Genetic analysis of instability in Petunia hybrida : 1. A highly unstable mutation induced by a transposable element inserted at the An1 locus for flower colour.

A hypothesis is proposed to interpret the results of crossing experiments with unstable mutants of Petunia hybrida having variegated flowers and showing variation in the colour intensity and the degree of spotting of the corolla. It is postulated that the An1 locus, which is involved in anthocyanin synthesis, is composed of a structural gene with an adjoining regulatory region, the latter in turn comprising two components, viz., the 'mutator', responsible for the activation of the structural gene, and the 'expressor', controlling the rate of activity. Unstable An1 alleles originate from deletions induced by a transposable element inserted within the regulatory region. Such deletions extend from one of the ends of the inserted element across the adjacent DNA and thus may include parts of the 'expressor' and the 'mutator'. Reversions result from repair of the deletions, the inserted element not necessarily becoming lost in the process.