AC-INDUCED INSTABILITY AT THE XANTHOPHYLLIC LOCUS OF TOMATO

To detect genomic instability caused by Ac elements in transgenic toma toes, we used the incompletely dominant mutation Xanthophyllic-1 (Xa-1 ) as a whole plant marker gene. Xa-1 is located on chromosome 10 and i n the heterozygote state causes leaves to be yellow. Transgenic Ac-con taining tomato plants which differed in the location and number of the ir Ac elements were crossed to Xa-1 tester lines and F1 progeny were s cored for aberrant somatic sectoring. Of 800 test and control F1 proge ny screened, only four plants had aberrantly high levels of somatic se ctors. Three of the plants had twin sectors consisting of green tissue adjacent to white tissue, and the other had twin sectors comprised of green tissue adjacent to tissue more yellow than the heterozygote bac kground. Sectoring was inherited and the two sectoring phenotypes mapp ed to opposite homologs of chromosome 10; the green/yellow sectoring p henotype mapped in coupling to Xa-1 while the green/white sectoring ph enotype mapped in repulsion. The two sectoring phenotypes cosegregated with different single, non-rearranged Acs, and loss of these Acs from the genome corresponded to the loss of sectoring. Sectoring was still observed after transposition of the Ac to a new site which indicated that sectoring was not limited to a single locus. In both sectored lin es, meiotic recombination of the sectoring Ac to the opposite homolog caused the phenotype to switch between the green/yellow and the green/ white phenotypes. Thus the two different sectoring phenotypes arose fr om the same Ac-induced mechanism; the phenotype depended on which chro mosome 10 homolog the Ac was on. We believe that the twin sectors resu lted from chromosome breakage mediated by a single intact, transpositi on-competent Ac element.