Chromosome 5A of bread wheat, Triticum aestivum carries the major gene, Vrnl, which is one of the main determinants of the winter/spring growth habit polymorphism in this species. Genetical analysis of this chromosome has been carried out using single-chromosome recombinant lines to establish the pleiotropic effects of this locus and two other major genes, q determining ear morphology and bl determining the presence of awns, on important agronomic characters. The three major genes were located on the long arm of chromosome 5A with a gene order of: centromere -bl-q-Vrnl. Analysis of quantitative characters from a winter sowing revealed pleiotropic effects of Vrnl or the effects of closely linked loci on the characters plant height, tiller number and spikelet number. However effects on ear emergence time were not associated with Vrnl but with q as were effects on spikelet number and ear length. In addition a locus determining yield/plant was located between Vrnl and q. Independant loci determining height and ear length were apparent on the short arm of chromosome 5A. From a spring sowing, however, there was a large pleiotropic effect of Vrnl on ear emergence time, as well as the effects previously detected. In addition, associated with q were effects on plant height and grain size which were not expressed from the winter sowing.

The use of irradiated pollen to bring about limited gene transfer in wheat has been investigated. Doses of X-rays of 2Kr, 3Kr and 5Kr were used to generate M1 progeny between maternal and paternal genotypes differing in quantitative and major gene characters. Cytological studies of M1 plants revealed hybrids with widespread aneuploidy and structural rearrangements in the paternal genome. These effects resulted in phenotypic variation between M1 progeny and complex multivalent formation at meiosis. All M1 plants at the 5Kr and 3Kr doses were sterile and all but 2 plants at the 2Kr dose.Studies of the two M2 families from these plants revealed disturbances in genotype frequencies for some of the marker loci with an excess of maternal homozygotes and a deficit of paternal homozygotes. This was also reflected in a more maternal appearance for quantitative characters. These results are interpreted as showing that irradiation damage to the paternal genome in M1 plants results in the differential transmission of maternal alleles.

Chromosome 5B of bread wheat is known to carry two major genes giving rise to genetic disorders, Ne1 for hybrid necrosis and Vg for winter variegation. Additionally, in many european winter wheat varieties this chromosome is represented in a translocated form, with 5BL-7BL, 5BL-7BS chromosomes rather than the normal 5B and 7B forms of the standard variety Chinese Spring. Genetic analysis has been carried out to map these genes and the translocation break point, and to investigate their pleiotropic effects or those of linked quantitative trait loci (qtl) for economically important characters. This was facilitated by the development of single chromosome recombinant lines between a normal and translocated karyotype, and growing these in field experiments over two seasons. There was differential segregation in favour of the translocated karyotype in the population of recombinant lines. Linkage analysis revealed that the two morphological markers and the isozyme locus Ibf-B1 were located on the long arm of 5B with a gene order of: breakpoint - Ne1 - Vg - Ibf-B1. Analysis of quantitative characters using these genes as landmarks showed pleiotropic effects of Ne1 or effects of tightly linked qtl on most of the quantitative characters related to grain yield. An additional qtl determining spikelet and grain number/ear appeared to be linked to the centromere. Effects on ear emergence time were associated with both Ne1 and Vg, and these interacted with environments. Similarly, effects on plant height were associated with Ne1 and Vg. In addition, there was a further unlocated locus (loci) for height acting independently of the markers.

SummaryF2, monosomic analysis involving crosses between the monosomic series of a resistant wheat variety, Chinese Spring, and a susceptible variety, Sicco, has located a major gene locus, designated Dfql, on chromosome 2B of wheat which determines the differential response of these varieties to treatment with the wild oat herbicide, difenzoquat. The allele from Chinese Spring conferring resistance is dominant and studies of the responses of Chinese Spring single chromosome substitution lines and nullisomic–tetrasomic lines for chromosome 2B indicate that this allele actively promotes resistance to the herbicide. It is suggested that this gene may prevent inhibition of DNA synthesis in the apical meristem, which is the site of action of the herbicide (Pallett & Caseley, 1980).Other chromosomes were also implicated as carrying ‘modifier genes’ which affect the ratio of resistant: susceptible plants in F2 monosomic families, namely 1D, 2D, 3A, 3B, 5B and 5D. These chromosomes may affect the retention and translocation of the herbicide to the target site and hence the threshold of response.The simple inheritance of difenzoquat resistance indicates that it should be easy by conventional breeding techniques to transfer the resistance into susceptible varieties.

The responses of wild populations of emmer wheat (Triticum dicoccoides), from different ecogeographical areas of Israel, to three herbicides, difenzoquat, chiortoluron and metoxuron, commonly used on cultivated wheats, were studied. Although cultivated wheats are polymorphic for a response to difenzoquat, all families of all populations of the wild species were resistant. The species was, however, polymorphic for response to both chiortoluron and metoxuron. In addition, there appeared to be differentiation between populations in the frequencies of resistant and susceptible morphs for these herbicides. There was also a close correspondence between the responses of individual families to chiortoluron and metoxuron, which suggests a common genetic control. The implications of these findings for understanding the evolution of herbicide resistance, and for developing strategies for breeding for resistance in the cultivated species are discussed.

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