Studies On Genetic Diversity And Phylogeography Of Wild Pyrus Betulaefolia In Northern China

Genetic diversity, described mainly as any measure that quantifies the magnitude of genetic variability within species, is a fundamental source of biodiversity. Genetic diversity within species has been recognized as especially important in striving to slow or halt the loss of biodiversity. Such intraspecific genetic diversity provided benefits for evolutionary trajectory of a species. Wild species generally contains more genetic variability than the cultivated ones. Naturally occurring variation among wild species is an underexploited resource in plant breeding. Pyrus betulaefolia Bunge, which has been considered as an intermediate species connecting European and Asian pears, is one of the most ancient wild species in the genus Pyrus L. Until now, little is known about genetic diversity and phylogeography of P. betulaefolia. In this study, filed investigation, cpDNA, and genomic SSR were used to detect genetic diversity and phylogeography of P. betulaefolia. The main results were listed as follows;1. P. betulaefolia mainly distributed in the area of32.94-39.74°N,107.54-119.16°E, with elevation from47to1545meters in northern China. Ziwu Mountain, Taihang Mountain, Funiu Mountain and Mengshan Mountain were four major distribution areas of P. betulaefolia. The age spectrum of population and morphology of leaf and fruit changed dramatically among different regions. The highest morphological diversity of leaf occurred in populations of P. betulaefolia located in Henan Province. Meanwhile, single fruit weight of P. betulaefolia from these locations was significantly higher than that from the others. Transverse and vertical diameters and single fruit weight of P. betulaefolia in Huachi and Ningxian in Gansu Province were remarkably smaller than those from other sites. Trees with diameter at breast height ranged from2.5to22.5cm were the primary constituents of P. betulaefolia populations.2. Based on two chloroplast DNA intergenic fragments (trnL-trnF and accD-psaI), P. betulaefolia populations sampled in northern China contained a high level of genetic diversity (HT=0.826). A significantly higher Nst than Gst reflected the (Nst> Gsx, P<0.05) phylogeographic structure in P. betulaefolia. A significant isolation-by-distance value (r=0.587, P<0.001,1,000permutations) among all18populations indicated a correlation between genetic divergence and geographic distance. Four population groups were identified in a neighbour-joining tree based on the genetic distance. Analyses of molecular variation (AMOVA) showed that the genetic variation mainly existed within population, representing56.67%和of the total variation; the remaining variation (43.33%) was from among populations. Phylogeographic analyses indicated that the populations of P. betulaefolia experienced a scenario of rapid range expansion, which probably occurred between608,000and204,580years ago. Meanwhile, both the restricted gene flow with isolation by distance and allopatric fragmentation were crucial processes responsible for shaping the genetic patterns of P. betulaefolia. The occurrence of specific haplotypes might be ascribed to an ancestral introgression or joint retention of an ancestral polymorphism with other Pyrus species at the northern edge of the distribution of P. betulaefolia.3. Sixteen microsatellite markers showed a high level of polymorphism. Three markers were excluded for further analyses because of significant linkage disequilibrium with other loci. The genetic diversity varied significantly among remaining13SSR loci and among populations. The number of different alleles for each locus ranged from3.4(CH01B12) to10.4(MES122), with an average of7.1. The mean number of effective alleles among loci was4.1, with a range of1.77-5.94. The overall mean values of expected and observed heterozygosity were0.702and0.687, respectively. Two gene pools and three regions were recognized according to the results given by Bayesian clustering and neighbor-joining tree. Results inferred from MIGRATE indicated asymmetrical gene flow among different population groups. Genetic composition of P. betulaefolia in east group changed evidently due to asymmetrical gene flow with other groups and/or introgression of cultivated species. Taihang Mountains were crucial geographical boundary in the formation of current genetic patterns of P. betulaefolia in northern China.4. Natural populations of P. betulaefolia and individuals within them are in danger or near extinct due to rapid environmental change and ongoing habitat fragmentation. This led to an urgent action to preserve the wild germplasm. Conservation priority should be given to populations with a high level of haplotype diversity and unique haplotypes such as SABT, GSZN, and HNQS. The individuals in those three populations could be used as elite materials in stock breeding of pear. For those populations that displayed diverse age spectrum, such as populations HNWG, HNLB, and HNLS, could be preserved in situ. In addition, more attention should be focused on the variant originated from seedlings when the conservation was performed by collecting seeds.