Linear differentiation of cereal chromosomes : I. Common wheat and its supposed ancestors

Cytological markers used for identification and transfer of Aegilops spp. chromatin carrying valuable genes into cultivated forms of Triticum

There are many reports describing chromosome structure, organization and evolution within goatgrasses (Aegilops spp.). Chromosome banding and fluorescence in situ hybridization techniques are main methods used to identify Aegilops Linnaeus, 1753 chromosomes. These data have essential value considering the close genetic and genomic relationship of goatgrasses with wheat (Triticumaestivum Linnaeus, 1753) and triticale (× Triticosecale Wittmack, 1899). A key question is whether those protocols are useful and effective for tracking Aegilops chromosomes or chromosome segments in genetic background of cultivated cereals. This article is a review of scientific reports describing chromosome identification methods, which were applied for development of prebreeding plant material and for transfer of desirable traits into Triticum Linnaeus, 1753 cultivated species. Moreover, this paper is a resume of the most efficient cytomolecular markers, which can be used to follow the introgression of Aegilops chromatin during the breeding process.

Aegilops tauschii Accessions with Geographically Diverse Origin Show Differences in Chromosome Organization and Polymorphism of Molecular Markers Linked to Leaf Rust and Powdery Mildew Resistance Genes

Aegilops tauschii (2n = 2x = 14) is a diploid wild species which is reported as a donor of the D-genome of cultivated bread wheat. The main goal of this study was to examine the differences and similarities in chromosomes organization among accessions of Ae. tauschii with geographically diversed origin, which is believed as a potential source of genes, especially determining resistance to fungal diseases (i.e., leaf rust and powdery mildew) for breeding of cereals. We established and compared the fluorescence in situ hybridization patterns of 21 accessions of Ae. tauschii using various repetitive sequences mainly from the BAC library of wheat cultivar Chinese Spring. Results obtained for Ae. tauschii chromosomes revealed many similarities between analyzed accessions, however, some hybridization patterns were specific for accessions, which become from cognate regions of the World. The most noticeable differences were observed for accessions from China which were characterized by presence of distinct signals of pTa-535 in the interstitial region of chromosome 3D, less intensity of pTa-86 signals in chromosome 2D, as well as lack of additional signals of pTa-86 in chromosomes 1D, 5D, or 6D. Ae. tauschii of Chinese origin appeared homogeneous and separate from landraces that originated in western Asia. Ae. tauschii chromosomes showed similar hybridization patterns to wheat D-genome chromosomes, but some differences were also observed among both species. What is more, we identified reciprocal translocation between short arm of chromosome 1D and long arm of chromosome 7D in accession with Iranian origin. High polymorphism between analyzed accessions and extensive allelic variation were revealed using molecular markers associated with resistance genes. Majority of the markers localized in chromosomes 1D and 2D showed the diversity of banding patterns between accessions. Obtained results imply, that there is a moderate or high level of polymorphism in the genome of Ae. tauschii determined by a geographical origin, which we proved by cytogenetic and molecular markers analysis. Therefore, selected accessions might constitute an accessible source of variation for improvement of Triticeae species like wheat and triticale.

C-banding pattern and polymorphism of Aegilops caudata and chromosomal constitutions of the amphiploid T. aestivum - Ae. caudata and six derived chromosome addition lines

C-banding patterns were analysed in 19 different accessions of Aegilops caudata (= Ae. markgrafii, = Triticum dichasians) (2n = 14, genomically CC) from Turkey, Greece and the USSR, and a generalized C-banded karyotype was established. Chromosome specific C-bands are present in all C-genome chromosomes, allowing the identification of each of the seven chromosome pairs. While only minor variations in the C-banding pattern was observed within the accessions, a large amount of polymorphic variation was found between different accessions. C-banding analysis was carried out to identify Ae. caudata chromosomes in the amphiploid Triticum aestivum cv 'Alcedo' - Ae. caudata and in six derived chromosome addition lines. The results show that the amphiploid carries the complete Ae. Caudate chromosome complement and that the addition lines I, II, III, IV, V and VIII carry the Ae. caudata chromosome pairs B, C, D, F, E and G, respectively. One of the two SAT chromosome pairs (A) is missing from the set. C-banding patterns of the added Ae. caudata chromosomes are identical to those present in the ancestor species, indicating that these chromosomes are not structurally rearranged. The results are discussed with respect to the homoeologous relationships of the Ae. caudata chromosomes.

