A GENETIC CHANGE ASSOCIATED WITH THE R LOCUS IN MAIZE WHICH IS DIRECTED AND POTENTIALLY REVERSIBLE

Abstract

INTEGRITY of the gene in heterozygotes, first inferred from MENDEL'S (1865) I Pisum data and repeatedly corhrmed since in a wide variety of organisms, is regarded as a universal principle in Mendelian heredity. Departures from the law have been reported from time to time, only to prove unfounded, in most instances a t least, on close analysis. The present study is concerned with a seeming exception to the rule at the well-known R locus in maize conditioning aleurone and plant color. All the RI c? gametes (self-colored aleurone) produced by RrRSt plants (heterozy-gous stippled aleurone) were found to give a different phenotype in testcrosses on rrrr 9 9 (colorless aleurone) than the R' 8 gametes formed by R'R' sibs. The altered form of R' in the Rrrr testcross progeny is transmitted through the succeeding sporo-phyte generation and, therefore, is heritable. The change in Rr is potentially reversible with equal regularity. R' gametes formed by all RrRI plants derived from RrRst heterozygotes by seliing were found to have regained the standard (or near-standard) level of pigment-producing action in test-crosses with rrrr 9 9. The present communication is a preliminary account of this unusual phenomenon. MATERIALS AND METHODS The R' gene used conditions self-colored (purple or red) aleurone when homozygous. The superscript in the symbol (EMERSON, ET AL. 1935) denotes that the allele also produces anthocyanin pigment in vegetative tissues of the plant. The other allele is Rat (stippled), which gives an irregular pattern of mostly small colored spots with sharply defined boundaries in the aleurone, the outer layer of endosperm cells. Aleurone color alone could be scored in the present experiment (see later). The plant color action of the RT and Rat genes was utilized only to identify R'R', &Rat, and RstRat plants in segregating families at the seedling and flowering stages. RrRr plants have red coleoptiles and anthers, whereas these tissues are green in RatRat individuals. The anthers in the heterozygote, RTRat, are light red, although there is overlap with the R'R' class. In order to improve readability of the text the plant color superscripts will be omitted from the gene symbols except as the complete designations are necessary to make the meaning clear. R, therefore, will be substituted for P, and r' will be represented by r. The R gene in question has been in the writer's cultures for ten years or more, without having disclosed any unusual properties. It is evident from STADLER'S (1951) consin.