青藏高原区杨属的系统发生及对分类和育种的启示

Taxonomy is the basis of recognizing,studying and employing living beings.The genus of Populus is considered as one of most difficult biotic group for classification in the world.There are a great divergence and many arguments in its taxonomy,which brings unbeneficial effects on research,breeding and cultivation of poplar.To establish a unified classification system based on phylogeny is ultimate goal for Populus classification.In an 18-year-long investigation of the systematics of all taxa of Populus,we found 22 described taxa and 8 new taxa of Populus in the Tibetan Plateau.Through an integrated study including morphology,whole-genome bioinformatics,ecology,biogeography and breeding,we have illustrated that P.wilsonii,P.lasiocarpa,P.simonii,P.szechuanica,P.cathayana and P.curviserrata are ancient pure species,and the residuary 23 taxa are all originated from interspecific hybridization among the above 8 pure species.Among them,there are 3 hybrid taxa between sect.Tacamahaca and sect.Tacamahaca.In addition to these taxa,there are also many difficult to name hybrid populations in the region,which are the ongoing hybrid speciation events.The diversification of Populus species is driven by repetitive interspecific hybridization,leading to that their phylogeny appears as an extraordinarily complicated network rather than a tree.According to the phylogeny,we put forward some proposals that P.kangdingensis,P.schneideri var.tibetica and P.qamdoensis should be merged into P.przewalskii,P.haoana should be merged into P.purdomii,P.wuana and P.yatungensis should be respectively merged into P.pseudoglauca and P.ciliate,P.szechuanica var.tibetica should be redefined and promoted as a new hybrid species,and so on.Meanwhile,we also found some phenomenon:within the distribution range of hybrid taxa,there is no distribution of their ancestral species;the hybrid taxa possessed stronger ecological adaptability to high altitude,cold and drought than their ancestral species.We further purposed a new model of hybrid speciation that can well explain some questionable issues,such as why the nucleo-cytoplasmic conflict of phylogeny is widely present,why ancestral species only reside in a few eastern and southern moist valleys of the Plateau,why the classification of P.szechuanica,P.purdomii,P.haoana,P.xiangchengensis and P.szechuanica var.tibetica is so confusing.With the temporal and spatial scale of evolution,the heterosis between sect.Tacamahaca and sect.Leucoides is very significant and important,and interspecific or intertaxonomic hybridization is the most important source for species and genetic diversity of Populus.Hereafter,we should specifically strengthen the protection of ancestral pure species and rare hybrid species,restrict the cultivation of extrinsical poplar species,think over the strategies of hybrid breeding under the phylogenetic background,and attach importance to breeding and cultivation of hybrid taxa with outstanding adaptability to high altitude,cold and drought.