, genetic diversity should be comparable in disjunct distribution areas resulting from vicariance, whereas reduced genetic diversity might be expected in colonized areas. Lindera obtusiloba (Lauraceae) is a deciduous shrub show disjunct distributions between north East Asia (NEA) and south East Asia (SEA). Four cpDNA sequences, 6 nuclear microsatellites (nSSR) and Ecological Niche Modeling (ENM) were combined to infer its phylogeography history. Based on different genetic predictions, we can tell whether its disjunct distributions were originated from vicariance or LDD, and which climate changes or geological movements that are related to can infer through divergence time estimations.
Significant phylogeography structure (NST = 0.884 > GST = 0.645, P < 0.01) was found and no cpDNA haplotypes were shared between NEA and SEA. Network and Bayesian tree derived two lineages corresponding to geological locations. 6 nSSRs had high genetic differentiation (FST = 0.58) and also revealed two distinct clusters with very little introgression. UPGMA tree show two reciprocal monophyletic lineages. Regional genetic diversity is significant higher in NEA in nSSRs and comparable in cpDNA sequences. Indicating that L. obtusiloba was originated from vicariance and northern microrefugia exist in NEA. Divergence times between two lineages were estimated at 7.49 Ma using cpDNA sequences and 2.6 Ma using nSSRs. 7.49 Ma and 2.6 Ma are both in relation to aridification in North China and global cooling, which may force ancestral continuous distributions of L. obtusiloba at high latitude southward migration and fragmented into NEA and SEA separate distributions according to a species living in warm and humid condition. ENM show two separated distributions during 3 Ma before, Last Interglacial (LIG) and Present but not during Last Glacial Maximum (LGM). No gene flow had happened between during the LGM may due to reproductive isolation after long-term isolation.