The effects of X rays on the bronze and shrunken loci in maize.

INCE the discovery by MULLER with Drosophila (1927) and by STADLER with barley (1928) that the frequency of visible and lethal mutations is greatly increased by X-irradiation, a wide variety of organisms has been subjected to X-ray treatment by different investigators to elucidate the nature of the induced genetic changes. Although it is generally accepted that the majority of these mutations are associated with chromosomal alterations ranging from gross structural changes to minute deletions and inversions, there are discordant results and divergent opinions on whether or not X rays can also effect intragenic changes. The earliest work by PATTERSON and MULLER (1930) and by TIMOFEEFF-RESSOVSKY (1932) indicated that reverse mutations could be induced at selected loci in Drosophila. These induced reversions were assumed to be diagnostic of intragenic mutations. Contrary results, however, were obtained by LEFEVRE (1950) in extensive experiments similar to those mentioned above. Subsequent data (MULLER and OSTER 1957; LEFEVRE and GREEN 1959; GREEN 1961) showed that X ray-induced reversions do occur in Drosophila and that the earlier negative data may be explained in terms of the stages at which cells were treated or by the particular alleles employed. The evidence from studies with maize is in agreement with that reported by LEFEVRE (1950) in his Drosophila work. In an extensive experiment, STADLER (1944) was unable to induce somatic reversions of the a, allele. Other investigations by STADLER and ROMAN (1948) dealt with the production by X rays of direct mutations at the A , locus. Among approximately 415 plants which exhibited loss of the A phenotype, three appeared as putative intragenic mutations and were subjected to extensive genetic and cytological tests. All three proved to be minute deficiencies which included the A locus, rather than intragenic alterations. From other studies (EMMERLING 1955; NUFFER 1957) no unequivocal evidence has been found for the induction by X rays of intragenic mutations in maize. Unlike the results in maize, evidence in Neurospora indicates that intragenic mutations are produced by X-irradiation (GILES, DE SERRES and PARTRIDGE 1955; DE SERRES 1957). In fact, such alterations have been partially characterized at the nucleotide level (MALLING and DE SERRES 1967).