Phylogenetic Analysis Of Leymus (Poaceae: Triticeae) Inferred From Mitochondrial CoⅫ Sequence

Leymus Hochst. is a perennial genus of grass in the tribe Triticeae(Poaceae). There are about 32 species and 19 subspecies in Leymus,and all the species are polyploids with the chromosome numbers ranging from 2n=4x=28 to 2n=12x=84. Many species of Leymus are not only main components of grasslands and grass marshlands, but also fine varieties of herbage and forge. Some species bear desirable traits, such as disease and insect resistance, bigger spikes, more and bigger grains and efficient photosynthesis for the improvement of forage and cereal crops. However, the origin and definition of the genus, precise taxonomic ranks and relationships among the species in the genus have been under discussion.The sequences of the mitochondrial Coâ…«region were analyzed for 23 Leymus species, together with three Pasthyrostachys species (Ns genome), three Pseudoroegneria (St genome) species, three Agropyron (P genome) species Lophopyrum elongatum (Ee genome), Eremopyrum triticeum (F genome) and Thinopyrum bessarabicum (Eb genome). The main results were as follows:1. the phylogenetic relationships among species within NJ, MP and ML were considered unresolved due to a high number of paraphyletic grade, low statistic support and zero-length branches. One explanation is that tree-based methods make the phylogenetic relationships among the species of Leymus ambigous due to the low-rate variation of plastid DNA. Ulike tree-based methods, network appoachs can reflect genealogical relationships of the mitochondrial Coâ…«haplotypes.2. In the MP, ML, NJ tree and MJ network inferred from Coâ…«sequences, most of the Leymus species and psathyrostaechys species were in one clade, indicating that Leymus species might derive their Ns genome from Psathyrostachys species. Different Leymus species was clustered with different Psathyrostachys species, indicating that different Psathyrostachys species could serve as the donor for different Leymus species.3. The network analysis showed that Leymus is closely to Pseudoroegneria, Agropyron and Eremopyrum. This result support the suggestion that the Xm genome in Leymus maybe originated from an ancestral lineage of Agropyron and Eremopyrum triticeum.4. Data from the Coâ…«sequence suggested that L. duthiei and L. komarovii were closely related to the species of Leymus. Network patterens indicated that Leymus species from the same areas or neighboring geographic regions are closely related to each other, and supported the suggestion that North American Leymus species might originate from recent colonization via the Bering land bridge.5. Psathyrostachys might serve as the maternal donor during the speciation of L. shanxiensis, L. secalinus, L. ramosus, L. ovatus, L. flexus, L. pendulus and L. qinghaicus. The progenitor of the Xm genome might serve as the maternal donor of L. angustus, L. leptostachys, L. yiwuensi, L. duthiei, L. pseudoracemosus, L. chinensis, L. alaicus and L. racemosus.