GrainGenes

III.1 Progress through loss.

G. A. Wiebe, United States Department of Agriculture, Agricultural Research Service, Beltsville, Maryland 20705, USA.

This is an odd and speculative title, but I wish to describe a possible way whereby barley cultivars may be improved by the inactivation of specific genes. The idea is based on the observation that evolution has endowed all organisms, including barley, with a broad variety of genes differing in worth. Consequently, improvements may be possible if we could remove the less worthy genes from the genome. I suggest this might be accomplished by using translocations.

Reciprocal translocations (or interchanges) occur naturally in plants and animals although at a rather low frequency. The frequency, however, can be enhanced by radiation or chemical mutagens. A translocation is a genetic interchange between two nonhomologous chromosomes. The point at which the exchange occurs is called a break point. These occur more or less at random along the length of the chromosome.

Chromosomes are made up of two kinds of chromatin: euchromatin and heterochomatin. The former contains genes that mendelize (genes used by the geneticist and plant breeder), while the latter is considered to be devoid of genes in the usual sense. Euchromatin usually makes up the larger part of most chromosomes.

Break points can be placed into three classes: (1) those where both breaks are in euchromatin (2) those where one break is in euchromatin and the other in heterochromatin and (3) those where both breaks are in heterochromatin. The frequency in each class will be related to the proportion of euchromatin to heterochromatin. In homozygous translocations: class (1) will have two split gene sites, class (2) one split gene site and class (3) no split gene site. The number of split gene sites in class (1) and (2) can conceivably be less in the rare event where the break point falls exactly between two adjacent gene sites for class (1) and between the end of a gene site and heterochromatin for class (2).

The phenotypic effect of an active gene can be described as carried out by transcription of a messenger RNA from the gene site, followed by translation in ribosomes, with the help of transfer RNAs, into an enzyme which operates through a metabolic pathway to give a specific phenotypic effect. No such a result is possible from a split gene site since no meaningful messenger RNA is transcribed. Let me illustrate this for the two gene sites case, class (1).

I visualize split gene sites as a kind of "deficiency" but without any loss of DNA from the chromosome. The exact status of a split gene is unknown. If a messenger RNA is still transcribed it must be of the nonsense type. If there is no transcription should the DNA be classified as heterochromatin?

We have about 400 homozygous translocations in barley and each has been identified as to which two chromosomes are involved. Most of these were produced by radiating pure line cultivars. This is fortunate, for now we can make isogenic comparisons between the pure line parental cultivar and the derived homozygous translocations.

Both favorable (and unfavorable) differences in performance should be related to the genetic alteration at the break point as described here. But this statement is incomplete, since the translocations were produced by radiation other genes in the genome may have mutated to give a difference in performance. Consequently, to rule out the effect of these genes, it is suggested the homozygous translocation be backcrossed to the original pure line cultivar to eliminate the mutated genes. Then, if the difference still exists, it must be due to the genetic alteration at the break point. Favorable translocations of this kind can then be backcrossed into other genotypes to see how universal this advantage is.

Among our collection of homozygous translocations we have those which are lethal and have to be maintained as heterozygotes, those with reduced vigor in varying degrees, those showing new morphological types, and no doubt many cryptic metabolic and physiological changes. Knowing evolution favors a diversity of genotypes, it is conceivable that some should be beneficial, and thus one could occasionally expect "progress through loss" as the title of this paper suggests.

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