Somatic and meiotic instability of R-stippled, an aleurone spotting factor in maize.

NTHOCYANIN pigmentation of the aleurone layer in maize endosperm A is influenced by the stippled allele of the R locus in two characteristic ways. First, it conditions the irregular distribution of seed pigmentation aptly described by its name (Figure 1). Occasionally the instability is lost in the germ line, yielding a strongly and uniformly pigmented form termed self-colored (Rc). In the nomenclature commonly applied to unstable loci such changes are subsumed under the name mutation. Secondly, stippled reduces the action of sensitive alleles in heterozygotes (BRINK 1956). The reduced level of action is heritable, and also potentially reversible. Studies concerning this unusual phenomenon, termed paramutation, have been reviewed by BRINK (1964) and by BRINK, STYLES and AXTELL (1968). Rather than its paramutagenic property, the primary consideration of the present article is the chromosomal basis for stippled to self-colored mutation and the relation of mutation to aleurone spotting. A useful concept for understanding mutable allele behavior is to consider the somatic and germinal instability in a given case as having a common basis. Those changes which occur early during sporophyte development lend the most direct support to this view. Meristems so affected give rise progressively, first to somatic tissues which evidence the change phenotypically, and then to reproductive tissues which transmit the altered form. This view is also supported, though less directly, by correlated levels of somatic variegation and germinal mutability. Such positive correlations have been observed both where the source of variation is genetic, as in the case of marbled aleurone (Rmb) in maize (WEYERS 1961), and where the variation is of environmental origin (HARRISON and FINCHAM 1964, for example). R-stippled, however, is exceptional in this regard. ASHMAN (1960, 1965) described a modifier of its expression (M") which markedly increased aleurone spotting without influencing germinal mutation. Mutation frequency, on the other hand, was,affected by R locus composition of the parent sporophyte, being elevated about three fold among Rst gametes from R"RSt, as compared with those from RStr plants. A class of self-colored mutations from both genotypes probably has a basis common to that which underlies aleurone spotting. The present study identifies a further class of mutations from RstRSt and certain RSt heterozygotes whose origin is associated with meiotic recombination.