Genetic Diversity, Gene Introgression And Homoplasy In Natural Sympatric Populations Of The Genus Actinidia

Gene introgression is usually resulted from spontaneous hybridization occurring among closely related species in sympatric populations and thus has great impact on genetic structure of natural populations. This phenomena has recently become one of the most attracted subjects in study of evolutionary and conservation biology. In the first study, eight chloroplast microsatellite (cpSSR) primers were used to examine genetic diversity, population genetic structure and hybridization introgression among seven Actinidia species with overlapped distributions in transitional regions ranging from western China plateau to middle-eastern China hilly mountains. The results showed: (1) A total of 42 haplotypes designated by 18 alleles at the six polymorphic loci were detected across 655 individuals of ten sympatric populations sampled. Although a reticulate evolutionary relationship was revealed among cpSSR haplotypes, there existed a strong species-specific tendency; (2) The intra-specific genetic diversity is generally high but varied greatly among different species. A. fulvicoma var. lanata showed the highest genetic diversity (PPL=62.50%, h_T = 0.173, H_T = 0.897) whereas A. deliciosa exhibited the lowest (PPL=37.5%, h_T = 0.041, H_T = 0.516); (3) High level of inter-population genetic differentiation (G_ST = 0.319-0.780, F_ST = 0.40-0.695) but limited gene flow (N_m = 0.219-0.747<1) were detected among the different species, of which A. deliciosa showed the highest inter-population genetic differentiation (G_ST =0.780, F_St =695); (4) No significant difference was found between Gst (unordered alleles) and Nst (ordered alleles), suggesting the lack of phylogeographical structure for most Actinidia species. This is consistent with the Mantel test which revealed no significant relationship between genetic distance and geographical distance; (5) Interestingly, a relatively high level of inter-specific genetic differentiation was detected among species in sympatric populations (F_St = 0.476-0.990), except for two populations (HW and GZ) of A. chinensis/A. deliciosa complex. The result was also well in accordance with UPGMA analysis which revealed that the populations tended to cluster together into the same species. Further analysis indicated that A. chinensis/A. deliciosa complex was probably highly influenced by both shared ancestral chloroplast polymorphism and recent hybridization introgression. However, hybridization introgression rarely occurred between less related species, suggesting homoplasy events may have also played an important role in shaping phylogenetic relationships in the genus. In the second study, the ploidy analysis, genetic diversity and hybridization introgression of A. chinensis and A. deliciosa, were investigated in one sympatric distributed complex populations and two single species allopatric populations using nuclei and chloroplast SSR markers. Previous investigation showed that A.delicious was always hexaploid, however, the individuals we sampled in Hunan Huangsang were either diploid or tetraploid. Little variance was detected between A.chinese and A.delicious, which indicated that the two species were closely related. The results have important implications for understanding the population genetics and evolutionary patterns in the genus Actinidia and provide baseline data for formulating the conservation and management strategies.