PARAMUTAGENIC SYSTEMS IN SOME SOUTH AMERICAN RACES OF CORN

Abstract

RINK (1956) reported the occurrence of an unusual phenomenon at the R locus in maize. The “standard” Rr (colored aleurone, colored plant) allele was observed to have decreased pigment producing capacity after being heterozygous (R‘ RSt ) with a “pattern” allele, stippled (R“) . Additional observations by BRINK and WEYERS (1957) showed that marbled (Rmb), another “pattern” allele, also produced the change if brought into the heterozygous condition with R‘. Both RSf and Rnlb produce green plants, but Rmb has a coarser pattern in the aleurone than R”t. The term paramutation was proposed to describe such changes observed at the R locus and the characteristics of the phenomenon have been reviewed elsewhere (BRINK 1958a). The change was found to be genetic, transmitted through the eggs and the sperm, to occur regularly in Rr gametes derived from R‘ RSt or R‘ Rmb heterozygotes and to revert partially toward the standard R‘ level when made homozygous. BRINK, BROWN, KERMICLE and WEYERS (1960) observed that Rg (colored aleurone, green plant) also could be paramutated. BROWN and BRINK (1960) observed that both paramutated alleles Rr.st and Rg:st acquired some paramutagenic properties from the inducing source. Other paramutagenic sources have been observed. ASHMAN (1960) isolated non-purple (T‘) mutants from Rr R”t heterozygotes and found some of these to be paramutagenic and others not. MCWHIRTER and BRINK (1962) tested 83 self-color (Rsc) mutants of RSt alleles and found that they varied from “strongly paramutagenic” and “weakly paramutagenic” to “non-paramutagenic” apparently in a continuous series. The basis of the phenomenon was found to be chromosomal at the R region and not the result of plastid or other cytoplasmic factor inheritance (BRINK 1958b). A series of 24 South American races of corn with variegated aleurones (marbled-like and stippled-like phenotypes) have been tested and their effects on the standard R‘ are described. The study reported here revealed that paramutagenicity is common in sources with variegated aleurones. A brief report of some of these results appeared earlier (LINDEN 1963).