Advances in the research on phylogeny of the genus Pyrus and the origin of pear cultivars native to East Asia

Abstract: Pear (Pyrus L.) is one of the most important temperate fruit trees in the world. The genus Pyrusis believed to have originated from the mountainous areas in West and Southwest China during Tertiary period. Traditionally Pyrus belongs to the subfamily Maloideae of the family Rosaceae which is divided intofour subfamilies on the basis of fruit types: Spiraeoideae (follicles or capsules), Rosoideae (achenes), Prunoideae (drupes) and Maloideae (pomes). Recent molecular phylogenic study of Rosaceae based on multi⁃ple DNA sequences divided Rosaceae into three subfamilies Dryadoideae, Rosoideae and Amygdaloideae(including the former Prunoideae, Spiraeoideae and Maloideae). Therefore, under new taxonomic systemof Rosaceae, Pyrus belongs to the subtribe Malinae of the tribe Maleae in the subfamily Amygdaloideae.The genus Pyrus species are widely spread through the Euro-Asian continent including North Africa andabout 20 primary species have been recognized. The Pryus species are geographically divided into two na⁃tive groups: occidental pears and oriental pears. Studies on genetic relationship of pear species and cultivars based on DNA markers and phylogenetics of Pyrus based on multiple DNA sequences supported thepoint of view of independent evolution of occidental pears and oriental pears. Phylogenetic trees inferredfrom nuclear LFY2int2-N (LN) data showed that reticulation caused by hybridization is one of the majorevolutionary processes for Pyrus species. Polytomies of the gene trees and star-like structure of cpDNAnetworks suggested that rapid radiation is another major evolutionary process, especially for the occidentalspecies. P. mamorensis, P. gharbiana, P. cossonii, P. regelii and P. betulaefolia were shown to be mono⁃phyletic in the LN tree. P. calleryana once treated as descendent of primitive stock Pyrus was found to beputative hybrid involving P. betulaefolia and P. pashia. Studies of genetic relationship of pears carried outby different research groups in the world who used different DNA markers and pear accessions revealedthat local cultivars of Chinese white pears, Chinese sand pears and Japanese pears might originate fromthe same ancestor - wild Pyrus pyrifolia and gene introgressions from local species could happen duringthe development of different P. pyrifolia cultivar groups. Based on the new research results and International Code of Nomenclature for Cultivated Plants, the new nomenclature for Chinese white pear has beensuggested: Pyrus pyrifolia White Pear Group. The newest researches proved that Ussurian pear cultivarsare of hybrid origin involved in wild P. ussuriensis and Chinese white pear cultivars or Chinese sand pearcultivars. Much more samples collected from wild pear species and new research strategies will be neededfor the reconstruction of Pyrus phylogeny and illumination of the origin of main pear cultivar group.