A comparison of the instabilities at the Nivea and Pallida loci in Antirrhinum majus

Abstract

The unstable recurrens alleles of nivea and pallida have been compared for sensitivity to temperature during development. Previous work showed that the palrec allele mutates germinally and somatically with a greater frequency at 15° than at 25° C. The nivrec allele is less sensitive to temperature but somatic instability is still inversely correlated with temperature. There is no difference in germinal instability at the two temperatures for nivrec. The genetic stabiliser controls for palrec and nivrec are independent. Chemical mutagens and X-rays are ineffective in inducing changes in instability of either allele. Plants doubly homozygous for the two rec alleles and for the aurone producing sulfurea gene show that nivrec usually mutates prior to palrec; instances of palrec mutating first have possibly been seen but the flush that frequently occurs in plants homozygous for nivrec prevents a definite conclusion as to whether there is an obligate mutational sequence. An example of paramutation is described. Results vary according to the temperature at the time of the cross and during seed maturation, and depend on the direction of the cross; paramutagenic female parents induce a higher frequency of paramutants. Lines of varying paramutability and paramutagenicity have been established. Somatic paramutation in homozygous nivrec regularly occurs and varies with plant development.