Cytological characterization, powdery mildew resistance and storage protein composition of tetraploid and hexaploid 1BL/1RS wheat-rye translocation lines

Progenies of a tetraploid 1BL/1RS wheat-rye translocation line, CV 256, selected from the cross 'Cando' x 'Veery', were analyzed by means of Giemsa C-banding. CV 256 is cytologically stable for the presence of the 1BL/1RS translocation but still segregating for A- and B-genome chromosomes of 'Cando' and 'Veery'. In CV 256, nucleolar activity of the 1RS NOR locus is suppressed, as judged by the absence of a secondary constriction in that rye segment and the capability of organizing nucleoli. PAGE analysis of prolamins confirmed the presence of two 1RS secalins in all single seeds analyzed. SDS-PAGE analysis of reduced glutenins of single seeds indicated that some seeds contained the 'Cando' Glu-B1 locus (subunits 6+8), some contained the 'Veery' Glu-B1 locus (subunits 7+9) while others contained all four subunits, indicating that the material was heterozygous. Pm8 resistance is expressed in the tetraploid 1BL/1RS translocation line based on the reactions of six well-defined powdery mildew isolates. However, Pm8 resistance is not expressed in the hexaploid wheat cultivars 'Olymp', 'Heinrich' and 'Florida', which also contain the 1BL/1RS translocation. Obviously, the existence of the 1BL/1RS translocation is not a proof for the expression of the associated genes. PAGE results did not show a clear linkage between powdery mildew resistance and the presence of 1RS secalins.

Identification of a complete set of isogenic wheat/rye D-genome substitution lines by means of Giemsa C-banding

A complete set of isogenic wheat/rye D-genome substitutions were produced by crossing an inbred line of spring rye Secale cereale L. cv. "Prolific" to a tetraploid wheat, the A-and B-genomes of which had previously been extracted from hexaploid wheat, Triticum aestivum L. em Thell. cv. "Thatcher". After chromosome doubling, the derived hexaploid triticale (x Triticosecale Wittmack) was backcrossed to 6x "Thatcher" and selection for wheat/rye substitution lines was carried out in BCF3 to BCF6 families by using Giemsa C-banding. Five fertile disomic wheat/rye D-genome substitution lines were obtained and their chromosomal constitution was determined to be 1D/1R, 2D/2R, 7D/4R, 6D/6R, 7D/7R. The two remaining 3R and 5R substitutions are at the moment in a monosomic condition. Another 1D/7R substitution was detected but this plant was very weak and sterile, indicating that only substitutions between homoeologous chromosomes result in fertile, vigorous plants. Furthermore, many rye telocentrics as well as rye-rye and rye-wheat translocations were selected. Since all lines selected in this program share the same genetic background of "Thatcher" wheat, genetic heterogeneity is excluded. The material is very useful, therefore, for analyzing the effects of different rye chromosomes or chromosome segments in an otherwise homozygous background.

Interspecific variation in C-banded chromosomes of diploid Aegilops species

Based on an improved C-banding technique, the C-banding patterns of all 11 diploid Aegilops species were described and compared. All diploid species exhibit characteristically different patterns which enable the chromosomes of any complement to be identified individually. These patterns confirm existing genome symbols and provide further evidence for the suggested changes in genome symbols of Ae. umbellulata and Ae. sharonensis, U and S(sh) respectively. Furthermore, Ae. uniaristata should be given a separate symbol, probably N. Aegilops speltoides and Ae. sharonensis could be possible donors to the B genome of wheat. Interspecific divergence in these diploid species has been accompanied by either amplification or deletion as well as massive repatterning of heterochromatin from the centromere to the telomere.

Investigation of a preferentially transmitted Aegilops sharonensis chromosome in wheat

An attempt to produce a set of addition lines of Aegilops sharonensis to the wheat variety 'Chinese Spring' produced only one addition line. This was due to preferential transmission of one chromosome from Ae. sharonensis. This chromosome was studied in detail by established cytological methods of chromosome observation and by the newer techniques of C-banding and in situ hybridization of a cloned DNA sequence. The chromosome was found to be partially homologous to an Ae. sharonensis chromosome of similar behaviour in another wheat addition line. The incomplete homology of the two Ae. sharonensis chromosomes was due to the presence of a translocated segment of a wheat chromosome. - Substitution lines of the Ae. sharonensis chromosome for wheat homoeologous group 4 were produced and the Ae. sharonensis chromosome thereby designated 4 S (l) .

THE RYE GENOME IN WINTER HEXAPLOID TRITICALES

In eight winter triticales (× Triticosecale Wittmack) chosen from the Cambridge crossing program all seven rye (Secale cereale L.) chromosomes were present. However, a number of rye chromosomes showed an obvious reduction in the size of C-bands at one or more telomeres, compared with normal rye. In two lines the C-band at the short arm telomere of 2R was obviously larger than normal in rye. Comparison of these results with those published for spring triticales suggests that divergent selection pressures may determine the number and structure of rye chromosomes in winter and spring lines. In particular it is suggested that the differences may represent environmental adaptation and reflect selection pressure on the rye genome in triticale to conform to the cline for DNA amount per diploid genome on latitude previously described in cereal grain crops.