Dipioid Hybrid Origin Of Picea Purpurea

It remains unclear how hybridization and introgression had taken part in complicated origin history of the extant species. In this study, we aimed to examine the evolutionary origin of Picea purpurea, a diploid spruce species in the eastern Qinghai-Tibet Plateau (QTP). Morphological evidence suggests the close relationship between Picea purpurea and P. likiangensis while molecular evidence grouped P. purpurea and P. wilsonii together. These inconsistent positions suggested its possible homoploid hybrid origin. The present study was designed to test this hypothesis and to examine the history of interspecific gene flow based on population genetic data from multiple loci of genomes of three diploid species.In order to avoid the recent introgressions, needle samples were collected only from the allopatric distributions of three species. A total of268individuals from29populations were sampled for P. wilsonii (10populations), P. likiangensis (10populations) and P. purpurea (9populations). Three chloroplast (cp) DNA fragments, two mitochondrial (mt) DNA fragments and twelve nuclear loci were sequenced for all collected populations. The genealogical relationships based on the haplotypes identified from both cp and mt DNA fragments suggested that P. purpurea and P. wilsonii were more closely related to each other than to P. likiangensis. In contrast, the haplotypes from the nuclear locus sel364supported that P. purpurea and P. likiangensis are closely related while the nuclear sequences from other loci foiled to distinguish three species. Further STRUCTURE clustering analyses based on population genetic data of all12nuclear loci indicated that P. purpurea has a mosaic composition from both P. wilsonii and P. likiangensis when K-2and three species were identified when K=3. The overall genetic distance between P. wilsonii and P. purpurea was significantly larger than that between P. likiangensis and P. purpurea. Model tests based on approximate Bayesian computation analyses also supported the hybrid origin of P. purpurea. Because mtDNA and cpDNA are mainly inherited maternally and paternally respectively, one parental species only provided once organeile origin. The close relationship of P. purpurea with P. wilsonii at these two markers suggested that the hybrid P.purpurea must have further been introgressed by one organelle genome of P. M-ilsonii again. Further analyses suggested the hybrid origin followed by immediate mt-genome introgressions of P. purpwea probably occurred during the climatic oscillations of the Pleistocene. The geological studies showed that from the Pliocene to Pleistocene, the Qinghai-Tibet Plateau uplifted several times and climates fluctuated greatly in this region. These changes might have promoted the allopatric divergence of two ancestral spruce species, and facilitated their secondary contacts, which further leaded to the hybrid origin and following mtDNA introgression of P. purpwea. This study firstly documented a special speciation mode.