A REINVESTIGATION OF THE INSTABILITY AT THE YV LOCUS IN TOMATO

The instability at the yv locus on chromosome 6 of Lycopersicon escule ntum has been re-investigated. Cytogenetic analysis of green, green-ye llow variegated and yellow plants showed that the instability is not c aused by the somatic segregation of extra chromosomal fragments as was proposed earlier by Hagemann. Instead, it is postulated that the vari egated line carries an unstable recessive allele of yv (yv(mut)) that mutates frequently from dominant green to recessive yellow on the basi s of the following observations: (i) variegated plants produced varieg ated and yellow offspring in aberrant ratios, and (ii) yellow male ste rile plants appeared in populations derived from crosses of the Yv(ms) line with wild-type plants and the line LA 780, being recessive yv. U sing the isozyme marker Aps-1 linked to yv, it was shown that the defi cit in variegated and yellow mutants, which frequently occurs in F-2 p opulations derived from crosses between the variegated mutant and wild type yv(+) plants, was due to certation. From the analysis of these F- 2 populations, it is concluded that the instability at yv is likely to be controlled autonomously. No evidence was obtained showing germinal reversion events to occur, as inferred from the absence of green plan ts among the selfed offspring. Variegated offspring were produced by a yellow plant which is interpreted to represent a somatic reversion ev ent from yellow to variegated. Remarkably, when variegated plants were crossed to wild-type plants, F-1 plants with a mutant phenotype appea red. On the basis of analogies with mutable alleles from maize and Dro sophila, alternative mechanisms underlying the mutability of yv are di scussed.