Studies On Genetic Diversity And Structure Of Pyrus Calleryana And P.Pashia In China

Genetic diversity is a critical factor for the long-term survival of a species or a population. To wild fruit tree species, understanding the genetic diversity is the primary task of rootstock selection. Pyrus calleryana Dcne and P. pashia D. Don are wild pear species which originate in China and widely used as rootstocks in Southern China. P. calleryana is one of the most ancestral pear species while P. pashia is thought to be the linkage between Oriental and Occidental Pear. Therefore, these two species also play important roles in the evolution of Pyrus. However, lack of sufficient research on P. calleryana and P. pashia limits the utilities of these two important pear species. In this study, a combined approach using cpDNA haplotypes and nSSRs was taken to study the genetic diversity and population structure of P. calleryana and P. pashia. Considering that P. calleryana and P. pashia are morphologically similar to each other, the phenotypic variation of them was also studied using six fruit phenotypic characters. The main results are listed as follows:1. Two cpDNA intergenic regions (accD-psaI and trnL-trnF) and14nSSR makers were used to study77individuals from8populations. A high level of genetic diversity was detected in P. calleryana (cpDNA:Hd=0.719,π=0.00105, nSSR: He=0.639). AMOVA analyses revealed that the majority of the genetic diversity occurred within population. Mantel tests showed that the genetic distance was significantly correlated with the geographic distance (cpDNA haplotype:r=0.332, p=0.048; nSSRs:r=0.592,p=0.008). STRUCTURE analysis exhibited that two clusters existed among the8populations. All the populations could be divided into two groups, correlating with geographic regions (northeast and southwest). Geographic distance was a key factor in shaping current genetic structure of P. calleryana. Pollen/seed flow (0.86) indicated that seed dispersal among populations was more frequent and pollen dispersal was limited to some extent. Population FY was the most diverse population and suitable population to be protected under in situ conservation strategy. 2. In this study, we investigated the genetic diversity and population structure of P. pashia using accD-psaI and trnL-trnF intergenic spacers and13nSSR makers for a total of327individuals over22populations. P. pashia contained a relatively high level of genetic diversiy (cpDNA:Hd=0.718, π=0.00085; nSSR:He=0.741). AMOVA analyses of molecular variance indicated that the genetic variation mainly existed within populations, representing59.61%and91.24%of the total variation on the cpDNA and nuclear DNA level, respectively. Mantel test of cpDNA did not show a correlation between the genetic and geographic distances (r=0.139, p=0.09) while the nSSR-based Mantel test showed that geographic distance was an important factor to shape the current population genetic structure (r=0.783,p=0.000). According to the cpDNA haplotype results, genetic differentiation among populations was high (Gst=0.404). However, nSSR only revealed moderate genetic differentiation (Fst=0.087). Nst (0.420) was significantly higher than Gst (0.402)(p<0.05), which was reflected on the phylogeographic structure in P. pashia. NCA analyses of clade1-2, clade2-1, and the total cladogram inferred contiguous range expansion events in P. pashia. The overall population expansion of P. pashia was estimated to occur between621,000and209,000years ago. NCA analyses of clade2-2showed that restricted gene flow existed among populations. STRUCTURE analysis showed that four clusters formed the current genetic structure of P. pashia. According to the Ennos’ formula, pollen flow was6.05times more than seed flow. Populations YN1(cpDNA) and YN8(nSSR) represented the highest genetic diversity of P. pashia and should have the priority when conservation strategy is implemented.3. All the cpDNA haplotypes of P. calleryana and P. pashia were used to construct Neighbor-Net network. Two haplotpyes were shared by P. calleryana and P. pashia (haplotype S1and S2), which were the most ancestral haplotypes of P. calleryana and P. pashia, respectively. Shared haplotypes and non-monophyly of these two species on the Neighbor-Net network indicated that they had a recent ancestor.4. Six phenotypic variations were analyzed in three P. calleryana populations and four P. pashia populations. Results showed that single fruit weight was the most diverse phenotypic character (P. calleryana:CV=26.37%; P. pashia:CV=37.06%). Four clusters were detected from UPGMA clusters, and all the samples of P. calleryana were grouped into one cluster. PCoA analysis also distinguished P. calleryana from P. pashia.