TOPOLOGICAL CONSTRAINTS ON TRANSVECTION BETWEEN WHITE GENES WITHIN THE TRANSPOSING ELEMENT TE35B IN DROSOPHILA-MELANOGASTER

The transposable element TE35B carries two copies of the white (w) gen e at 35B1.2 on the second chromosome. These w genes are suppressed in a zeste-1 (z(1)) mutant background in a synapsis-dependent manner. Sin gle-copy derivatives of the original TE35B stock give red eyes when he terozygous, but zeste eyes when homozygous. TE35B derivatives carrying single, double or triple copies of w were crossed to generate flies c arrying from two to five ectopic zu genes. Within this range, z(1)-med iated suppression is insensitive to copynumber and does not distinguis h between w genes that are in cis or in trans. Suppression does not re quire the juxtaposition of even numbers of zu genes, but is extremely sensitive to chromosomal topology. When arranged in a tight cluster, i n triple-copy TE derivatives, w genes are nonsuppressible. Breakpoints falling within TE35B and separating two functional w genes act as par tial suppressors of z(1). Similarly, breakpoints immediately proximal or distal to both w genes give partial suppression. This transvection- dependent downregulation of w genes may result from mis-activation of the X-chromosome dosage compensation mechanism.