[Somatic Conversion (Paramutation) at the sulfurea Locus of Lycopersicon esculentum Mill. : IV. The Genotypic Determination of the Frequency of Conversion].

1. Several lines of Lycopersicon esculentum, which are heterozygous for a mutant sulfurea (sulf) allele vary greatly in the percentage of variegated plants among the heterozygotes. This variegation is caused by somatic conversion (paramutation). The different frequency of conversion is due to the presence of different sulf alleles. Within the sulfurea (pura) (sulf (pura) ) and sulfurea (variegata) (sulf (vag) ) groups there are different alleles, which - though indistinguishable in homozygous condition - can be distinguished by their different conversion activity (paramutagenicity) in heterozygotes with sulf (+). 2. The conversion activity (paramutagenicity) of an allele is expressed by the percentage of green-yellow variegated plants among the heterozygotes (e. g. sulf (pura-90%) means: 90 plants out of 100, which are heterozygous for this particular sulf (pura) allele, are variegated, and 10 are entirely green). 3. The conversion activity (paramutagenicity) of a particular sulf allele can be changed by mutations; it can be either increased or decreased. 4. Crosses have been made between sulf homozygotes (Lycopersicon esculentum, variety Lukullus) and different taxa of the subgenus Eulycopersicon (L. esculentum: marker stocks, German tomato varieties, distantly related varieties from South and Central America; L. pimpinellifolium). Within the subgenus Eulycopersicon the frequency of somatic conversion (paramutation) is - within the range of random and modificative fluctuations - determined only by the conversion activity (paramutagenicity) of the special sulf allele present. Effects of the genetic background could not be demonstrated. Conversion-stable (non-paramutable) sulf (+) isoalleles have not been found in this subgenus. 5. The sulf (pura) group consists of alleles with all possible degrees of conversion activity (paramutagenicity) between 0% and 100% for particular years and average conversion values between 3,6% and 92,9% for several years. The sulf (vag) alleles have a lower conversion activity; its maximum is about 12%. No sulf alleles have been found which have entirely lost their conversion activity. 6. After crossing sulf heterozygotes (L. esculentum) with the distantly related species Lycopersicon hirsutum (subgenus Eriopersicon) and Solanum pennellii significant deviations from the expected 3∶1 segregation for sulf have been found in F 1 and F 3; there is a distinct deficit of sulf sulf seedlings. In F 1 species hybrids somatic conversion (paramutation) occurred very seldom (less than 2%). However in F 2 of both crosses some progenies had frequencies of conversion up to 9,3% (L. esc. x L. hirs.) and 8,5% (L. esc. x Sol. pen.). In F 3 some progenies had frequencies which were slightly higher than those in F 2. In F 4 a progeny has had a frequency of conversion of 61,7%. 7. In F 1, F 2, F 3and F 4 of these species hybrids the sulf (+) allele is from L. hirsutum or S. pennellii and the sulf allele is from L. esculentum; i.e. the system sulf (+) - sulf is always the same. Therefore the differences in the conversion frequency between F 1 and F 2, F 3and F 4 respectively indicate an influence of the genetic background. The genetic background of the subgenus Eulycopersicon allows the full expression of the conversion system sulf (+) - sulf. Genes of L. hirsutum or S. pennellii, however, intensely inhibit the occurrence of somatic conversion in F 1. Genetic recombination in the species hybrids leads to the occurrence of genotypes in F 2, F 3and F 4 which allow conversion to take place more frequently. 8. In the discussion the results obtained with the sulf system of the tomato are compared with those of the analysis of the paramutation systems at the R and B locus in Zea mays, at the cruciata locus in Oenothera and in the rogue heterozygotes of Pisum sativum.