Cytogenetic analysis and mapping of leaf rust resistance in Aegilops speltoides Tausch derived bread wheat line Selection2427 carrying putative gametocidal gene(s)

Leaf rust (Puccinia triticina) is a major biotic stress affecting wheat yields worldwide. Host-plant resistance is the best method for controlling leaf rust. Aegilops speltoides is a good source of resistance against wheat rusts. To date, five Lr genes, Lr28, Lr35, Lr36, Lr47, and Lr51, have been transferred from Ae. speltoides to bread wheat. In Selection2427, a bread wheat introgresed line with Ae. speltoides as the donor parent, a dominant gene for leaf rust resistance was mapped to the long arm of chromosome 3B (LrS2427). None of the Lr genes introgressed from Ae. speltoides have been mapped to chromosome 3B. Since none of the designated seedling leaf rust resistance genes have been located on chromosome 3B, LrS2427 seems to be a novel gene. Selection2427 showed a unique property typical of gametocidal genes, that when crossed to other bread wheat cultivars, the F1 showed partial pollen sterility and poor seed setting, whilst Selection2427 showed reasonable male and female fertility. Accidental co-transfer of gametocidal genes with LrS2427 may have occurred in Selection2427. Though LrS2427 did not show any segregation distortion and assorted independently of putative gametocidal gene(s), its utilization will be difficult due to the selfish behavior of gametocidal genes.

Large-scale selection of lines with deletions in chromosome 1B in wheat and applications for fine deletion mapping

Terminal deletions of chromosome 1B in common wheat were selected on a large scale. The gametocidal gene of Aegilops cylindrica was used as the inducer of chromosome breakage. First, genes for endosperm storage proteins located on both arms of chromosome 1B were used as the selection markers. However, it was found that the chromosome breakage occurred during female gametogenesis, causing genotypic inconsistency between the embryo and endosperm. Thus, we isolated plants with terminal deletions in chromosome 1B by C-banding. Of 1327 plants examined, 128 showed aberrations in chromosome 1B: 47 in the short arm, 76 in the long arm, and 5 in both arms. The present deletions tended to have the breakpoint at more proximal regions than those produced previously by T.R. Endo and B.S. Gill. Using 33 deletion lines produced in this study and 34 lines previously produced, we mapped 39 RFLP loci and a nucleolar organizer region (NOR) on a specific region of chromosome 1B. The NOR was found to consist of two subregions with different repetitive units, which were termed NOR-B1d and NOR-B1p. Based on this fine deletion map and genotypic inconsistency between embryo and endosperm, the features of the gametocidal gene are discussed.Key words: deletion line, gametocidal gene, Triticum aestivum, deletion map, nucleolar organizer region.

Development and identification of a complete set of Triticum aestivum - Aegilops geniculata chromosome addition lines

The production and identification of a complete set of intact Aegilops geniculata chromosome and telosome additions to common wheat is described. All Ug and Mg genome chromosomes were tentatively assigned to their homoeologous groups based on C-banding, meiotic metaphase I pairing analyses and plant morphologies. Thirteen disomic and one monosomic wheat-Ae. geniculata chromosome additions were identified. Furthermore, two monotelosomic (MtA7UgL, MtA7MgL) and nine ditelosomic (DtA1UgS, DtA1UgL, DtA2UgS, DtA1MgL, DtA2MgL, DtA3MgS, DtA5MgS, DtA6MgL, DtA7MgS) wheat-Ae. geniculata additions were recovered. C-banding and meiotic pairing analyses revealed that all added Ug and Mg genome chromosomes are structurally unaltered compared to the Ae. geniculata parent accession. Chromosome 4Mg has a strong gametocidal gene that, when transferred to wheat, causes extensive chromosome breakage mainly in gametes lacking it. The relationships of Ae. geniculata chromosomes with those of the diploid progenitor species and derived polyploids is discussed.Key words: Triticum aestivum, Aegilops geniculata, chromosome addition lines, C-banding, genome evolution.

