THE ROLE of Aegilops SPECIES in the ORIGIN and IMPROVEMENT of COMMON WHEAT
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ACTA AGROBOTANICA Vol. 66 (4), 2013: 7–14 Reviews DOI: 10.5586/aa.2013.046 THE ROLE OF Aegilops SPECIES IN THE ORIGIN AND IMPROVEMENT OF COMMON WHEAT Roman Prażak University of Life Sciences in Lublin, Subdepartment of Plant Biology, Faculty of Agricultural Sciences Szczebrzeska 102, 22-400 Zamość, Poland e-mail: [email protected] Received: 22.04.2013 Abstract rust [9,10,11,12,13,14,15,16,17,18,19,20,21,22], to Some Aegilops species participated in wheat evolu- powdery mildew (Blumeria graminis) [23,24,25], tion playing a major role in wheat domestication and therefore to eyespot (Pseudocercosporella herpotrichoides) the genus Aegilops represents a big part of the additional gene [26,27], to tan spot (Pyrenophora tritici-repentis) [28], pool determining important traits of wheat. Breeders have been to nematodes and insects [29,30,31], and to pre-harvest using these genes for many years to produce improved cultivars. sprouting [32]; and tolerance for soil salinity, drought Wide crosses between its wild relatives are sources of desira- [33,34] and soil acidification [35,10]. ble characteristics for genetic improvement of common wheat. An example of the transfer of beneficial traits Triticum aestivum evolution and methods for transfer of alien from Aegilops species to cultivated wheat is the trans- material into wheat, briefly reviewed in this article, include in- location from Ae. ventricosa introduced in France to the corporation of the whole genomes, single chromosomes, small winter variety VPM1, conferring resistance – owing to chromosomal segments, single genes and cytoplasm substitu- tion in wheat. the genes Pch1 and Pch2 – to eyespot, caused by Pseu- docercosporella herpotrichoides [26]. In Great Brita- in an enzyme marker of the eyespot resistance gene Key words: Aegilops, evolution, gene transfer, Triticum aestivum, wheat improvement, wide crosses was developed and the Rendezvous variety was pro- duced, which is found in the lineage of many British, French, German and Swiss winter wheats as well as Aegilops species as a source of valuable in Australian and North American wheats. In addition traits for common wheat to resistance to Pseudocercosporella herpotrichoides, In the last few decades, the biodiversity of culti- a translocation from Ae. ventricosa was also found to vated wheat varieties has become significantly impove- occur in these varieties, on chromosome 2B, containing rished. It can be increased by introducing economically a group of three rust resistance genes: Lr 37, Yr 17 and important genes from wild species, including genes Sr 38 [36]. from the genus Aegilops, to the wheat genome [1] The Aegilops and Triticum genera belong to the same family The role of Aegilops species of grasses (Poaceae). Aegilops species are annual gras- in the evolution of common wheat ses characterized by strong tillering, found mainly in Both selection by man and natural cross-bree- the Mediterranean Basin, southern Asia, the mountains ding between primitive wheats and wild grasses of of the Caucasus and Kashmir, and the Near East. They the genus Aegilops occurring as weeds in wheat crops grow at various altitudes, from 0 to 2,000 m, in dry and have played a significant role in the evolution of wheat. degraded environments, at field edges, on roadsides, in Some researchers estimate that hexaploid wheats evo- grassland, and in or near cultivated fields [2, 3]. lved over 10,000 years [37]. The donor of the A geno- Aegilops species are a source of valuable traits me for common wheat is probably the diploid wheat for wheat, including long ears [4]; a high content of T. urartu [38]. The origin of the B genome is still a mat- protein, the amino acid lysine, and the macronutrients ter of controversy. Huang et al. [39] believe it to be iron and zinc in the kernels [5, 6, 7, 8]; resistance to similar to the S genome of Aegilops species belonging © The Author(s) 2013 Published by Polish Botanical Society 8 Roman Prażak to the Sitopsis section. The S genome of Ae. speltoides Difficulties in obtaining F1 intergeneric hybrids is most similar to the B genome of polyploid wheats. also result from the genetic barrier posed by the group This can be seen in the fact that the brittle rachis gene of Kr genes in common wheat. The system controlling Br3 has the same position on the short arm of chromo- the capacity for intergeneric cross-breeding of T. aesti- some 3S in Ae. speltoides [40] and on the short arm vum with rye includes four dominant genes, Kr1, Kr2, of chromosome 3B in the tetraploid wheat T. durum Kr3 and Kr4, of which Kr1 produces the strongest ef- [41,42]. The locus of another brittle rachis gene, Br2, fect and Kr3 the weakest. In their dominant form, these was localized on the short arm of chromosome 3A in genes inhibit seed-setting ability in the F1 generation T. durum [42]. The B genome, which originated in Ae. [49, 