The regulation of gene mutation in plants

Abstract

This discussion is based largely on work on mutable genes in maize by McClintock (1946-68, 1965), Brink and his associates (Barclay & Brink 1952; Brink & Nilan 1952), and Peterson (1966), with some points illustrated by reference to my own studies on Antirrhinum majus in collaboration with B. J. Harrison at the John Innes Institute and G. R. K. Sastry at the University of Leeds. The phenomenon of high mutability is, I believe, likely to have the same basis in Antirrhinum (and in numerous other flowering plants) as in maize. I shall not be dealing with the equally interesting and possibly related phenomenon of paramutation , described by Brink (1964). The mutability we are concerned with is set apart from most gene mutation by its extraordinarily high frequency. One finds alleles at well-known gene loci in both maize and Antirrhinum which mutate thousands of times in the development of every plant carrying them so that, if the gene in question controls pigmentation, numerous differently coloured spots and sectors of various sizes appear throughout the plant. All the examples I shall be mentioning concern anthocyanin pigmentation in the maize seed or in the epidermal cell layer of Antirrhinum. The mutations affect not only these visibly pigmented tissues but also the subepidermal cells, which do not normally form pigment themselves but give rise to germ cells which can transmit an altered capacity for pigment synthesis to the entire plant in the next generation. In some cases the frequency of mutations among the germ cells can amount to several per cent and in our Antirrhinum system we have occasionally encountered frequencies of more than 50 %. We are dealing here with a process which is at least 3 or 4 orders of magnitude more frequent than any reasonable estimate of the general frequency of errors in DNA replication.