Advances in phylogeny of the genus Pyrus and genetic relationships of Asian pear cultivars

Pyrus species are widespread throughout the Eurasian continent. More than 35 species are recognized, of which approximately 20 are treated as primary species. Pyrus species are geographically divided into two native groups: occidental and oriental pears. However, elucidation of Pyrus phylogenetic relationships has proven challenging because of the lack of distinguishing characters among species and frequent hybridization in the genus. Recent studies on Pyrus phylogeny, based on multiple DNA sequences, supported the hypothesis that occidental and oriental pears are independently evolving. Phylogenetic trees inferred from nuclear LFY2int2-N (LN) data showed that reticulate evolution caused by hybridization is a major evolutionary strategy of Pyrus species. Four occidental species, P. mamorensis, P. gharbiana, P. cossonii, and P. regelii, and one oriental species, P. betulifolia, were shown to be monophyletic in the LN tree. Pyrus calleryana, which was previously considered as a descendent of primitive Pyrus stock, was found to be polyphyletic. Further study using retrotransposon-based SSAP markers revealed that P. calleryana was a putative hybrid between P. betulifolia and P. pashia. Moreover, pear is one of the most widely planted fruit trees in the world. Pyrus communis is widely cultivated throughout much of the world, but is not extensively cultivated in East Asia. In East Asia, different cultivated species or types have been recognized. However, their origin is still controversial. Using different DNA markers and pear accessions, various research groups revealed that Chinese white pear (CWP) and Chinese sand pear (CSP) cultivars are closely related, although they are traditionally assigned to different species. Alternatively, Japanese pear (JP) and CSP cultivars belong to the same species, P. pyrifolia, but they do not typically cluster together in dendrograms. Further genetic structure analysis of Asian pear cultivars showed that CWP, CSP, and JP did not significantly differ from each other. Researches also showed that Ussurian pear cultivars resulted from hybridization between wild P. ussuriensis and cultivated P. pyrifolia. Additional wild Pyrus samples and research strategies are needed to further elucidate Pyrus phylogeny and the origin of Asian pear cultivar groups.