The incorporation of alien disease resistance in wheat by genetic interference with the regulation of meiotic chromosome synapsis

1. Triticum aestivum ssp. vulgare variety Chinese Spring (2 n = 6 x = 42) is susceptible to yellow rust caused by Puccinia striiformis while the wild annual grass Aegilops comosa (2 n = 14) is resistant to all the physiologic races for which it has been tested. 2. By a backcrossing programme initiated from Chinese Spring × Ae. comosa hybrids, using Chinese Spring as the recurrent parent, a line was isolated with a single chromosome of Ae. comosa , determining rust resistance, added to the full complement of Chinese Spring. 3. The alien chromosome substituted with good genetic compensation only for the chromosomes of homoeologous group 2 of Chinese Spring. This demonstrated that the chromosome determining rust resistance is in homoeologous group 2. It was designated 2M since Ae. comosa has the M genome. 4. In order to induce recombination between 2M and its wheat homoeologues, hybrids were made using Ae. speltoides which has the capacity to suppress the activity of chromosome 5B that normally prevents homoeologous synapsis. A backcrossing programme, using Chinese Spring as the recurrent parent, was reinitiated from the 29-chromosome hybrids carrying chromosome 2M and the haploid complements of Chinese Spring and Ae. speltoides . 5. Selection was practised for rust resistance and ultimately a resistant plant with 42 chromosomes, that formed 21 bivalents at meiosis, was isolated. This plant was heterozygous for a dominant rust resistance allele ( Yr 8 ) derived from Ae. comosa . Homozygotes were isolated in its progeny and in this way the rust resistant breeder's variety, Compair, was established. 6. Compair differs from Chinese Spring in its yellow rust resistance which was shown to be determined by a chromosome corresponding to 2D of Chinese Spring. This chromosome of Compair has the short arm, the centromere and a proximal segment of the long arm of chromosome 2M and a distal segment of the right arm of chromosome 2D. The modified chromosome, which is designated 2M/D, arose by homoeologous recombination in the Ae. speltoides hybrid or in the immediately succeeding backcross generations. Chromosome 2M/D carries the Yr 8 gene in the proximal segment of the long arm derived from chromosome 2M. 7. In hybrids between Compair and standard wheat varieties, chromosome 2M/D pairs regularly with chromosome 2D so that regular segregation of Yr 8 can be expected and Compair treated like any other parental variety in wheat hybridization programmes. 8. This work illustrates the way that homoeologous recombination can be induced and exploited both in cytogenetic analysis in wheat and in practical breeding work. The nature of the meiotic synapsis of chromosome 2M/D with its partial homologues raises questions concerning the means by which chromosome 5B influences the specificity of meiotic synapsis.