The mutational potential of single R alleles.

 

The mechanisms responsible for the wide array of allelic variability found among cultivated races of maize are as yet uninvestigated. The present study was projected in the hope that some insight relative to problems of gene evolution might be gained.

 

Careful selection of parental allelonorphs, in any attempt to evaluate the mutational potential of single loci, merits full consideration. An underlying consideration is the likelihood that different alleles are derived from one another by single or repeated mutations, or by unit steps from some single or few archetype genes. On the basis of what is now known concerning the spontaneous mutational behavior of the gene Rr (Stadler, Genetics 31: 1946), it is conceivable that some parental alleles might yield mutants with intermediate phenotypic effects, while others might either fail to mutate at all, or yield a restrictive class of gene alterations indistinguishable from the bottom recessive. Mendelizing variations of the latter class may well represent intragenic changes, but, in the absence of wholly satisfactory criteria, it is equally plausible that they are, in effect, minute chromtin losses beyond the present level of cytological detection.

 

As a preview to the outcome of this projected study, in which it is planned to test something on the order of half a million gametes for mutation at a single locus, all previously identified mutants from rch were assembled in a single planting for comparison and further study of their properties. The writers are deeply grateful to Professor L. J. Stadler, who generously provided laboratory and field facilities for this study during the summer of 1948.

 

All mutants described here derived from a single extracted rch allele. Consequently, they are somewhat indicative of the evolutionary mutational potential for the locus.

 

From among all Rr and rr alleles studied to date, rch was selected as the parental allelomorph for intensive study, since in plant color phenotype it represents the closest approximation to the wild type form. It is characterized by the broadest, most intense pigmentation pattern in the majority of susceptible tissues of the seedling and flowering plant. But, because this allele falls into the rr category (colorless aleurone series), our study is restricted to mutational changes resulting in altered pigmentation patterns in only the vegetative, diploid tissues. It is hoped that further surveys of new collections will disclose Rr strains equivalent to rch in plant color phenotype. A suitable Rch stock, if found, would provide the nearly ideal parental gene for a study of gene evolution at this locus. Mutational changes, and their interactions, as they affect all aspects of Rch action in the plant, seed, and pericarp, would then be available for analysis. It is a pleasure to acknowledge here the cošperation of Dr. E. G. Anderson, who has supplied us with a large number of new accessions among which Rch alleles are being sought. Seed samples from other investigators, particularly of colored aleurone and cherry pericarp effect, would be appreciated.

 

The tentative designations and salient features of the mutants studied are given in table 1. Mutants, which arose in crosses affected with irradiated pollen, cannot yet be considered as alterations induced by the treatment. Further data bearing on the relative frequencies of the specific mutational changes in treated and controlled material are required before this question can be settled effectively. Such cautioned interpretation applies, in particular, to mutants distinguishable from the bottom recessive by their positive action, and which, moreover, are free from associated changes in gametophytic viability.

 

Table 1. Mutations of rch

 

Mutant

Treatment

Remarks

 

rch-S1

none

transmits low rate of sectorials

rch-S2

"

transmits high rate of sectorials

rch-S5

"

"

rch-X32

X-ray

mutant lost - not pollen transmissible

rch-V1

U.V.

original mutant sectorial - stabilized to rch - not pollen transmissible

rch-V2

"

tests not completed

rch-V10

U.V.

stable intermediate allele

rg -S2

none

wholly green - male, female transmission normal

rg -S3

"

"

rg -S4

"

tests not completed

rg- -X1

X-ray

wholly green - 50% pollen abortion, female transmissible

rg -X6

"

wholly green - tests not completed

rg X7

"

wholly green - approx. 60% pollen abortion

rg -V1

U.V.

wholly green - approx. 30% pollen abortion

rg -V2

"

wholly green - approx. 40% pollen abortion

 

As given in table 1 the mutants fall into four more or less well-defined categories: A. mutation to unstable alleles, B. mutation to stable alleles with intermediate effects, C. mutations to the bottom recessive with normal viability in the gametophyte, D. mutations to the bottom recessive, rg, associated with reduced haplophase viability.