The Evolution Patterns Of GBSS â…  Gene And Phylogenetic Study In Leymus Hochst.

For the variable rates of exon and intron, nuclear gene in eukaryotes can be utilized for phylogenetic reconstruction of various taxonomic levels. When cpDNA and ITS sequence can not be used to resolve for maternal inheritance and paralogy, single-and low-copy genes are the most potential tools for reconstructing phylogeny. Because it is less likely subject to concerted evolution, low-copy nuclear gene granule-bound starch synthase gene (GBSSI) is more reliable for phylogenetic study for its considerable information than multi-copy nuclear genes such as ITS and cpDNA. Moreover, the coding sequence and non-coding sequence perform different evolutionary speed, which provides diverse traits for phylogeny of variable taxonomic levels.As a allopolyploid genus in the tribe Triticeae (Poaceae), Leymus Hochst. is a perennial genus of grass with a diverse array of morphology, ecology and distribution. The chromosome numbers in Leymus species are 2n=28-84. The genomic constitution of Leymus was assigned as NsXm, where Ns was presumed to be originated from Psathyrostachys Nevski, while Xm represented a genome of unknown origin. In this study, GBSSI sequences originate from 23 Leymus species and 28 diploid taxa were used to reconstruct phylogeny and analyse evolutionary history. In addition, molecular evolution of GBSSI in Leymus was also discussed. The main results were shown as follows:1. In the GBSSI trees, Two homoeologues of the GBSSI gene were isolated from the most of sampled Leymus species. Nearly all the Ns-type of Leymus were clustered with Psathyrostachys lanugiosa,Psa. huashanica and Psa. frigilis. Psa. juncea was located in the base, which indicated that Psa. juncea was differentiated from Leymus early. Psa. juncea may serve as a donor for the Leymus species from the Qinghai-Tibetan Plateau and Xinjiang.2. All the Leymus species from the Qinghai-Tibetan Plateau and North America are in the same clade with Psa. lanugiosa and Psa. huashanica, which suggested these species were close related. Perhaps Psa. lanugiosa and Psa. huashanica were involved in the formation of these Leymus species. Besides, the origination of Leymus from Xinjiang and Middle East might refer to Psa. huashanica and Psa. frigilis.3. All the Xm-type of Leymus species except for L. salinus were clustered independently, which indicated there was an unknown genome involving in the Xm genome formation. Futhermore, one copy of L. racemosus,L. alaicus were claded with Pseudoroegneria spicata, suggesting the St genome from Pseudoroegneria and Xm from L. racemosus,L. alaicus shared the same origination.4. In the branchs of Ns,17 Leymus species exhibited short branch lengths and little sequence variation. After mismatch distribution test, it shows unimodal. According to the test of nucleotide polymorphisms,πandθ(?) is small. It may be inferred that adaptive radiation had occurred in the group.5. By testing the nucleotide polymorphisms, Xm branch exhibiting higher value than Ns branch. It was inferred that Xm genome beared more selection pressure, and ployploid might suffer from a genetic bottleneck.