Phylogeography And Speciation Of Four Pine Species In Northeast China

Although homoploid hybrid speciation in plants is probably more common than previously realized, there are few well-documented cases of homoploid hybrid origin in conifers. We examined genetic divergence between two currently widespread pines in northeast China, Pinus sylvestris var. mongolica and P. densiflora, and also whether two narrowly distributed pines in the same region, P. funebris and P. takahasii, might have originated from the two widespread species by homoploid hybrid speciation. Our results, based on population genetic analysis of chloroplast (cp), mitochondrial (mt) DNA, and nuclear gene sequence variation, showed that the two widespread species were divergent for both cp- and mtDNA variation, and also for haplotype variation at two of eight nuclear gene loci surveyed. Our analysis further indicated that P. sylvestris var. mongolica and P. densiflora remained allopatric during the most severe Quaternary glacial period that occurred in northeast China, but subsequently exhibited rapid range expansions. Pinus funebris and P. takahasii, were found to contain a mixture of chlorotypes and nuclear haplotypes that distinguish P. sylvestris var. mongolica and P. densiflora, in support of the hypothesis that they probably originated via homoploid hybrid speciation following secondary contact and hybridization between P. sylvestris var. mongolica and P. densiflora. Mitotype variation indicated that P. densiflora was the maternal parent of both putative homoploid hybrid species. Overall, this study highlights the important role of Quaternary climatic oscillations in causing range shifts that can result in contacts between previously isolated species and the origin of new hybrid species.