Heredity66 (1991) 251 —257 Received 25 June 1990 Genetical Society of Great Britain Herbicide response polymorphism in wild populations of emmer wheat J. W. SNAPE, E. NEVO,* B. B. PARKER, D. LECKIE & A. MORGUNOV Cambridge Laboratory, JI Centre for Plant Science Research, Colrey Lane, Norwich NR4 9UJ, UK and * Institute of Evolution, University of Haifa, Mt Carmel, Haifa 31999, Israel Theresponses of wild populations of emmer wheat (Triticum dicoccoides), from different eco- geographical 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 sus- ceptible 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. Keywords:emmerwheat, herbicides, polymorphisms. Introduction wheat, Gale & Miller, 1987). Finally, if no variation Theuse of effective and environmentally safe herbi- exists within related species then the techniques of cides is an important component of maintaining high genetic engineering must be used to introduce genes and stable agricultural production. Not infrequently, from wider biological sources including other plant however, new varieties of crop species are susceptible species or bacteria (Oxtoby & Hughes, 1989). to environmentally acceptable and/or widely used Wild emmer wheat (Triticum dicoccoides) is a chemicals for which resistance would be desirable. tetraploid species which is a progenitor of both durum Consequently there is, and will continue to be, a wheats, Triticum durum and bread wheats, Triticum recurring need to introduce herbicide resistance genes aestivum. This species has been shown to be highly into plant breeding programmes as new safer herbi- polymorphic for major gene variation and also highly cides are developed and as weed spectra change. variable for quantitative characters (Nevo et a!., 1984; For any particular crop species the available sources Nevo, 1987, 1988; Nevo & Beiles, 1989). Much of this of genes for herbicide resistance will depend on the variation appears to have an adaptive significance for types of herbicides used for weed control and their the species, and in addition, it can be an important mode of action. A crop series may indeed itself be source of genetic variation for improvement of the polymorphic for responses, as, for example, wheat is cultivated species. As the species is already demonstra- for response to some phenylurea herbicides (Snape & tively variable for a wide range of characteristics, it is of Parker, 1988). In this case, the introduction of resis- interest to examine whether it could be a source of tance is straightforward by utilizing conventional variation for characters not yet examined, such as methods for sexual hybridization in backcrossing pro- resistance to widely used herbicides to which the grammes. If such variation does not exist within the cultivated species are polymorphic or uniformly species then alternative technologies must be susceptible. The present experiments were initiated, employed. As a first step the responses of related, wild therefore, to examine this possibility by studying the species can be examined. If resistance exists in these, response of wild populations of emmer wheat from then the techniques of interspecific hybridization and different ecogeographical areas of Israel to three chromosome engineering can be used to introgress widely used herbicides of cultivated wheat, difenzo- genes for resistance into the crop (see for example in quat, chiortoluron, and metoxuron, to which the 251 252 J. W. SNAPE etal. cultivated wheats display herbicide response poly- field under natural conditions or in a growth room morphisms. under controlled conditions. Field experiments were grown in 1986, 1987 and Materials and methods 1988. Each experiment consisted of a random block design where individual families were represented by microplot rows of five plants. The numbers of repli- Populations studied cates of each family varied depending on the avail- Seventeen populations, taken as representatives of ability of seed, from 1 to 4. Control plots of known different ecogeographic regions of Israel, were chosen resistant or susceptible varieties of bread wheat were for study. These populations were a subset of a much randomized between the experimental families. larger study of genetic variation in T dicoccoides of the The experiments were sown either in the autumn or Middle East region (Nevo & Beiles, 1989). In the late winter and sprayed with the appropriate herbicide experiments each population was represented by a in late spring when the majority of plants had reached sample of families, where each family consisted of the the 4—5 leaf stage, and when weather conditions were progeny produced by the controlled self-pollination of appropriate. Difenzoquat was sprayed at a dose rate of individual plants collected in the wild. In all, 336 2kg ha1 active ingredient (a.i.), equivalent to twice the separate families were tested. recommended dose. Chlortoluron and metoxuron were also sprayed at dose rates twice that recom- mended for farmers, namely 5.5 kg a.i. ha-' and 8.25 Herbicidesused kg a.i. ha ,respectively.These doses were chosen on Threeherbicides commonly used for weed control in the results of previous experiments, which showed that crops of cultivated bread and durum wheat were they gave maximum differentiation between resistant examined. First, two phenylurea herbicides, chlor- and susceptible genotypes. Individual plants of each toluron [3-( 3-chloro-p-tolyl)- 1, 1 -dimethylurea] and plot were scored for response 3—4 weeks after applica- metoxuron [3-( 3-chloro-4-methoxyphenyl)- 1,1 -di- tion. methylurea]. These are widely used for the control of Growth-room experiments were carried out under annual grass and broad-leaved weeds of wheat and simulated day conditions of 16 h light at 20°C and 8 h barley. Variation in the tolerance of bread wheat dark at 15°C. Initially, seed from each family was germi- varieties to chiortoluron and metoxuron has been nated in Petri dishes and then transplanted into 10-cm reported (van Heile et al., 1970), and Snape & Parker pots containing John limes Number 1 potting compost. (1988) showed that the polymorphism for response to Between 4 and 10 plants from each family were sown, chioroluron is controlled by a single major gene locus together with appropriate controls. All pots were then on chromosome 6B. It is of interest, therefore, to individually randomized for each experiment in one examine whether the wild progenitor species is poly- large block. When the majority of plants had reached morphic or whether the polymorphism arose during the 4—5 leaf stage they were removed from the growth cultivation. room and sprayed with the appropriate herbicide at the Difenzoquat [1,2-dimethyl-3,5-diphenylpyrazolium] same concentrations used for the field experiments. is used to control wild oat species in wheat, barley and They were then replaced and rerandomized. Individual rye. Again, differential responses are exhibited within plants were scored for response 3 weeks after applica- bread wheat and have been shown to be controlled by tion. variation at a major gene locus on chromosome 2B In both environments, field and growth room, quali- (Snape et al., 1987). However, varieties of durum tative (resistant or susceptible) scores were taken, rela- wheat appear to be almost uniformly susceptible to this tive to the performance of the controls, as well as a herbicide and only one resistant variety has been found quantitative score of damage, on a 1—9 scale. A score among a sample of world varieties (Leckie, 1989). It is of 1 indicated no perceivable effect of the herbicide of interest therefore to examine whether resistance and a score of 9 severe damage or plant death. In most exists in the wild progenitor species or evolved during experiments it was possible to classify individual cultivation of the bread wheats. families unambiguously and qualitatively as being uniformly resistant, intermediate, or susceptible, or segregating relative to the bread wheat controls. Experimentaltechniques Generally, resistant families correlated with quantita- Totest the responses to all three herbicides, individual tive scores of 1—3, intermediate as 4—7, and susceptible plants for each population were grown either in the as 8—9. HERBICIDE RESPONSES IN WILD EMMER WHEAT 253 Results tions appeared homogenous for resistant genotypes. This result is surprising in view of the almost uniform Response to difenzoquat susceptibility previously observed in tests of cultivated durum wheat varieties (Leckie, 1989). It appears, Totest the responses to difenzoquat, a single experi- ment was grown in the field in 1986. This contained a therefore, that this wild species may be monomorphic sample of 157 families, representatives of 17 different for the resistance allele at the locus for response to populations, together with