Genetic variation and natural hybridization among sympatric Actinidia species and the implications for introgression breeding of kiwifruit

The overlapping ranges of closely related species provide a natural setting for the investigation of reticulate hybridization and other evolutionary processes. In the present study, we examined the pattern of genetic variation and interspecific gene flow in seven Actinidia species across ten localities in which sympatry among at least two species occurs. Our results showed that 48.7% of the alleles across the nine nuclear microsatellite loci examined were shared among the seven Actinidia species. Moreover, at the species level, Actinidia chinensis and Actinidia deliciosa exhibited the highest genetic similarity, with a large percentage of shared alleles (P (s) = 81.3%) and a significant consistency between the distribution frequency of their allele sizes (r = 0.859, P = 0.045). Yet, the genetic distinctions between species are obvious except for the species pair A. chinensis and A. deliciosa. Interspecific introgression was detected among the two main species pairs (Actinidia latifolia-Actinidia eriantha and A. chinensis-A. deliciosa), but more apparent in the latter in which 30% of the A. chinensis individuals and 49% of the A. deliciosa individuals showed genetic admixture in the STRUCTURE analysis. Possibly active hybrid zones relating to the two main species pairs were discussed at last, which are expected to pave the way for the introgression breeding of kiwifruit from natural sympatric populations.