Plasmon diversity in Triticum and Aegilops and its implication in wheat evolution

Recent results on the plastome, chondriome, and plasmon diversity among Triticum (wheat) and Aegilops species have been reviewed. As to the plastome diversity, 17 different chloroplast DNA types were found among 32 species (42 accessions in total) by restriction endonuclease analysis of chloroplast DNA. Referring to the physical map of common wheat chloroplast DNA, 46 plastome mutations have been identified, and based on the number of the mutations detected between every pair of the plastomes, their phylogenetic tree has been constructed. Complete base sequence of five rbcL genes encoding rubisco large subunit, which were isolated from five different plastomes, clarified the amino acid differences that are responsible for the electrophoretic difference between the H- and L-type large subunits. A phylogenetic tree was constructed for these genes, including their flanking regions, based on the number of mutations observed between them. This tree closely resembled the phylogenetic tree of their carrier plastomes, except in one minor point. As to the chondriome diversity, both the restriction endonuclease analysis and restriction fragment length polymorphism analysis of mitochondrial DNA demonstrated higher variability of mitochondrial DNA than chloroplast DNA, proving its usefulness in elucidation of the plasmon differentiation between closely related species or even within a species. Plasmon diversity revealed by comparing phenotypes of eu- and allo-plasmic lines of emmer, timopheevi, or common wheat indicated that all characters observed are affected by alien plasmons, among which male sterility, pistillody, haploid parthenogenesis, prematurity sprouting, variegation, and growth depression are specifically expressed by different genome–plasmon interactions. Nuclear genes responsible for their expression are determined in limited cases. As to the intraspecific variation of the plasmon, Aegilops speltoides showed large intraspecific variation, whereas two wild tetraploid wheats, Triticum dicoccoides and Triticum araraticum, revealed little variation. Aegilops triuncialis was dimorphic and proved to have originated from the reciprocal crosses between Aegilops caudata and Aegilops umbellulata. Combining all the data available, the plasmon in Triticum and Aegilops species has been classified into 16 types. Plasmon diversity decreases with the increase in the ploidy. Among the diploids, differentiation of the plasmon is paralleled with that of the nuclear genome. Origin of the plasmon of most polyploid species has been determined definitely, and for the remaining, their plasmon origin could be postulated. Thus, the maternal lineage of the species in these two genera has been clarified to a great extent.Key words: Triticum, Aegilops, plasmon, plastome, chondriome, rbcL gene, variation, phylogeny, maternal lineage, restriction endonuclease analysis, restriction fragment length polymorphism analysis, base sequencing, alloplasmic wheat, phenotypic effect.