Molecular Phylogeny And Evolution Of The Genus Elymus L. In Tribe Triticeae

The wheat tribe Triticeae Damort., in the subfamily Pooideae, Poaceae, includes three of the most important cereal crops, i.e., wheat, barley, and rye, as well as many economically valuable forage grasses. The genus Elymus L. is the largest genus in the Triticeae Damort., following the circumscription of Love and Dewey, encompassing approximately 150 taxa, and occurring from the Arcic and temperate to subtropical regions. As an exclusively allopolyploid genus, Elymus has its origin in other groups, and thus it has close relationships with other genera in Triticeae. Cytological studies suggest that five basic genomes, namely, the St, Y, H, P, and W in various combinations constitute Elymus species, i.e. the StH and StY genomes in tetraploids, and the StStY, StHY, StPY, and StWY in hexaploids. The St genome is a fundamental genome that exists in all Elymus species and was donated by the genus Pseudoroegneria (Nevski) A.Love. The H, P, and W genomes are derived from the genera Hordeum L., Agropyron (L.) Gaertner, and Australopyrum (Tsvelev) A. Love of Triticeae, respectively. However, the donor of the Y genome that is present in the majority of the Asiatic Elymus species has not yet been identified, although extensive investigations have been carried out.Asia, particularly the central Asiatic mountain region, is an important center for the origin and diversity of Elymus L., which is reflected by the following facts: (1) more than half of the world number of Elymus species occurs in this area; (2) many different morphological forms, which are used as key characters in the taxonomy, are found in this region; (3) the different polyploid levels have been reported from Asia; (4) most genomic combinations known in this genus are found in Asia; (5) the "Y"genome, which is present in the majority of Asiatic Elymus species, is thought to have its origin in Central Asia or the Himalayan region. The StY-genome Elymus species, being one of the most important genomic combinations, are restricted to Asia with one exception of E. panormitanus (Parl.) Tzvelev, which is also found in South East part of Europe.Data from extensive morphological and cytogenetic analyses have been used to illustrate systematic relationships of the genus and to clarify the ancestry of polyploid species. Compared with StH-genome species, StY-genome species show unique systematic relationships. There had three distinct morphological groups. The cytogenetic analyses showed the StY-genome Elymus species from the same geographic regions, e.g. within eastern or western Asia, have a relatively closegenomic relationship, whereas those from different regions have a more distant relationship, and the St and Y genomes have close affinities. In this work, we used the sequences of nuclear genes (ITS, Adh2, Adh3 and Waxy) and cpDNA (trnL-F and matK) to explore the origin of Elymus species and the Y genome, and to reveal the mechanism of the geographical differentiation of the StY-genome Elymus species. The main results are summarized as follows:1. Based on multiple-gene sequence data, including one chloroplst trnL-F gene, one ITS fragments of nuclear rDNA, and three single copy nuclear gene Adh2, Adh3, and Waxy, the phylogenetic relationships of the four related genera diploids of Elymus were inferred. The phylogenic trees indicated that: (i) each genome group is monophyletic; (ii) H genome is basal; (iii) P genome shows a closer affinity to W genome; (iv) St genome has a close affinity to P-W genome clade.2. To explore the origin of Elymus species and the mechanism of the geographical differentiation of StY-genome Elymus species, we selected 54 taxa with St, H, P, StSt, StY, StH, StStY, StHY, StPY and StWY genome constitutions, including 45 Elymus taxa. In the present study, the ITS, trnL-F and matK were sequenced. The ITS sequences revealed a distinct phylogenetic relationship of the polyploid Elymus and their diploid donor genera, although the St and Y genomes did not show any separation. The ITS tree indicated a certain degree of geographic differentiation of the StY-genome species. The trnL-F and matK sequences revealed an especially close relationship of Pseudoroegneria to all Elymus species included. All these trees indicated multiple origins and recurrent hybridization of Elymus species.3. In the ITS sequence analysis, the phylogenetic tree indicated the correlation between geographic distribution and species relationships exists, and the Y and St genomes may have the same origin. Considered the weak support in the ITS tree, we selected single copy genes (Adh2, Adh3, and Waxy) to give further research about these scientific questions. The three single copy genes had higher phylogenetic information sites and gave good support for these two questions. In phylogenetic trees, there had distinct genomic clades clustered the diploid genomic species and the polyploids with one of genomes similar to the corresponding diploid genome. In other words, the StH- and StHY-genome polyploid species were grouped with St- and H-genome diploids, respectively; the StPY-genome polyploid species clustered with St- and P-genome diploids, respectively; the StY-genomepolyploid species grouped with St- and StSt- genome species. Still, there had no Y genome clade. Based on the ample data about these three genes, combined with the ITS sequences, and cytological data and C values, we thought the Y genome may be the differentiated St genome after St genome polyploidization. So, the StY-genome allotetraploids may be the StSt-genome autotetraploids or segmental autotetraploids.The phylogenetic trees showed the species distributed in western Asia all grouped with the central Asiatic species and formed the basal clade in the Adh gene trees, and some central Asiatic species grouped with eastern Asiatic species. These indicated the genetic diversity is the highest in the central Asiatic species, and the Central Asia may be the original center.In summary, this study suggests that: (i) Elymus L. has its origin through a typical intergeneric/interspecific hybridization, and the diploids of Pseudoroegneria (Nevski) A.Love, Hordeum L., Agropyron (L.) Gaertner, and Australopyrum (Tsvelev) A. Love of Triticeae are its ancestors; (ii) Pseudoroegneria (Nevski) A.Love was the maternal donor of the polyploid Elymus; (iii) the "Y" genome existed in the StY-genome Elymus may be differentiated from the St genome, i.e., the autopolyploid StSt-genome species probably experienced genomic differentiation within a species after autopolyploidy, leading to the significant genomic change and resulting the StY-genome allotetraploids; (iv) the StY genome species distributed in the Central Asia with a high level of genetic diversity radiated to the Western and Eastern Asia, under the environmental selection and other factors, and only a few species spread to the western Asia and most species have remained in the Central and eastern Asia.