Association of three highly mutable genes involving pigmentation in Anthirrhinum majus

Abstract

Unstable alleles at three unlinked loci ( delilah, nivea and pallida) can mutate somatically to allow the production of clones of pigmented cells (“flakes”) in particular parts of the corolla. The size of the flakes depends on the timing of the mutations during the development of the flower. The allele delilah results in a lack of purple anthocyanin pigment in the corolla tube; delilah recurrens is an unstable allele manifesting itself by streaks of pigmentation in the corolla tube where mutations to Delilah + occur. The alleles nivea and pallida each result in a lack of purple anthocyanin pigment throughout the corolla, and nivea in addition abolishes the yellow aurone pigment which occurs instead of anthocyanin on the corolla face of the normal flower. The unstable alleles nivea recurrens and pallida recurrens, when mutating somatically to Nivea + and Pallida +, respectively, cause purple flakes throughout the corolla (except the face area), and nivea recurrens when mutating to Nivea + additionally gives rise to yellow flakes on the face. When all three unstable alleles are combined in the homozygous condition in one plant, the frequencies and sized of pigment flakes reflect different combinations of mutational events. The aurone-producing face area shows flakes due to mutation of nivea recurrens alone; the upper and lower lobes exhibit anthocyanin flakes at a rate governed by the timing and frequency of coincident mutations of nivea recurrens and pallida recurrens; and the corolla tube shows cells with pigment only when the three unstable genes have all mutated in the same tissue. It was found that the unstable alleles at the three loci appear to behave independently of one another in two ways: ( 1) the mutation rate of each unstable allele was unaffected by the presence of other unstable alleles in the genotype of the plant; ( 2) coincident mutation of different unstable alleles in the same tissue occurred at a rate approximating randomness. However, preliminary tests indicated that in the doubly homozygous pal-rec, niv-rec plants the niv-rec allele had to mutate prior to pal-rec if anthocyanin was to be produced. This sequence also gave the best fit for the theoretical prediction of the number of coincident mutations. The implications for other systems of these consequences of combined mutation of different genes are discussed.