A. S. 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.

Leaf rust and stripe rust resistance genes derived from Aegilops sharonensis

SummaryLinked leaf and stripe rust resistance genes introgressed into hexaploid wheat from Aegilops sharonensis provided protection in the seedling stage to a wide range of pathotypes of the two diseases. Monosomic and telosomic analyses showed that the resistance genes occur on wheat chromosome 6A. This result could be confirmed making use of mapped chromosome 6A microsatellite markers. The introgressed chromatin appeared to involve the proximal part of 6AL and the complete 6AS arm and it was thus not possible to deduce the chromosome arm harbouring the resistance genes. The resistance showed non-Mendelian transmission. The genetic background of a heterozygote interacted with the introgressed region to result in either preferential or impaired female transmission. Male transmission appeared to be affected in a different way from female transmission and was exclusive in the genetic background studied. Symbols Lr56 and Yr38 are proposed to designate the respective genes of which line 0352-4 is the appropriate source material.

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.

A study of modified forms of the Lr19 translocation of common wheat

Abstract Following the induction of allosyndetic pairing between the Thinopyrum-derived Lr19 translocation in ‘Indis’ wheat and homoeologous wheat chromatin, eight suspected recombinants for the Lr19 region were recovered. These selections were characterised for marker loci that were previously used to construct a physical map of the Lr19 segment. At the same time near-isogenic lines were developed for some of the selected segments and tested for seedling leaf-rust resistance in order to confirm the presence of Lr19. It appeared that three of the four white-endosperm selections do not possess Lr19 and only one, 88M22-149, is a true Lr19 recombinant. The resistance gene in the three non-Lr19 selections resides on chromosome 6B, appears to derive from ‘Indis’, and was selected unintentionally during backcrossing. The pedigree of ‘Indis’ is suspect and it is believed that the Lr19 translocation in ‘Indis’ is in reality the Th. ponticum-derived (T4) segment rather than being of Th. distichum origin as was believed earlier. The white-endosperm recombinant, 88M22-149, retained the complete Lr19 resistance and was apparently re-located to chromosome arm 7BL in a double-crossover event. 88M22-149 has lost the Sd1 gene and often shows strong self-elimination in translocation heterozygotes. This effect may result from additional gametocidal loci or from an altered chromosome structure following re-location of the segment. 88M22-149 in fact contains a duplicated region involving the Wsp-B1 locus. Three selections had partially white endosperms and expressed Lr19 and other Thinopyrum marker alleles. Polymorphisms for the available markers confirmed that the translocated segment in at least one of them had been shortened through recombination with chromosome arm 7DL. Further markers need to be studied in order to determine whether the translocation in the remaining two partially white recombinants had also undergone recombination with wheat. The eighth selection has yellow endosperm and appears to self-eliminate in certain translocation heterozygotes. No evidence of recombination could be found with the markers used. If the latter selections are in fact recombinants they may prove useful in attempts to unravel the complex segregation distortion mechanism.

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.

Attempts to remove gametocidal genes co-transferred to common wheat with rust resistance from Aegilops speltoides

Rust resistance genes (introgressions S24 and S13) transferred to hexaploid wheat from two Aegilops speltoides accessions could not be used commercially due to associated gametocidal (Gc) genes. Crosses to wheat followed by rigorous selection for increased fertility were employed in an attempt to separate the unmapped S24 stem rust resistance from the Gc gene(s). However, improved fertility of the better selections could not be maintained in subsequent generations. Since the S13 introgression (leaf, stripe and stem rust resistances) mapped to chromosome 3A, allosyndetic pairing induction was used in an attempt to remove the Gc gene(s). This produced putative primary recombinants with improved fertility and plant type, the best of which had exchanged a small region of Ae. speltoides chromatin, yet was still associated with (reduced) Gc effects. This selection (04M127-3, which appears to have the Su1-Ph1 suppressor) was then crossed with wheat. Surprisingly, the 04M127-3 gametocidal effect differed drastically from that of the original introgression allowing the recovery of 35 recombinant, leaf rust resistant progeny. Microsatellite and DArT markers showed that each secondary recombinant had exchanged most of the Ae. speltoides chromatin. Although the data suggested that a complex multigenic interaction may govern the gametocidal response, preliminary indications are that the Gc effect had largely been removed and it now seems possible to completely separate the gametocidal genes from the S13 leaf rust resistance gene (here designated Lr66). The associated (S13) stripe rust and stem rust resistance genes were lost during recombination.

Extension and use of a physical map of the Thinopyrum-derived Lr19 translocation.

Twenty-nine deletion mutant lines were used to extend a physical map of the Lr19 translocated chromosome segment. One hundred and forty-four Sse8387I/MseI and 32 EcoRI/MseI primer combinations were used to obtain 95 Thinopyrum-specific AFLP markers. The physical map confirmed that terminal deletions had mostly occurred, however, it appears that intercalary deletions and primer or restriction site mutations were also induced. The markers allowed for grouping of the deletion mutant lines into 19 clusters, with 7 AFLP markers mapping in the same marker bin as Lr19. Primary and secondary Lr19 allosyndetic recombinants were subsequently physically mapped employing AFLP, RFLP, SCAR and microsatellite markers and the data integrated with the deletion map. A further shortened, tertiary Lr19 recombinant was derived following homologous recombination between the proximally shortest secondary recombinant, Lr19-149-299, and distally shortest recombinant, Lr19-149-478. The tertiary recombinant could be confirmed employing the mapped markers and it was possible to identify new markers on this recombinant that can be used to reduce the translocation still further.

Identification of Thinopyrum distichum chromosomes responsible for its salt tolerance.

A Thinopyrum distichuml 4x rye (Secale cereale) hybrid with genomes J1 dJ2 dRR was pollinated with diploid rye and mostly yielded F1 offspring with 21 chromosomes (two complete rye genomes and seven Thinopyrum chromosomes). Apparently, the closely related homoeologous chromosomes of the J1 d and J2 d genomes regularly formed bivalents during megasporogenesis, and egg cells mostly received a random, yet balanced set of seven Thinopyrum chromosomes. F, plants were tested for salt tolerance and a set of fifteen highly salt-tolerant F1 plants were selected and maintained as clones for several years. These were C-banded and the Thinopyrum chromosomes contained in each were determined. By comparing segregation patterns it was now possible to group the Thinopyrum chromosomes into seven homoeologous pairs. For each of four homoeologous pairs, one of its members occurred at a higher than expected frequency, implying that these chromosomes are expressed under salt stress conditions. The results could be confirmed by backcrossing two of the most tolerant F1 plants to diploid rye. While the critical chromosomes can be identified through C-banding, an attempt was made to also find a RFLP marker for each. RFLP probes, diagnostic for the group 2, 3, 4 and 5 homoeologues of wheat, detected polymorphisms on the respective critical Thinopyrum chromosomes. However, the preliminary allocation of the critical chromosomes to homoeology groups needs to be confirmed using more and varied markers.

SCAR markers that specifically tag chromosomes 2D of wheat and 2J1 d of Thinopyrum distichum

A SCAR marker for Thinopyrum distichum chromosome 2J1 d (involved in salt tolerance) also amplified a slightly larger fragment in chromosome 2D of common wheat and substituted hexaploid triticale. The Thinopyrum and wheat derived fragments were isolated and used to develop two new and highly specific markers for 2J1 dL and 2DL, respectively. The chromosome 2J1 dL marker is useful in attempts to introgress salt tolerance into cultivated wheat and triticale whereas the 2DL marker can be used for rapid identification of hexaploid triticales with the 2D(2R) chromosome substitution.