Study of intralocus events leading to R stippled (R ^st) recombinants in Zea mays

The two main components of the R locus, designated P and S condition the production of a red pigment, chemically identifiable with cyanidin 3-glucoside, in the plant and seed tissues respectively. In this paper we present some data on the structural organization in terms of its P and S components of an unstable form of R, designated R ^st, and of two of its derivatives. The superscript st (stippled) refers to a distinctive pattern of aleurone variegation conditioned by R ^st. The instability is seemingly the result of the interaction of the S component of R ^st with an adjacent element affecting its gene action.

The two stippled derivatives analyzed are R ^sk (smoky) that conditions a different pattern of aleurone variegation and R ^sc (self-colored) characterized by a homogeneous aleurone pigmentation. Assuming that R ^st is a gene complex, p S ^st in structure, heterozygous P s / p S ^st individuals should yield P S ^st and p s recombinants, besides the parental P s and p S ^st strands.

Accordingly Ps / p S ^st male parents, carrying appropriate outside markers, were crossed with a p s / p s line. Among the progeny of this cross the exceptional individuals carrying nonparental strands were isolated as presumed intralocus recombinants and their flanking markers constitution was established. This analysis, extended to the p S ^sk and p S ^sc complex shows:

• (1) a disparity in the production of the two nonparental strands, with an excess of p s over P S ^st

• (2) the occurrence of a distinctive morphology in about one half of the exceptional individuals carrying a p s strand

• (3) negative interference in conjunction with intralocus events, on the hypothesis that the latter are the result of reciprocal exchange between genes or the R complex.

The possible origin of the nonparental strands through a mechanism of gene conversion or chromosomal deletion is considered and the evidence in favour or against either hypothesis is discussed.

The occurrence of P S ^st strands offers the opportunity to analyze the effect of S ^st on P gene action.

This test was performed by comparing the pigment concentration and distribution of individuals genotypically P S ^st/p s and p S ^st / P s. The comparison shows that seedlings of the two genotypes differ significantly both in the amount of pigment produced and its distribution. While the former determines a weak pigmentation, phenotypically recognisable as sectors or fine stripes along the primary root, the latter shows a homogencous pigmentation amounting to six times that found in P S ^st seedlings.

These observations seem to indicate that both P and S, when lying in coupling on the same chromosome together with the instability factor, are coordingted in their action by the latter.