50, 51]. There are, however, varieties of wheat, speltoides, probably underwent a secondary modifica- e.g. Chinese Spring with the genotype kr1 kr1 kr2 kr2, tion in wheat [43]. Tetraploid wheats (AABB) emer- which are distinguished by adequate ability to cross ged as the result of chance pollination of Ae. speltoides breed with rye [52]. with T. urartu pollen. Later, the number of chromoso- To overcome barriers to cross-breeding, various mes in the resulting hybrid must have doubled spon- methods and techniques are used for transferring ge- taneously. DNA analysis by Haider [44] revealed nes from Aegilops species to wheat. These include the high similarity of the S genome of Ae. speltoides to the following: B genome of tetraploid and hexaploid wheat. Hexa- – adding the entire genome of the Aegilops spe- ploid wheats are presumed to have arisen from chance cies to wheat genomes, i.e. obtaining amphi- pollination of the tetraploid wheat T. dicoccoides with ploids, and from these, via backcrossing with pollen of the diploid goatgrass Ae. squarrosa (Ae. tau- wheat, addition and substitution lines; schii) (DD), followed by spontaneous doubling of the – one of the recombination methods, i.e. cros- number of chromosomes or the pairing of unreduced sover resulting from homologous or homeolo- gametes in the resulting hybrid. Thus, they are natural gous pairing of chromosomes; amphiploids. The similarity between the D genomes – translocations induced by ionizing radiation, of wheat and Ae. squarrosa can be seen in the posi- caused by gametocidal chromosomes, or re- tion of the Br1 brittle rachis gene on the short arm of sulting from somaclonal variation; chromosome 3D of T. aestivum and of the gene Brt – obtaining alloplasmic forms of wheat with the on the short arm of chromosome 3D in Ae. squarrosa cytoplasm of Aegilops species; (Ae. tauschii) [41,42]. – transferring single genes to wheat via genetic engineering methods. Methods and techniques of gene transfer Chromosome doubling using colchicin in F1 from Aegilops species to common wheat hybrids makes it possible to obtain amphiploids [53], Some Aegilops species occurring as weeds in and from these, via backcrossing with wheat, addition wheat crops or on unploughed land between fields can and substitution lines [54,55]. Amphiploids can also spontaneously cross breed with common wheat [45]. arise spontaneously via pairing of unreduced male and The effectiveness of such spontaneous cross-breedings female gametes formed by intergeneric or interspecific is similar to that of artificial ones. Seed setting in hy- F1 hybrids [56]. brids takes place more often when the maternal form By backcrossing amphiploids with wheat and is Aegilops and the paternal form is wheat [46,45,3]. carrying out selection for 43-chromosome plants, addi- Most easily cross-bred with wheat are Aegilops species tion lines can be produced. One Aegilops chromosome containing the D genome, which has the highest homo- is added to the complete set of wheat chromosomes, logy to the D genome of wheat [3]. and following pollination of plants with 43 chromoso- Failures in cross-breeding of wheat with many mes, forms arise that have two Aegilops chromosomes. Aegilops species are due to low homology or lack of Although addition lines have not found wide applica- homology between their genomes and those of whe- tion in practice due to disturbances in meiotic divisions at, and to an improper number of chromosomes in the leading to the loss of the added chromosome and to endosperm. Underdevelopment of the endosperm le- low fertility, they are used to identify foreign chromo- ads to the death of the embryos in the early stage of somes and as initial forms for transferring foreign ge- development. Embryos can be isolated and grown on netic material to wheat [55,57]. artificial media in vitro. Sterility in F1 hybrids of di- Addition lines are used to obtain substitution li- stant forms is due to disturbances in the meiosis pro- nes. These are more stable than addition lines and have cess [47], which results in a lack of functional gametes. more genotypic variation. Substitution lines were obta- Backcrossing with wheat or colchicination is necessa- ined in which Aegilops chromosomes were substituted ry to obtain kernels [48]. in place of wheat chromosomes [6,58,59,60]. Owing © The Author(s) 2013 Published by Polish Botanical Society The role of Aegilops species in the origin and improvement of common wheat 9 to the foreign chromosomes, these lines are resistant to ph2 (nulli-3D) to be used in breeding. Subsequent mo- fungal pathogens, nematodes, and insects. nosomic series of wheat were obtained in the cultivars The simplest method for transferring foreign ‘Drabant’, ‘Jara’, ‘Favorit’ and others [52]. genes to wheat is recombination resulting from homo- Using induced homeologous pairing and cros- logous pairing of chromosomes, usually chromosome sing over, a number of wheat varieties with Aegilops D of Aegilops and chromosome D of wheat [3].