EFFECT OF AN ABNORMAL KNOB-CARRYING CHROMOSOME 10 ON PARAMUTATION OF Rr IN MAIZE

Abstract

THE R' factor in maize, conditioning anthocyanin formation in seed and plant, is highly sensitive to alteration in pigment-producing action in heterozygotes with the stippled (R) or the marbled (Rmb) allele, when present in a structurally normal chromosome 10 (BRINK 1956; BRINK and WEYERS 1957). The R'rV kernels resulting from testcrosses of standard R'R', RrrT, or R'rg plants on rgfl females (colorless aleurone, green plant) are darkly mottled, whereas the corresponding class of seeds from r+OO x RrRst8 and TOflO x R'Rmb 6 matings are weakly colored. The reduction in pigment-producing action that R' invariably undergoes in RTRst and R'Rmb plants is heritable and has been attributed to a kind of genetic change at, or near, the R' locus termed paramutation (BRINK 1958). Recent studies, the detailed results of which are unpublished, show that the sensitivity of R' to paramutation in heterozygotes with stippled is greatly altered by reciprocal translocations involving the long arm of chromosome 10, in which the R locus resides. The present report is concerned with a related investigation, namely, the effects on R' action following insertion of the factor into a chromosome 10 differing from the normal in possessing a unique terminal segment on the long arm that contains a large heterochromatic knob. Abnormal chromosome 10: The knob-bearing, abnormal chromosome 10 in question, designated K10, has been found in several races of maize from Latin America and the southwestern United States and in a teosinte strain from Chapingo, Mexico (LONGLEY 1937,1938; RHOADES 1952). RHOADES (1942) observed that the proximal portion and a small distal portion of the distinctive terminal segment on the long arm of K10 are euchromatic. A conspicuous heterochromatic knob, about equal in length at pachytene to the sum of these euchromatic parts, lies between the latter. RHOADES found also that the proximal euchromatic part of K10 which is associated at pachytene with the distal one sixth of the normal chromosome 10, in K10/10 heterozygotes, differed in chromomere pattern. EMMERLING (1959) has identified three prominent chromomeres in this region that are not present in normal 10 and has observed that another, less conspicuous, chromomere lies at the base of the knob. Pairing between K10 and normal 10, in K10/10 heterozygotes, ends in the region between