RESURGENCE OF GENETIC INSTABILITY IN ANTIRRHINUM-MAJUS

The frequent somatic change pallida-recurrens (pal-rec) → Pallida (Pal) in Antirrhinum majus to give full or spotted pigmentation varies inversely with temperature and is regulated by a stabiliser (st-St) system. Some Pal/Pal lines derived from highly unstable pal-rec/pal-rec had many pale sites indicating a resurgence of genetic instability. F1s between these resurgent (R) lines and lines homozygous recessive for different pigment-controlling genes showed numerous acyanic and occasional unstable rec areas. The frequency of these sites on the flowers also varied inversely with temperature and with the pal-rec stabiliser constitution. The application of 0.25% caffeine quadrupled the frequency of pale sites in R/R plants. Unstable somatic rec-type sites occurred de novo in crosses of R/R/ with 4 acyanic (or partially acyanic) genotypes. The administration of chemically defined precursors to the sites identified the genes involved somatically. Two contrasting types of instability in A. majus are described and compared. In the first, a blocked pigment-producing gene resumes its function (e.g. pal-rec → Pal) to give pigmented sites against an acyanic background. In the second, acyanic (or pale) sites are seen against an otherwise fully pigmented background. These occurrences together with the occasional appearance of new rec-type sites in crosses with the resurgent R/R lines are considered in terms of transposition of controlling elements. © 1979.