G. F. Marais

Leaf rust and stripe rust resistance genes transferred to common wheat from Triticum dicoccoides

Linked leaf rust and stripe rust resistance genes introduced from Triticum dicoccoides protected common wheat seedlings against a range of pathotypes of the respective pathogens. The genes were chromosomally mapped using monosomic and telosomic analyses, C-banding and RFLPs. The data indicated that an introgressed region is located on wheat chromosome arm 6BS. The introgressed region did not pair with the ‘Chinese Spring’ 6BS arm during meiosis possibly as a result of reduced homology, but appeared to pair with 6BS of W84-17 (57% of pollen mother cells) and ‘Avocet S’. The introgressed region had a very strong preferential pollen transmission (0.96–0.98) whereas its transmission through egg cells (0.41–0.66) varied with the genetic background of the heterozygote. Homozygous resistant plants had a normal phenotype, were fertile and produced plump seeds. Symbols Lr53 and Yr35 are proposed to designate the respective genes.

Transfer of rust resistance genes from Triticum species to common wheat.

A programme aiming to transfer leaf rust resistance genes identified in a collection of wild Triticum species was initiated in 1993. In 2000, 25 promising backcross populations were available, 19 of which bred true for resistance. Seedlings of the above lines were tested with nine leaf rust, four stem rust and two stripe rust pathotypes endemic to South Africa. A subset of five lines in which resistance (derived from T. dicoccoides, T. sharonense, T. speltoides and T. peregrinum) appeared to be integrated on wheat chromosomes and six addition lines with added chromosomes from T. kotschyi, T. peregrinum, T. umbellulatum, T. macrochaetum and T. neglectum appeared to have wide spectrum resistances, and were retained. In several instances promising stem rust and/or stripe rust resistance genes were co-transferred with leaf rust resistance. The stripe rust resistance was also effective to four Australian pathotypes and appeared to be novel. Temporary gene designations were assigned to the resistance genes in four euploid derivatives.

Association of a stem rust resistance gene (Sr45) and two Russian wheat aphid resistance genes (Dn5 and Dn7) with mapped structural loci in common wheat

A stem rust resistance gene, originally derived from Triticum tauschii accession RL5289 and present in the germplasm line 87M66-2-1, is here designated Sr45. Sr45 was found to be closely linked to Sr33 (9 ± 1.9 map units) and the centromere (21 ± 3.4 map units) on chromosome arm 1DS. Sr45 is believed to be the same gene as SrX. The Russian wheat aphid resistance gene, Dn5, was loosely linked (32 ± 5 map units) to Ep-D1b, which occurs on a translocation derived from T. ventricosum, and to the cn?D1 locus (37 ± 6.3 map units) on chromosome arm 7DL. Dn5 derives from T. aestivum accession Pl294994 which was found to express two novel Ep-1 alleles (proposed designations Ep-A1d and Ep-D1e). A gene (here designated Dn7) for Russian wheat aphid resistance that was derived from the rye accession, Turkey 77, mapped 14.5 ± 3.9 map units from Lr26 on the 1BL.1RS translocation.

Evaluation and reduction of Lr19-149, a recombined form of the Lr19 translocation of wheat

The Lr19 translocation was introgressed from Thinopyrum ponticum in 1966. It has not been used in wheat breeding in many countries despite it being an excellent source of leaf rust resistance as it carries an undesirable gene(s) coding for yellow endosperm pigmentation. A shortened form, Lr19-149, was since produced and lacks the yellow pigment genes. A yield trial with near isogenic lines of both the original and shortened translocations suggested that Lr19 may cause a small reduction in kernel size and anincrease in loaf volume, effects which are not associated with Lr19-149. In Lr19-149 heterozygotes the translocation generally showed reduced pollen transmission whereas its transmission through egg cells was mostly normal. An attempt to shorten Lr19-149 through allosyndetic recombination in the absence of Ph1b produced four recombinants which were characterized by means of RFLP and AFLP polymorphisms and physically mapped with a set of 27 deletion lines. In three recombinants (252, 299 and 462) Thinopyrum chromatin proximally to Lr19 was exchanged for wheat chromatin. In one recombinant (478) chromatin distally from Lr19 was replaced. Based on physical map distance estimates it appears that the Lr19 translocation in the shortest recombinant (299) may have been reduced to about one third or less of its original size. It may now be possible to obtain a further, albeit relatively small, decrease in the size of the translocation through homologous crossover between recombinants 299 and 478. Similar to Lr19-149, the new recombinants show self elimination in heterozygotes and they have apparently retained the Sd2 locus.

The derivation of compensating translocations involving homoeologous group 3 chromosomes of wheat and rye

SummaryThe Sr27 translocation in WRT238 was found to consist of chromosome arms 3RS of rye and 3AS of common wheat. An attempt was made to purposely produce compensating translocations having 3RS and a wheat homoeologous group 3L arm. To achieve this, plants, double monosomic for 3R and a wheat homoeologous group 3 chromosome, were irradiated (7.5 Gy gamma rays) or left untreated before being used to pollinate stem rust susceptible testers. Segregation for stem rust resistance was studied to identify F2 families with Sr27-carrying translocated chromosomes, these were confirmed by means of C-banding. Compensating translocations 3RS3AL and 3RS3BL) were obtained readily and at similar frequencies from untreated and irradiated plants (respectively, 7.2% and 9.3%). Both translocation types have impaired transmission and segregate approximately 3: 2 (present: absent) in the F2.

