| Malus toringoides (Rehd.) Hughes was collected by Wilson in western Sichuan Province of China, in 1904, and Render (1915) described it as a variety of M. transitoria. It was later renamed as M. toringoides (Rehd.) Hughes by Eglish botanist Hughes in 1920. M. toringoides belongs to Series kansuenses Rehd. of the genus Malus Miller of Rosaceae family incurrent taxonomy. M. toringoides is a facultative apomictic species. However, it shows high diversities in morphology, germplasm characteristics, and ecological habitats. Used as rootstock, M. toringoides showed high resistance to various environmental stresses (such as tolerances to salt, drought, heat, cold, water-logging, and resistance to valsa mail Miyabe et Yamada and Xylaria mali Fromme), and had important effects on the grafted trees including dwarfing, early-fruiting, improvement of production and fruit quality. Therefore, M. toringoides was considered as a very important apple germplasm resource in the genus Malus. A better understanding of the origin and evolution of M. toringoides and its closely-related species has important implications for future germplasm conservation and utilization.The morphological variation of M. toringoides ranges from M. transitoria Schneid to M. kansuensis Rehd. The patterns of the morphological variations of M. toringoides were similar to the results of introgressive hybridization. Field investigation revealed that scatterly distributed M. kansuensis Rehd and (or) M. transitoria Schneid were found in the population of M. toringoides. The three species overlapped in the area of weastern Sichun Province and the Southern part of Gansu Province, China, and then showed geogeraphic affinity, and the distribution pattern of the three species showed the characteristics of geographic vicariation (vertical vicariation) and they are, therefore, called vicarious species. The AFLP (amplified fragment length polymorphism) molecular marker analysis showed that there were three types of bands existed in M. toringoides, M. transitoria Schneid and M. kansuensis Rehd.: 1) the bands shared by M. transitoria and M. kansuensis, 2) the bands shared by M. toringoides and M. transitoria or M. Kansuensis, and 3) the bands only presented in M. toringoides. These results suggested that the AFLP bands of M. toringoides were the results of partial additionof the bands of M. transitoria and M. kansuensis and its bands were not derived from their common ancestor. Up till now, however, there still lacks of convincing evidence for the origin of M. toringoides in current literature, and this hinder the utilization of the M. toringoides germplasm. In the present study, the sequence variation of the internal transcribed spacer (ITS) of nuclear ribosomal DNA (nrDNA) was used to re-examine the origin of M. toringoides. A total of 69 accessions from three natural populations (Marerkang, Xiaba and Kehe, Aba Autonomous Region, Sichuan, China) of M. toringoides and 10 accessions of its putative parents were analyzed. Using Malus angustifolia Michx., Malus ioensis Britt. and Malus doumeri Chev. as outgroups, our phylogenetic analysis of the ITS sequences of M toringoides and its putative parents showed that M. toringoides was not monophyletic, and two different types of ITS sequences which were obtained from each of the six accessions of M toringoides were found to have clustered separately with those of the two putative parent species on the gene tree. A comparison of the sequence variation between M. toringoides and its putative parents revealed an additive variation pattern of ITS sequences in the putative hybrid species. These results are consistent with the previous morphological and amplified fragment length polymorphism (AFLP) data which suggested that M. toringoides was of hybrid origin. Our ITS data provide new molecular evidence for the hybrid origin hypothesis of M. toringoides and these results are of great importance for future study on hybridization, polyploid speciation and evolution of the genus Malus Miller. |
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