A Study Of Molecular Phylogenetics Between Malus Domestica Borkh. And Its Closely-related Wild Relatives

The cultivated apple under the scientific name of Malus domestica Borkh.includes all the cultivated types in the genus M.Miller which belongs to the subfamily Maloideae of Rosaceae.The genus M.Miller was established by Philip Miller in 1768,and since thena series of study from morphology to molecular markers on the origin and evolution of the M.domestica Borkh.have been carried out both in China and abroad in the past more 250 years.Because of long cultivation history and sexual compatibility between M. domestica and its closely related species,the questions concerning the wild ancestor of the cultivated apple, the time and place(s) of its origin have remain unresolved.In early studies several M.species,such as M. sieversii(Ledeb.) Roem.,M.sylvestris Miller,M.orientalis Uglitzk were suggested to have been involved in the origin of the cultivated apple.In current literature,there are also two points of view about how the cultivated apple was originated.One suggested that the cultivated apple was directly domesticated from M. sieversii,while the other contended that the cultivated apple was of hybrid origin between M.sieversii and its close relatives in the genus M.Miller.In respect to the question concerning where the cultivated apple was domesticated,Central Asia,south Asia,West Asia,the Caucasus,Pakistan,Western China and Yili prefecture of Xinjing Automous Region were suggested to be the possible origin place(s).Apple is one of the most important fruit crops cultivated in the temperate region of the world.A better understanding of the phylogenetic relationship between M.domestica Borkh.and its closely related species is of great theoretical and practical meanings for future variety breeding,germplasm investigation,collection, conservation and utilization of apple.In the present study,the SbeI gene fragments and two chloroplast DNA(cpDNA) regions(TrnS-G, matK) of the cultivated apple(M.domestica Borkh.cv.'Golden Delicious') and all its putative ancestors were amplified,cloned and sequenced.Using M.toringoides(Rehd.) Hughes as outgroup,we constructed the maximum parsimony(MP) tree of the materials analyzed with PAUP~* computer software.Both our cpDNA and SheI gene trees indicated that M.sieversii and M.sieversii subsp,kirghisorum(AI.) Ponom. were the species most closely related to M.domestica Borkh.Our results also showed that M.orientalis Uglitzk is more closely related to M.domestica Borkh than M.sylvestris Miller.Our data supported that Yili area of Xinjiang Autonomous Region was the diversity center of M.sieversii and the origin place of the cultivated apple.In this study we reconstructed for the first time the molecular phylogenetic relationship between M.domestica Borkh and its candidate wild ancestors based on a combined analysis of both nuclear and chloroplast genome sequences.Our data provided new evidence for the origin and evolution of the cultivated apple.