Molecular Phylogeny Of Section Leuce And The Hybridization Origin Of Hybrids In Section Leuce Of Populus

Section Leuce has the most widely distribution in the genus Populus. The species from which can be found in most of the Northern Hemisphere. The species relationships have not been fullly resolved due to different taxa used and frequent hybridization and introgression. P.canescens, P. hopeiensis and P. tomentosa are recognized to be hybrids based on the existing studies, and genetic mechanism of P. canescens has been deciphered. P. hopeiensis and P.tomentosa are endemic to China with high quality timber and long life. However, it is far from clear whether their origin has been the subject of intense study, historically and recently. In this research, the phylogeny and divergence time of P. alba, P. adenopoda, P. davidiana, P. tremula,P. tremuloides and P. gradidentata was reconstructed with 26 cpDNA. To reveal the origin of P.hopeiensis and P. tomentosa, 392 individuals in 36 populations of 8 species(hybrids) were analyzed with 10 nDNA and 6 cpDNA. This study will lay an important foundation for the scientific breeding, the utilization of wild resources and further understanding the hybrid origin.The main results are:(1) The same topology was acquired with MP, ML and BI, with high support values, and the species relationships avoided being affected by different outgroups. The phylogeny of section Leuce was a robust monophyly, which indicated the species shared the common maternal ancestor. The maternal ancestor isolated from the genus Populus in middle Miocene(11.25Myr) and split into 2 big groups in 7.60 Myr. P. alba and P. adenopoda occupied the first branch and separated in 6.04 Myr with the lift of Qinghai-Tibet Plateau. The second big group was divided into 2 small groups in 6.59 Myr, P. tremuloides and P. gradidentata was one of the small groups and was divergent in 5.91 Myr, the last small group comprised P. davidiana and P.tremula, and they began segregate in 5.28 Myr. Obviously, the geographic isolation, the lift of Qinghai-Tibet Plateau in particularly, had the critical role in the progress of speciation.(2) The fact that significant differentiation was detected between hybrid and putative parents by Fst indicated they have different hereditary basis. P. tomentosa was divided into 2groups in the cpDNA tree. P. tomentosa Ⅰ grouped with P. adenopoda, while P. tomentosaⅡ and P. hopeiensis formed a cluster with P. davidiana. All the individuals of P. tomentosa and P. hopeiensis joined P. alba in the nDNA tree. The placement of P. tomentosa and P.hopeiensis changed in the nucleoplasm phylogeny, which implied they had a hybrid origin probably. Taken nucleoplasm phylogeny and STRUCTURE into consideration, P. hopeiensis was a hybrid with the unidirectional pollination from P. alba to P. davidiana. Similarly, P.tomentosa Ⅰ was also a hybrid with the unidirectional pollination from P. alba to P.adenopoda. P. tomentosa Ⅱwas complex hybrid and the origin patterns was(P. davidiana ×P. adenopoda) × P. alba. The same origin patterns of P. hopeiensis, P. tomentosa Ⅰ and P.tomentosa Ⅱwere certified and they occurred in 1.37 Myr, 0.59 Myr and 0.897 Myr respectively with DIYABC. Most of the individuals of P. hopeiensis and all of the P. tomentosa Ⅰ were F1 generation, 2 individuals of P. hopeneisis were F2 generation.(3)In the analysis of ENM, The distribution of P. adenopoda, P. davidiana, P. tomentosa and P. hopeiensis almost has not been changed since LIG, but ice age played a crucial role in the distribution of P. alba. The habitats between P. hopeiensis, P. tomentosa and their putative parents differentiated significantly, which offered the opportunity for existence and multiplication of them.