On the genome constitution and evolution of intermediate wheatgrass (Thinopyrum intermedium: Poaceae, Triticeae)

Abstract

BackgroundThe wheat tribe Triticeae (Poaceae) is a diverse group of grasses representing a textbook example of reticulate evolution. Apart from globally important grain crops, there are also wild grasses which are of great practical value. Allohexaploid intermediate wheatgrass, Thinopyrum intermedium (2n = 6x = 42), possesses many desirable agronomic traits that make it an invaluable source of genetic material useful in wheat improvement. Although the identification of its genomic components has been the object of considerable investigation, the complete genomic constitution and its potential variability are still being unravelled. To identify the genomic constitution of this allohexaploid, four accessions of intermediate wheatgrass from its native area were analysed by sequencing of chloroplast trnL-F and partial nuclear GBSSI, and genomic in situ hybridization.ResultsThe results confirmed the allopolyploid origin of Thinopyrum intermedium and revealed new aspects in its genomic composition. Genomic heterogeneity suggests a more complex origin of the species than would be expected if it originated through allohexaploidy alone. While Pseudoroegneria is the most probable maternal parent of the accessions analysed, nuclear GBSSI sequences suggested the contribution of distinct lineages corresponding to the following present-day genera: Pseudoroegneria, Dasypyrum, Taeniatherum, Aegilops and Thinopyrum. Two subgenomes of the hexaploid have most probably been contributed by Pseudoroegneria and Dasypyrum, but the identity of the third subgenome remains unresolved satisfactorily. Possibly it is of hybridogenous origin, with contributions from Thinopyrum and Aegilops. Surprising diversity of GBSSI copies corresponding to a Dasypyrum-like progenitor indicates either multiple contributions from different sources close to Dasypyrum and maintenance of divergent copies or the presence of divergent paralogs, or a combination of both. Taeniatherum-like GBSSI copies are most probably pseudogenic, and the mode of their acquisition by Th. intermedium remains unclear.ConclusionsHybridization has played a key role in the evolution of the Triticeae. Transfer of genetic material via extensive interspecific hybridization and/or introgression could have enriched the species' gene pools significantly. We have shown that the genomic heterogeneity of intermediate wheatgrass is higher than has been previously assumed, which is of particular concern to wheat breeders, who frequently use it as a source of desirable traits in wheat improvement.

Sequence variation at the three chloroplast loci (matK, rbcL, trnH-psbA) in the Triticeae tribe (Poaceae): comments on the relationships and utility in DNA barcoding of selected species

“…The previous cpDNA studies on the Triticeae (e.g. Adderley and Sun 2014;Mahelka et al 2011;Mason-Gamer 2013;Redinbaugh et al 2000;Yan and Sun 2012) have shown that during the allopolyploid formation the chloroplast genomes were inherited from the parent carrying the St (nuclear) genome. Hence, the allopolyploid St-containing genera, delimited on the basis of genomic composition, probably share the same (or very closely related) chloroplast genomes that originate from Pseudoroegneria.…”

“…Such species-specific mutations were already reported for the chloroplast genome. Mahelka et al (2011) separated L. elongatum and T. bessarabicum from P. strigosa on the basis of the trnL-trnF sequences. MasonGamer (2013) found chloroplast mutations that separated E. caninus and E. repens from L. elongatum, T. bessarabicum and P. strigosa.…”

Reticulate Evolution of the Rye Genome

“…Reticulate evolution or hybrid speciation was postulated to have occurred frequently during plant evolution (Kellogg and Bennetzen, 2004;Linder and Rieseberg, 2004;Mallet, 2005). In the Triticeae, it may have occurred in diploid species (Kellogg et al, 1996;Escobar et al, 2011), but it has been most frequently postulated for allopolyploid Triticeae genera (Kellogg et al, 1996;Mason-Gamer, 2004;Mason-Gamer et al, 2010;Mahelka et al, 2011). Reticulate or hybrid speciation can occur (reviewed in Linder and Rieseberg, 2004) as a consequence of allopolyploidization, which involves fusion of unreduced gametes, or instant genome duplication after fusion of haploid gametes, giving rise to a fertile hybrid species in which diploid parental genomes are maintained.…”

VI. Subfamily Pooideae Benth. (1861)

“…It is clearly one progenitor of most of the polyploids in Elymus s.l. , 2010a, 2010b and Thinopyrum (Liu and Wang 1993;Mahelka et al 2011). Not surprisingly, because it has so many polyploid derivatives, the exact delimitation of Pseudoroegneria and its descendants has been controversial, with some species placed in Elytrigia and others in Elymus.…”