BGN 2: Building up genetic stocks of marker genes. I. Transfer to the Bonus genotype BARLEY GENETICS NEWSLETTER, VOL. 2, III. GENETIC AND CYTOLOGICAL TECHNIQUES
Persson and Hagberg, pp. 115-119

III.3. Building up genetic stocks of marker genes. I. Transfer to the Bonus genotype.

G. Persson and A. Hagberg. Swedish Seed Association, Svalöv, Sweden.

Introduction

In a recent paper Persson (1969) outlined methods to study recombination between heteroalleles in ert-loci in barley. The use of closely linked markers was discussed as one way to distinguish interallelic recombination from outcrossing. Before the ert-genes can be marked, they must be properly located, and suitable markers must be available The investigations to locate ert-genes have been in progress for a number of years, and now a stage has been reached that will permit the initiation of a program to mark these genes. Almost all the barley chromosomes are well supplied with markers, several of which are closely linked with ert-factors. The stocks carrying the markers have very different genotypic backgrounds. Thus, some of them have more or less dense spikes. As density is a quantitative character, the marker stocks and ert-mutants should be more or less isogenic to avoid interference of modifying genes affecting spike density. Since most of the ert-alleles studied have been induced in the Bonus genotype, the marker genes have been transferred to this genotype.

The marker genes have been transferred to the Bonus genotype by repeated backcrossing, using Bonus as the recurrent parent. Five backcrosses or totally, six crosses to Bonus were considered sufficient. To insure the complete recovery of the Bonus genotype, a greater number of crosses are desirable, as some blocks of linked genes may be difficult to break. Selection in segregating generations after the first backcrosses is recommended (Briggs and Allard, 1953). This kind of selection has not always been carried out in the present project, but as far as possible one backcross was made every year. The ultimately backcrossed lines, consequently, cannot be expected to be completely isogenic with the ert-mutants. For the further localization of ert-genes and for marking them, however, five backcrosses of the original marker lines should be sufficient in most cases.

Thus, a backcross program involving 21 marker genes was started in 1956. The complete work was carried out on field material with one generation being grown every year. A second program involving nine genes was started in 1963, when the first program was more or less completed.

There is a great difference in flowering time between Bonus and several marker stocks, the markers often being more than one week earlier. This caused some difficulties in getting both parents in the proper stages simultaneously for crossing. To facilitate the first crosses, ert-o16 was used instead of Bonus. Ert-o16, being one week earlier than Bonus, is an early induced ert-mutant in Maja barley. The last four crosses were always made with Bonus as the recurrent parent, and selection was made for late phenotypes to eliminate the factor for earliness.

The markers transferred

Of the factors in chromosome 1, three are chlorophyll factors, viz. YcYc, Ac2ac2 and Fcfc. YcYc and Ac2ac2 are not viable as homozygotes, but have to be increased as heterozygotes. Fcfc is viable as the homozygote, but fcfc plants are rather weak, and a deficit of this genotype in segregating populations is common. In addition, heading in the homozygotes is late and consequently the homozygote is often difficult to use for crossing.

The most significant characters of brbr plants are short straw and short awns. In populations segregating for Nn, normal vs. naked kernel, some plants may be found that are partially hulless. Probably this depends on partial dominance of the gene. All tested plants with nn constitution have been completely naked. Thus, in classifying plants, intermediate individuals should be classified as normal to obtain the best 3:1 segregation. Lb3lb3 is a factor for long, weak basal internodes in the spike. The phenotypic expression of this gene is distinct. The mutant has been described in detail by Kasha et al. (1960).

Four genes in chromosome 2 Oror, Vv, Trtr and Lklk were back-crossed to Bonus; and backcrossing still another, Lili, is under way. The homozygotes of orange seedling (Oror) are lethal under field conditions, but it is possible to obtain viable and fertile oror plants in the greenhouse. In backcrossing the factor for two-row vs. six-row phenotype, it was somewhat difficult to select plants with fully developed lateral kernels. According to Woodward (1947), vv plants are six-row, independent of which Ii-allele is present. It is clear, however, that vv plants without fully developed kernels can be isolated. Selection of plants with well-developed kernels in the lateral rows had to be made, and at least two such lines were obtained. The penetrance and expressivity of Trtr are complete and non-variable in the original stock. In segregating generations, however, plants with low expressivity are common. This was more and more pronounced as backcrossing progressed, and in the last backcross generations, only two or three flowers in the spike had two or three awns. Apparently, the gene is complex and modifiers are present in the original stock. The best 3:1 distribution is obtained if all plants with a tendency of the mutant character are classified as recessives. Lklk is dominant and the phenotype is very distinct.

