Paramutation at the sulfurea locus of Lycopersicon esculentum Mill. : VII. Determination of the time of occurrence of paramutation by the quantitative evaluation of the variegation.

1. In tomato plants heterozygous for a mutant allele of the sulfurea (sulf) locus paramutation may take place: under the influence of a paramutant sulf allele, the paramutable wild type allele sulf (+), which is present in the same nucleus, is heritably altered with a definite frequency to a sulf mutant allele, either of the sulf (pura)or the sulf (vag)group. 2. A number of the sulf (+) sulf heterozygotes remain entirely green during their whole ontogenetic development (type I plants, without paramutation). However, others of the plants become variegated: these variegated plants contain-apart from green sectors - only yellow-green speckled sulf (vag)sectors (type II plants), or only pure yellow sulf (pura)sectors (type III plants) or both sulf (vag)and sulf (pura)sectors side by side (type IV plants). 3. For all variegated plants (types II, III and IV) we determined the sizes of the green and of the paramutant sulf (vag)and sulf (pura)sectors and made a statistical analysis of the values obtained. 4. We conducted observations over a period of three years and obtained following findings: type II plants (with sulf (vag)sectors) have an average size of the paramutant sectors of 27.9% (the whole plant being 100%). Type III plants (with sulf (pura)sectors) have an average sector's size of 25.7%, whereas the size of the paramutant sectors in type IV plants (with both sulf (vag)and sulf (pura)sectors) amounts to 54.4% (35.7% sulf (vag)and 18.7% sulf (pura)). Thus, the occurrence of tissues of both phenotypes in one plant has, on the average, been found to be correlated with a doubling of the proportion of paramutant sectors in that plant. 5. Within sulf (+) sulf heterozygotes there is, in general, a positive correlation between the frequency of paramutant plants and the proportion of paramutant sectors within the plants. This is mainly due to the fact that there is a significant positive correlation between the frequency of type IV plants and the frequency of paramutant plants, i.e. the more plants within a progeny variegated, the greater the frequency of type IV plants containing both sulf (vag)and sulf (pura)sectors. 6. These findings (mathematically analysed and compared with the consequences of several models) may result in the following concept: the paramutation processes in sulf (+) sulf heterozygotes are restricted to a small group of cells (16 cells at the most) during a short period of about three cell generations after seed germination and expansion of the cotyledons. In the course of which, the probability for the occurrence of paramutation decreases rather quickly from one cell generation to the next. These characteristics of paramutation processes mentioned cause the occurrence of rather large and well defined sectors of paramutant tissue.