The chlorophyll-deficient mutant sulfurea
(an example of somatic gene conversion)

Hagemann, R. (submitted by H. Stubbe)

A chlorophyll deficient tomato plant, which appeared in R2 in 1949, was studied genetically and cytologically in the following years. The name sulfurea and the symbol sulf have been applied to this mutant. Some of the results, obtained by genetic analysis, are reported preliminarily as follows.

The homozygotes:

The descendants of the mutant were crossed to control plants of the same line (variety Lukullus). From the F2 and F3 of these crosses several different alleles were, isolated.

    I. Group of alleles sulfureapura (sulfpura1, sulfpura2, etc.) The cotyledons of homozygous sulfpura seedlings being greenish-yellow immediately after germination, soon become pale yellow (=sulfurea). These seedlings die within a fortnight. However, homozygous sulfpura plants may be kept alive by grafting them on control stocks and then they are able to form fruits. The grafted homozygous sulfpura plants have pure (=pura) yellow leaves.

    II. Allele sulfureavariegata (sulfvag) many of the homozygous seedlings die, as in the case of sulfpura. But some homozygotes survive and grow into plants with variegated leaves, which may also have fruits. When grafted for keeping all alive, homozygous sulfvag plants form green-white variegated (=variegata) leaves. This variegation seems to be a modificative pattern occurring in idiotypically identical cells.

The heterozygotes:

The heterozygotes (sulf+sulf) have green cotyledons not to be distinguished from those of normal (sulf+sulf+) plants. In further development a part of the heterozygotes show green-white variegated foliage leaves. The cooperation of mutated non-mendelian, extrachromosomal factors in this variegation could be excluded. The progenies of green heterozygous plants and the progenies of green and variegated branches of variegated plants have been tested. This analysis demonstrated the variegation of the foliage leaves to be produced by mutation of the normal allele (sulf+) to a sulf allele. In the pale or white regions of the leaves the genotype (sulf+sulf) has altered to (sulf sulf). This mutation occurs in the somatic tissues of the shoot; it also extends to generative tissue, if the mutated regions take part in the formation of reproductive organs. The sulf+ allele is by no means unstable or mutable in itself. On the contrary, in homozygous condition it is stable. In heterozygous condition, however, with a definite frequency this sulf+ allele begins to mutate to a sulf allele under the influence of the already present sulf allele. Such a type of mutation has been termed "conversion" by H. Winkler (1930). In the present case it is a somatic conversion.

The sulf alleles, as isolated till now, differ by the percentage in which they produce variegation, i.e. conversions, in heterozygotes. For example:

sulfpura385%
sulfpura470%
sulfpura16%
sulfvag1.5%
The opposite process, the mutation of (sulf+sulf) to (sulf+sulf+), according to all hitherto existing results, is completely lacking. The numerical proofs of the findings reported above together with other genetic and cytological studies will be presented in a subsequent paper.