GrainGenes

II.45. Eight paired barley lines.

G. A. Wiebe, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland 20705, U.S.A.; R. T. Ramage, Agricultural Research Service, U.S. Department of Agriculture, Department of Agronomy and Plant Genetics, University of Arizona, Tucson, Arizona 85721, U.S.A.; and R. F. Eslick, Department of Plant and Soil Science, Montana State University, Bozeman, Montana 59715, U.S.A.

Barley lines with eight pairs of chromosomes have been obtained from balanced tertiary trisomics. These lines are genetically stable. The extra pair of chromosomes is derived from a T5-7a translocation.

The steps in going from a normal seven pair barley to one with eight pairs are as follows: In the F2 of the cross, normal x homozygous T5-7a translocation, an occasional plant is obtained with 2n+1 chromosomes, that is, seven normal pairs plus one extra translocated chromosome. Such plants are called balanced tertiary trisomics (BTT), Ramage 1963, 1965, Wiebe and Ramage 1970. In the present case, the extra chromosome is about the same length as a normal chromosome. BTT plants are weak, and the extra chromosome is usually not transmitted through the pollen but is functional in the egg. When selfed, BTT plants give about 70 percent normal diploids and 30 percent BTT plants in their progeny. After several generations of selfing, we found several BTT plants nearly equal in vigor to normal diploids. The progeny from these vigorous BTT's showed two interesting differences from their earlier relatives (1) the ratio of diploids to BTT plants now was 55:45 and (2) true breeding lines with reduced vigor (similar to the original BTT lines) were obtained. Cytological observations showed the extra translocated chromosome had been reduced in length to about 1/3 of its original size. We call this change the "chopper" effect. This reduction in length is interpreted as a probable reason for the increased vigor and for an occasional transmittal of the extra chromosome through the pollen. The latter event results in true breeding eight paired lines when the pollen fertilizes an egg with the same extra chromosome.

At this time, we have no explanation why "chopping" occurs or what the causal agent is. The genes in the extra chromosome are all duplicated on the normal chromosomes of the genome. Therefore, losses sustained by "chopping" are covered, and a lethal condition should not arise. The reduced vigor and partial sterility of the true breeding eight paired lines is likewise not clear since the BTT's from which they came are nearly normal. We have noted, however, a gradual improvement in the eight paired lines with advance in generations. For example, in 1973, at eight locations in Montana, the best eight paired line yielded from 30 to 89 percent of the yield of the local cultivar Unitan at the individual locations and with an overall average of 64 percent. Improved vigor and fertility could be due to many things, including meiotic stability, a further reduction in length of the extra chromosome, adjustments in the background genotype of the diploid complement of the genome, the selection of favorable interactions between the genes on the extra chromosome and those in the normal genome, etc.

Using eight paired lines, one can ask--is it worthwhile to increase the gene pool of barley by five percent, which is approximately the amount added in this case. If one accepts this as a desirable objective and worthy of experimental investigation, then two things must be stabilized (1) meiotic regularity of the extra pair of chromosomes and in synchrony with the normal chromosomes and (2) some chosen fixed length for the extra chromosome in order to prevent it ultimate loss by continuous "chopping".

We feel meiotic regularity can be achieved by "chopping" both ends of the extra chromosome. Pairing of homologous chromosomes during meiosis begins at the ends of the chromosomes and proceeds toward the centromere. Consequently, when both ends of the extra chromosome are missing, no pairing is possible with either of the two parental normal chromosomes from which they arose. However, pairing of the two extra chromosomes can occur because they have similar ends. Cytological observations confirm this.

To maintain a fixed length for the extra chromosome, we propose to use two lethal genes (albinos) as "chopper stoppers". Using the T5-7a case as an example, one of the albino genes, say ax is located on both of the normal five chromosomes and the other albino ay is located on both of the normal seven chromosomes. The corresponding genes for green Ax and Ay are located on the two extra chromosomes in the five and seven parts respectively. The loci of the albino genes are chosen to bracket a segment of the extra chromosome containing the desired genetic information from chromosome five and seven. With this arrangement, a "chop" at either end of the extra chromosome that cuts into the bracketed segment will result in progeny plants that are albino and die. The net result is a population of plants where the length of the extra chromosome is stablized around the length of the bracketed segment.

Our current work on eight paired barleys consists of introducing the extra chromosomes into a large number of different genotypes. This involves crossing normal X eight paired lines and backcrossing the F1 x eight paired lines. Eight paired lines can be obtained from either cross since it has been shown the extra chromosome is transmitted through the pollen though in a low frequency and is obligatory when eight paired plants are used as males. Furthermore, a male sterile gene, not located on chromosome five or seven, is being introduced to facilitate outcrossing in a bulk population at the eight pair level. Later, selections will be made in this population. The objective is to find eight paired plants where the added extra chromosome has found a "happy home"

Twenty-one translocation groups are possible with the seven chromosomes of barley. Within each group, the translocations can have a large part of one chromosome and a small part of the other and vice versa. This can total to a large number of different translocations, and each conceivably could be investigated in a similar way as for T5-7a discussed here. In addition, one can consider making nine paired barleys and even higher numbers. And setting each in a wide variety of genotypes.

The idea of enlarging the genome of barley is an attractive one. The redundancy of genes occurs in evolution, Ohno 1970; and the extra chromosome does this as discussed here. Will this help barley if we add the right things? What are the right things? We have multiple choices in what to add and now a method for doing this. For seed of eight paired lines, write to G. A. Wiebe after the 1974 harvest.

References:

Ramage, R. T. 1963. Chromosome aberrations and their use in genetics and breeding--translocations. Barley Genetics I. Proc. 1st Intern. Barley Genet. Symp., Wageningen 1963. Poduc, Wageningen, The Netherlands.

Ramage, R. T. 1965. Balanced tertiary trisomics for use in hybrid seed production. Crop Sci. 5:177-178.

Wiebe, G. A. and R. T. Ramage. 1970. Hybrid Barley. Barley Genetics II. Proc. 2nd Intern. Barley Genet. Symp., Washington State University, Pullman, Washington.

Ohno, Susumu. 1970. Evolution by gene duplication. 160 pp, illus. Springer-Verlog, New York.

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