In Drosophila, insertions of the P{lacW} transposon led to uncommon express
ion patterns of the reporter genes white and lacZ: gene activity was gradua
lly reduced in the neighboring cells along the dorsal-ventral or proximal-d
istal axes. Using in situ hybridization, the gradual gene expression was sh
own to be caused by P{lacW} insertions in regions 40A (lines G1 and G2) and
69C (line G3) of the Drosophila genome. The expression patterns of genes l
acZ and white were similar in eye cells of each line. In salivary gland pol
ytene cells of G3 flies, the lacZ expression was also gradually reduced, wh
ich indicates the existence of a mechanism maintaining the gene activity gr
adient endomitoses.