Oror is in the short arm of chromosome 2, while Vv, Lklk, Lili and Trtr are in the long arm. Vv and Lklk are very closely linked, and it was very difficult to prove recombination between these two genes (Nilan, 1964).

Backcrossed genes in chromosome 3 are Uzuz, Xcxc, Acac and Anan. The last three are lethal chlorophyll factors. Like br, uz is a dwarf factor. It is associated with a factor for dense spikes, which is a disadvantage in studying ert-genes.

Backcrossing four more genes in chromosome 3 was initiated. These are Ysys (a chlorophyll factor), Stst, Zbzb and Alsals. The expressivity of Alsals in two-row barley seems to be low, and it is probable that the program cannot be accomplished for this gene.

One gene Kk, in chromosome 4 was backcrossed and work is under way for still another, Lb2lb2. K is completely dominant, and the heterozygotes cannot be identified from the morphology of the plants. Recent investigations indicate, however, that identification is possible from chemical analyses. Kk is located in the short arm and Lb2lb2 is probably in the long arm of chromosome 4.

Three genes in chromosome 5 were backcrossed, Trdtrd, Bb and Atat. Atat is a chlorophyll factor, that is not viable as a homozygote. B is dominant and heterozygotes (Bb) are distinguishable from BB plants. All three genes are in the short arm and the sequence is trd-B-at-centromere. Mt3mt3 might also be in chromosome 5 (Kasha et al., 1960). It is inherited independently of Bb and Trdtrd and is therefore possibly a mophological marker in the long arm. Backcrossing was started in 1963.

Chromosome 6 is scantily supplied with genetic markers. Two genes have been backcrossed to Bonus. These are oo and Xnxn. Xnxn is a chlorophyll factor, that is not viable as a homozygote. Oo is closely linked to the centromere, while Xnxn is assigned to the distal part of the long arm.

Of the genes in chromosome 7, Ss and Rr have been backcrossed. Ss is an excellent marker: but Rr apparently is affected by modifiers, just as Trtr is in chromosome 2. Both genes are in the long arm, Rr being distal to Ss.

Linkage data on the genes described above have been published elsewhere (e.g., Robertson et al., 1941, 1947, 1955, 1965; Kasha et al., 1960; Persson, 1969).

Table 1 and Table 2

Table 2 (continued)

Literature cited:

Briggs, F. N. and Allard, R. W. 1953. The current status of the backcross method of plant breeding. Agron. J. 45:131-138.

Kasha, K. J. and Walker, G. W. R. 1960. Several recent barley mutants and their linkages. Canad. J. Genet. Cyt. 2:397-415.

Nilan, R. A. 1964 The cytology and genetics of barley 1951-1962. Monographic Supplement No. 3 Research Studies, Washington State University. 32,1.

Persson, G. 1969. An attempt to find suitable genetic markers for dense ear loci in barley I. Hereditas 62:25-96.

Robertson, D. W., Wiebe, G. A. and Immer, F. R. 1941. A summary of linkage studies in barley. J. Am. Soc. Agron. 33:47-64.

Robertson, D. W., Wiebe, G. A. and Shands, R. G. 1947. A summary of linkage studies in barley: supplement I, 1940-1946. Ibid. 39:464-473.

Robertson, D, W,, Wiebe, G. A. and Shands, R. G. 1955 A summary of linkage studies in barley: supplement II, 1947-1953. Agron. J. 47:418-425.

Robertson, D. W., Wiebe, G. A., Shands, R. G. and Hagberg, A. 1965. A summary of linkage studies in cultivated barley, Hordeum species: supplement III, 1954-1963. Crop Sci. 5:33-43.

Woodward, R. W. 1947. The Ih, I, i alleles in Hordeum deficiens genotypes of barley. J. Am Soc. Agron 39-474-482.

BGN 2 toc
BGN Main Index