Perspectives in chromosome manipulation

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Abstract
Three categories of chromosome manipulation are discussed, with examples from hexaploid wheat. First, uncontrolled events, such as intergeneric translocations induced by mutagenic agents, have been frequently isolated but have been infrequently incorporated into widely grown varieties. A method proposed by K. W. Shepherd is designed to select an accommodating genetic background for these interchanges and thereby overcome this deficiency. This relies on selection for yield among many selections that are homozygous for a translocation but segregating for many other genetic components. The second category involves manipulations associated with the distinctive genetic activity of chromosome 5B. ph mutants are expected to play an important role in the transfer of genetic material from other genera to wheat. A method described by E. R. Sears is designed to isolate a small intercalated alien segment. This relies on crossing-over in an alien segment that is common to two distinct types of homoeologous-exchange chromosomes. The third category involves manipulations associated with a male-sterility mutation on chromosome 4A and the distinctive genetic activities of this chromosome. A method, described by the author, is designed to produce hybrid wheat with an induced male-sterility mutation on this chromosome. Fertility restoration is being attempted with four chromosomes, a restriction on which is that they must not pair with the other chromosomes of the wheat complement. These are a modified 4A -2R translocation chromosome, part of the cereal rye genome, chromosome 4 of barley, and chromosome 4 of diploid wheat. It appears that chromosome 4A originated elsewhere than from diploid wheat; thus the A genome of common wheat arose from at least two species. As detected by M. A. Hossain and the author, part of the genetic material for male fertility on 4A has a counterpart on rye chromosome 2R. An exchange between the chromosomes of homoeologous groups 2 and 4 appears to have occurred, perhaps at the diploid level.

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