Hybridization And Gene Flow Between Populus Nigra L.and P. Laurifolia Ldb.

Populus×jrtyschensis C.Y. Yang is a hybrid poplar in Populus section Aigeiros Duby that naturally distributed in the Irtysh valley. P.×jrtyschensis trees are valuable resources for breeding program of poplar cultivars due to their tolerance to cold environment and various types of soil. Morphological characters of P.×jrtyschensis (including traits of leaves, buds, branchlets, barks, etc.) are intermediate between P. nigra L. and P. laurifolia Ldb., whose distribution are sympatric or parapatric with P.×jrtyschensis. Therefore, it was proposed that P.×jrtyschensis is a natural hybrid between those two species. However, considering that P.×jrtyschensis occupies different habitat comparing to its two putative parental species, it is unclear that whether P.×jrtyschensis is a hybrid species which has its unique ecological niche or a series of hybrid swarm which occur at the overlapping area of their putative parental species.In this study, we surveyed the genetic variation of P. x jrtyschensis, P. nigra and P. laurifolia by sequencing eight single (low) copy nuclear genes (screened out of30) of168trees from31populations. Based on these DNA sequence data, various population genetic approaches were employed to detect the nucleotide diversity and population structure of P.×jrtyschensis and both of its putative parental species, and gene flow between these latter two. We were aimed to determine the genetic status of P.×jrtyschensis and to understand the roles it played in the hybridization process between P. nigra and P. laurifolia.Our results suggested that the genetic composition of P.×jrtyschensis is contradictory with the hybrid species hypothesis but fit with a multi-generation hybrid swarm hypothesis. First, most P.×jrtyschensis trees possess well-diverged genotypes from both of its putative parent species, but intermediate and distinct genotypes are expected under a hybrid species hypothesis; secondly, Bayesian clustering indicated that P.×jrtyschensis combines genetic components from both of its putative parental species; thirdly, P. x jrtyschensis experienced more recombination events than its putative parental species, indicating that it may comprise multi-generation hybrids; lastly, geographic distribution pattern of genotypes for each locus illustrates that P.×jrtyschensis at different sites were derived via local hybridization between P. nigra and P. laurifolia. The genetic composition of P.×jrtyschensis shows greater proportion from P. nigra, this is in agreement with previous observation that these hybrids are morphologically closer to P. nigra; this may have been caused by the fact that P. xjrtyschensis co-occur with P. nigra more frequently than P. laurifolia, which could have facilitate more backcrosses with P. nigra. Meanwhile, Isolation-Migration model detected introgression from P. laurifolia and P. nigra, this is in agreement with a theory that hybrid swarm bridges gene flow between hybridizing species pair. And the effective population size of these two species are3%-5%of their common ancestor’s, indicating that both species may have experienced bottlenecks in their past demographic history. In addition, neutral tests suggested that one locus(PAL) most likely experienced balancing selection in P. nigra and two loci (AREB1and EIN3) may have experienced balancing selection in both P. laurifolia and P. nigra. Our study provided another case of interspecific (intersectional) hybridization and introgression in Populus, our findings will also contributed to the utilization and conservation of P×jrtyschensis, P. laurifolia and P. nigra, a very important group of cold-tolerant forest trees.