Gametocidal genes in wheat and its relatives. IV. Functional relationships between six gametocidal genes

Gametocidal (Gc) genes in Aegilops species are known to cause gamete abortion and chromosome breakage when they are introduced into the wheat genetic background. Interactions of five Gc genes so far identified were investigated by analysis of wheat hybrids among lines carrying different gametocidal genes. As a result, the genes were classified into three functional groups. The first group includes two Gc genes of Ae. speltoides (Gc1a and Gc1b) and one gene (Gc-Sl3) on chromosome 2S1 of Ae. sharonensis. These genes were hypostatic to the genes (Gc-Sl1, Gc-Sl2) on chromosome 4S1 of Ae. longissima and Ae. sharonensis, which constitute the second group. In addition, plants carrying Gc genes of both the first and the second group produced progeny with higher frequencies of chromosome breakage than those found in the progeny of single gene carriers. It was concluded that there were specific interactions between these genes to enhance chromosome breakage. On the other hand, there was no interaction between the Gc gene (Gc-C) of Ae. triuncialis, the third group, and Gc genes belonging to the former two groups. These functional groups might be a reflection of the mechanisms by which Gc genes induce gamete abortion and chromosome breakage. Based on functional and local relationships, the symbols of the Gc genes were systematically redesignated.Key words: wheat, Triticum aestivum, Aegilops, gametocidal gene, chromosome breakage.

Structure of chromosome 5A of wheat speltoid mutants induced by the gametocidal genes of Aegilops speltoides

Speltoid spikes occur when the dose of Q, (suppressor of speltoidy) located on the long (q) arm of chromosome 5A, decreases from the normal two to one or none. In crossed or selfed progeny of wheat lines that carry the gametocidal genes, Gc1a or Gc1b, 35 speltoid mutants appeared independently. The nature of the mutants, whether due to gene mutation or due to chromosome deletion, including the Q gene, was investigated. The mutants in the M1 generation were self-pollinated and segregation of the character in the M2 was observed. Speltoid plants in the M2 were crossed with ditelosomic 5AL (20II + tII), and pairing between the telosome 5AL and the chromosome that caused speltoidy was observed. Of the 25 lines that were analyzed by the two methods, 22 had a chromosome deletion on the long arm of chromosome 5A, the length of which was variable. The chromosome with deletion could be classified into at least three groups by comparison with the length of telo-univalent 5AL. In the M3 generation, six homozygous deletion lines were selected. Chromosome breakage that resulted in the deletions could be due to the involvement of a transposable element or a restriction – modification system.Key words: Triticum aestivum, Aegilops speltoides, gametocidal gene, chromosome breakage, mutation.

Gametocidal genes in wheat and its relatives. III. Chromosome location and effects of two Aegilops speltoides-derived gametocidal genes in common wheat

A new gametocidal gene was found in a common wheat cultivar Chinese Spring carrying the cytoplasm of Aegilops speltoides strain KU 5725 (Plant Germplasm Institute, Kyoto University). Monosomic analysis revealed that this gene and a previously known gametocidal gene Gc1, both derived from Ae. speltoides, are located on chromosome 2B. The two genes appear to be allelic and so have been designated as Gc1a (previous gene) and Gc1b (present gene). The two genes differ in their ability to induce hybrid dysgenesis in wheat: Gc1a causes endosperm degeneration and chromosome aberrations, whereas Gc1b results in abnormal seed lacking the shoot primodium. No correlation between embryo or endosperm degeneration and chromosome breakage was observed.Key words: wheat, Aegilops speltoides, gametocidal gene, hybrid dysgenesis, chromosome breakage.

Gametocidal genes in wheat and its relatives. II. Suppressor of the chromosome 3C gametocidal gene of Aegilops triuncialis

Chromosome 3C of Aegilops triuncialis, in the monosomic state, causes semisterility in common wheat cultivars (Triticum aestivum). The Japanese cultivar 'Norin 26' carries a gene that acts as dominant suppressor of the gametocidal gene on chromosome 3C. However, this suppressor gene does not alter the functions of the gametocidal gene on chromosome 4S1 of Ae. longissima or Ae. sharonensis. By means of monosomic analyses, the suppressor has been located on chromosome 3B and is designated Igc1. This gene is distributed widely among wheat cultivars bred in central and southwest Japan. In the presence of Igc1 chromosome 3C shows no preferential transmission, but is transmitted to offspring at a frequency similar to those of other alien chromosomes. The progeny of plants that carry a single chromosome 3C exhibit chromosome aberrations, and possibly mutations, at high frequencies. Thus, the gametocidal gene on chromosome 3C causes a syndrome similar to hybrid dysgenesis in common wheat.Key words: common wheat, Aegilops triuncialis, gametocidal gene, suppressor, hybrid dysgenesis.