Study On The Hybrid Origin Of Malus Toringoides(Rehd.) Hughes And Its Closely Related Species

Malus toringoides (Rehd.) Hughes belongs to Series kansuenses Rehd. of the genus Malus Miller of Rosaceae family. It was first collected by Wilson in western Sichuan province of China in1904, and then was described as a variety of M. transitoria by Rehder in1915. In1920, it was renamed as Malus toringoides (Rehd.) Hughes by English botanist named Hughes. M. toringoides is a facultative apomictic species, and shows complex diversities in morphological characteristics, germplasm characters and ecological environments. Used as rootstock, M. toringoides showed high resistance to various environmental stresses (like 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 semi-dwarfing, improvement of fruit yield and quality. Therefore, M. toringoides was regarded as a very important germplasm resource of genus Malus. In the present studies, M. transitoria and M. kansuensis were considered as two parents of M. toringoides. However, the taxonomy of the closely related species of M. toringoides, such as M. maerkangensis, M. setok and M. xiaojinensis, were still in disagreement. In this study, M. toringoides and its closely related species like M. transitoria, M. kansuensis, M. maerkangensis, M. setok and M. xiaojinensis were collected as plant materials. The hybrid origin of M. toringoides and its closely related species were investigated based on sequences variation of3chloroplast DNA fragments(trnV(UAC)-ndhC, atpI-atpH and ndhA intron) and nuclear genes (nrDNA ITS region and single-copy nuclear gene SbeY). The main results are summarized as follows:(1) Pyrus pyrifolia was used as outgroup, and the phylogenetic analysis of M. toringoides and its closely related species along with some other plant species of genus Malus was carried out based on cpDNA sequences. Results showed that a monophyletic group was formed by sequences of M. toringoides, M. maerkangensis, M. setok, M. xiaojinensis, M. sikkimensis, M. spectabilis and M. rockii, while the sequences of M. transitoria and M. kansuensis formed a single group, respectively. It was indicated that the maternal origin of M. toringoides was closely related to M. sikkimensis, M. spectabilis and M. rockii, and M. transitoria was not likely to be the maternal origin of M. toringoides.(2) M. komarovii was used as outgroup, and the phylogenetic analysis of M. toringoides and its closely related species was carried out based on nrDNA ITS sequences. Meanwhile, The Sbel sequence of peach was used as outgroup, the phylogenetic analysis of M. toringoides and its closely related species along with some other plant species of genus Malus was also carried out based on single-copy nuclear gene Sbel sequences. Results showed that:in the phylogenetic tree based on nrDNA ITS sequences, M. toringoides, M. setok and M. transitoria were clustered in Clade A, M. xiaojinensis and M. kansuensis were clustered in Clade B, while M. maerkangensis was clustered in Clade A an Clade B, which indicated that M. toringoides and M. setok were closely related to M. transitoria, and M. xiaojinensis was closely related to M. kansuensis. In addtion, the hybrid origin of M. maerkangensis was corroborated, and M. kansuensis could be one parent of M. maerkangensis, and the other parent could be M. toringoides, M. setok or M. transitoria. However, none of the ITS copy type of M. xiaojinensis were clustered in Clade A, which couldn’t explain the evolutionary relationship between M. xiaojinensis and M. toringoides or M. setok. In the phylogenetic tree based on SbeI sequences, M. toringoides, M. maerkangensis, M. setok and M. xiaojinensis were grouped in more than one clade indicating the hybrid origin of the four species descridbed above. M. toringoides and M. setok were clustered in Clade Ⅰ with M. sieboldii, and clustered in Clade Ⅱ with M. transitoria, revealing that M. transitoria was likely to be one parent of M. toringoides (or M. setok) which was consistent with the results inferred from ITS sequences, and the other parent was closely related to M. sieboldii. M. maerkangensis and M. xiaojinensis were clustered in3groups (Clade Ⅰ, Clade Ⅱ and Clade Ⅲ for type S1, S2and S3SbeI sequences, respectively). As M. kansuensis had type S3SbeI sequences, M. toringoides and M. setok had type S1and S2SbeI sequences, indicating there was a chance that M. kansuensis was one parent of M. maerkangensis and M. xiaojinensis, which was consistent with the results inferred from ITS sequences, and M. toringoides (or M. setok) was probable the other parent of M. maerkangensis and M. xiaojinensis, this was at odds with the results inferred from ITS sequences.The results of our study provide new information for the hybrid origin of M. toringoides and its closely related species, and have theoretical and practical implication for a further development and utilizaon of M. toringoides and its closely related species. However, the application of SbeI and ITS gene sequences in the analysis of the origin of M. toringoides and its closely related species still need further discussion.