A genetic study of the gametocidal effect of the Lr19 translocation of common wheat

The Lr19 translocation is preferentially transmitted to the progeny of a heterozygote due to the actions of at least two genes, Sd1 and Sd2 (new designation). However, only Sd2 occurs in the recombinant, Lr19-149, and often causes self-elimination of the recombined translocated segment in heterozygotes. The degree of segregation distortion is determined by the interaction of the Sd genes with polygenes (responder genes) on various wheat chromosomes. In this study suspected responder alleles derived from ‘Inia 66’ or ‘Indis’ (chromosomes 2A, 2B, 3B, 5B, 5D and 6D) appeared mostly to be partially dominant to overdominant over the ‘Chinese Spring’ derived alleles. A specific allele may not necessarily hafe-tfoe same effect (suppression or enhancement) in different genetic backgrounds. Responder genes may not fully compensate for the absence of a homologue in a hemizygote which may then produce effects quite different from those of the homo- and heterozygotes.

An extended deletion map of the Lr19 translocation and modified forms

A physical deletion map of the Lr19 translocated chromosome segment was extended by mapping three additional Thinopyrum RFLP loci. The relative locations of the marker loci on the translocated segment were determined as: centromere, Sd1, Xpsr165, Xpsr105, Xpsr129, XcsIH81-1, Xwg380, Xmwg2062, Lr19, Wsp-D1, Sr25/Y. Various recombinants, putative recombinats and mutants of the Lr19 segment were also characterised with respect to the additional markers.

Identification of leaf rust resistance genes in Triticum species for transfer to common wheat

A total of 877 Triticum accessions (27 species) were screened for resistance to leaf rust (Puccinia recondita f. sp. tritici) using mixed inoculum of pathotypes UVPrt2, UVPrt3, UVPrt8, UVPrt9 and UVPrt13. Of these, 206 accessions were resistant/moderately resistant to all races. An attempt was made to cross each resistant accession with common wheat and to determine if resistance is expressed sufficiently in the presence of wheat genomes. Seventy nine accessions have not yet been crossed successfully, while the remaining 127 (representing 19 species) were crossed with common wheat. A number of transfer attempts failed in the F, as a result of suppression of resistance (44 accessions) or formation of embryoless or non-viable seeds/seedlings (13 accessions). The resistance of 70 hybrids is fully expressed and these are now in various stages of back-crossing to wheat. The resistance was confirmed by retesting the species sources of the 70 successful combinations with the individual leaf rust pathotypes.

An assessment of the variation for stem rust resistance in the progeny of a cross involving the Triticum species aestivum, turgidum and tauschii

Accession RL5289 of Triticum tauschii and Triticum turgidum var. durum cv. Cando showed resistance to the prevailing stem rust races of South Africa. RL5289 was crossed with Cando. An amphiploid was produced and crossed with the common wheat line W107. Three cycles of crosses of resistant derivatives to the common wheat cvs. Inia 66 and SST3 were then made. Five resistant F1 plants from the final crosses were planted and used to obtain five sets of F2-derived F3 populations which were tested for their stem rust resistance, agronomic and quality characteristics. Two F4 populations (87M66-2-1 and 87M66-5-6) were then derived as agronomically superior families that appeared to breed true for individual stem rust resistance genes. Monosomie analyses of 87M66-2-1 and 87M66-5-6 implicated chromosome 1D as carrying stem rust resistance genes. Selection 87M66-2-1 has a dominant resistance gene which appears to derive from Triticum tauschii accession RL5289. While the gene is not very effective against Canadian is...

Gametocidal effects and resistance to wheat leaf rust and stem rust in derivatives of a Triticum turgidum ssp. durum/Aegilops speltoides hybrid

SummaryGenes for leaf rust and stem rust resistance and segregation distortion (Gc), that seemed to derive from an Aegilops spetroides ssp. ligustica accession, were transferred to common wheat. While the advanced backcrosses had normal meioses and 42 chromosomes, high levels of male and female sterility, abnormal endosperm development and chromosome aberrations were evident. These effects were more pronounced in Gc-heterozygotes than in homozygotes. Gametes without Gc genes did not survive, and the Gc-associated defects were always inherited with the resistance. Since the resistance genes were effective against local pathotypes of the leaf rust and stem rust pathogens, an attempt was made to disrupt the Gc-system through irradiation, treatment with the mutagen N-nitroso-N-methyl-urea or growing the material at elevated temperatures. A very low frequency of the treated material showed slightly better fertility and seed development. However, these effects did not persist in subsequent generations and were apparently not strong enough to allow the recovery of segregates which had lost the Gc gene(s).