Biosystematics and evolutionary relationships of perennial Triticeae species revealed by genomic analyses

Understanding the classification and biosystematics of species in Triticeae Dumort., an economically important tribe in the grass family (Poaceae), is not an easy task, particularly for some perennial species. Does genomic analysis facilitate the understanding of evolutionary relationships of these Triticeae species? We reviewed literature published after 1984 to address questions concerning: (1) genome relationships among the monogenomic diploid species; (2) progenitors of the unknown Y genome in Elymus polyploids, X genome in Thinopyrum intermedium, and Xm genome in Leymus; and (3) genome constitutions of some perennial Triticeae species that were unknown or misidentified. Amajority of publications have substantiated the close affinity of the E-b and E-e genomes in Th. bessarabicum and Th. elongatum, supporting the use of a common basic genome symbol. The E genome is close to the St genome of Pseudoroegneria and ABD genomes of Triticum/Aegilops complex, providing an explanation for transferring genes from the E to ABD genomes with relative ease. Although the solid proof is still lacking, theW, P, and especially Xp genomes are possible origins for the Y genome of polyploid Elymus. The absence of the E genome and the allopolyploidy nature of tetraploid Leymus species have been unequivocally confirmed by both cytogenetic and molecular studies. However, the donor of the Xm genomes of Leymus was only speculated to be related to the P genome of Agropyron and F genome of Eremopyrum. Intermediate wheatgrass (Th. intermedium) has been extensively studied. The presence of the St (as the previously designated X) genome in Th. intermedium is now unequivocal. Its two more closely related E-1 and E-2 genomes are shown to be older versions of the E genome rather than the current E-b and E-e genomes. Speciation of Th. intermedium was similar to that of Triticum aestivum, in which the J(s)/E-s (like B) genomes had the greatest differentiation from the current J (E-b) genome owning to repetitive sequences of the V genome, whereas its St (like D) had the least differentiation from the current St genome. Species with unknown or misidentified genomes have been correctly designated, including those with the ESt, StP, StPY, StWY, EStP, HW, StYHW, and NsXm genomes. Some of those species have been transferred to and renamed in appropriate genera.