No abstract

Extensive research has confirmed that hybrid speciation exists widely, yet how it occurs remains unclear. The Qinghai-Tibet Plateau (QTP) is an evolutionary center for Populus and an ideal place to study evolution. We identified 23 known and 7 new taxa of Populus through a systematic investigation on the plateau. We introduce three new taxa, P. gonggaensis, P. butuoensis, and P. dafengensis. They have unique morphological characteristics and a narrow geographical distribution. We resequenced the whole genome of 150 individuals from typical populations of 30 taxa and constructed the phylogenetic tree of Populus in the region with 2.28 million SNPs. The three new taxa, together with P. wilsonii and P. lasiocarpa, constituted the base clade, and each formed an independent terminal clade. Further analysis of genetic structure, gene flow, hybridization, and historical effective population size proved that they were all intersectional hybrid species originating from the distant hybridization between sect. Leucoides and sect. Tacamahaca in the Middle Pleistocene. Therefore, taxonomically, it is appropriate to regard them as three new species of sect. Leucoides. Moreover, we found that three new taxa were endowed heterosis, no matter from the results of field investigation or homogenous garden experiments, they all had better adaptability than their parents, and most of the positive selected genes responded to stress. The study provides comprehensive evidence for the hybrid speciation of Populus and proposes three new taxa. It also presents a novel and comprehensive method for studying the phylogeny of Populus.

Poplar not only has important ecological and economic value, but also is a model woody plant in scientific research. However, due to the rich morphological variation and extensive interspecific hybridization, the taxonomy of the genus Populus is very confused, especially in the Sect. Tacamahaca. Based on the extensive investigation of Populus on the Plateau and its surrounding areas, two taxa (Populus kangdingensis and Populus schneideri var. tibetica) that are very similar in morphology and habitat and are in doubt in taxonomy were found. First of all, we set up 14 sample sites, carried out morphological investigation and statistics, and found that there were a few morphological traits that could be distinguished between the two taxa. Further phylogenetic analysis based on the whole genome resequencing data showed that the two taxa were hybrid progenies of P. xiangchengensis and P. simonii. Through gene flow detection and genetic differentiation analysis, it was found that there was still strong gene flow from P. xiangchengensis to the two taxa, and there was almost no differentiation between the two taxa. Therefore, P. schneideri var. tibetica should be classified into P. kangdingensis as same taxa. Finally, the population history was reconstructed by PSMC and ABC models, and it was found that they all belonged to a hybrid origin, and the change in population size was closely related to the Quaternary ice age. In addition, the hybrid population has better adaptability, and the suitable distribution area may expand in the future. This study provided a novel and comprehensive method for the phylogeny of Populus and laid a foundation for the development and utilization of poplar resources.

Background: Trees such as Populus are planted extensively for reforestation and afforestation. However, their successful establishment greatly depends upon ambient environmental conditions and their relative resistance to abiotic as well as biotic stress. Polyphenol oxidase (PPO) is a ubiquitous metalloproteinase in plants, which plays crucial roles in plant resistance against biotic and abiotic stresses. Although the whole genome sequence of populus trichocarpa has long been published, little is known about the PPO genes in Populus, especially those related to drought stress, mechanical damage, insect feeding and hormone response at the whole genome level. Results: In the recent research, a genome-wide analysis of the Poplar PPOs family was finished and 18 PtrPPOs gene were identified. Then, bioinformatics and qRT-PCR were applied to analyze the gene structure, phylogeny, chromosomal localization, gene replication, Cis-elements, expression patterns of PtrPPOs. Sequence analysis revealed that 2/3 of the PtrPPO genes not contained introns. Phylogenetic analysis revealed that all PPOs gene were split into 11 subfamilies, and woody plants differentiated a large number of PPO genes. 18 PtrPPOs gene were disproportionally apportioned on 19 chromosomes, and the number of three pairs of segmented replication genes and four tandem repeat genomes were detected in poplars. Cis-acting element analysis identified a large number elements of growth and development, secondary metabolism process, and stress-related elements on the promoters of different PPO members. Furthermore, PtrPPO genes were expressed preferably in young plant tissues and fruits. In addition, some PtrPPOs could be significantly induced by PEG, ABA and JA, thus revealing their potential role in regulating stress response. Conclusions: Comprehensive analysis is helpful to select candidate PPO genes for the follow-up study of the biological function, and molecular genetic progress of stress resistance in forest trees provides genetic resources.