Species Delimitation And Speciation Of Spruce Species

Spruces(Picea), a genus of Pinaceae, are important components of the forests in the Northern Hemisphere. Due to the large effective population size, long generation time, radiative diversification and frequent interspecific hybridization and introgression, it is relatively difficult to distinguish species by using traditional morphological method and universal DNA barcodes. In addition, genetic drift due to geographical isolation, gene flow and mutation rates together make it difficult to determine the evolutionary histories of spruce species, the interspecific relationships between some species are still unclear.At frist, we aimed to explore the discriminatory effectiveness of candidate DNA barcodes and test whether certain nuclear genes are more powerful tools for barcoding spruce species than chloroplast and mitochondrial DNA fragments(DNAs). We sequenced 20 DNAs, including five chloroplast DNAs, two mitochondrial DNAs and 13 nuclear genes, for a total of 123 individuals representing 37 spruce species and subspecies. We found that some nuclear genes did show higher discriminatory power than any of the chloroplast or mitochondrial DNA fragments we examined. However, no single DNA fragment could discriminate more than half of the spruce species, although a combination of all the nuclear genes greatly increased discrimination success(up to more than 89% of the total number of species). These findings suggested that most spruce species are derived from recent diversification and still on their way to the final stage of speciation. A low level of accumulation of genetic variation along each species lineage renders it impossible to develop any single nuclear DNA fragment as a barcode. However, combining variations from multiple nuclear genes makes it possible to discriminate between most spruce species. A tiered barcoding system is therefore needed for spruces, the first tier could use traditional cytoplasmic DNAs, and the second tier should comprise multiple nuclear DNAs developed specifically for this genus.Secondly, in order to explore the complex evolutionary histories between three close related spruce species with a similar morphology, Picea wilsonii, P. neoveitchii and P. morrisonicola, we combined population genetic data from three different genomes(including chloroplast, mitochondrial and nuclear genome) to decipher interspecific relationship, speciation pattern and gene flow between those three species. P. wilsonii and P. neoveitchii occur from central to northwest China, where they have overlapping distributions. However, P. morrisonicola is restricted solely to the island of Taiwan and is isolated from the other two species by a long distance. We examined sequence variations of 18 DNA fragments for 22 populations, including three from chloroplast(cp) genome, two from mitochondrial(mt) genome and 13 from nuclear genome. In both the cpDNA and the mtDNA, P. morrisonicola accumulated more species-specific mutations than the other two species. However, most nuclear haplotypes of P. morrisonicola were shared by P. wilsonii, or derived from the dominant haplotypes found in that species. Further phylogenetic analyses based on all nuclear sequences suggested that P. morrisonicola is more closely related to P. wilsonii than to P. neoveitchii. Our modeling results also supported the hypothesis that P. morrisonicola derived from P. wilsonii within the more recent past, most probably indicating progenitor-derivative speciation with a distinct bottleneck, although further gene flow from the progenitor to the derivative continued. In addition, we detected the occurrence of an obvious mtDNA introgression from P. neoveitchii to P. wilsonii despite their early divergence. The extent of mutation, introgression and lineage sorting taking place during interspecific divergence and demographic changes in the three species had varied greatly between the three genomes. Our findings highlight the complex evolutionary histories of these three Asian spruce species.In this study, our findings not only provide a case study on discriminating and barcoding recently diverged species, especially for those tree species, like Picea, with large effective population sizes and long life histories, but also provide a good example for understanding the speciation and evolutionary history of Pciea by using population genetics data. All of our findings had been published on the journal of Trees and Annals of Botany.