A test for cell autonomy, based on di-cistronic messenger translation

We have devised a test for cell autonomy of a gene that is switched on ectopically in a clone of cells, allowing us to ask whether the wild-type activity of this gene can influence neighbouring cells. To switch on the test gene, we used the yeast FRT system, and marked the FRT-generated cell clone by co-expressing beta-galactosidase. Co-expression is achieved by a stretch of 5' untranslated mRNA from the homeotic gene Ultrabithorax (Ubx), which is inserted between the two coding sequences. We show that this Ubx sequence mediates efficient and reliable di-cistronic mRNA translation in wing imaginal discs of Drosophila. Applying our test to Ubx, we find that ectopic Ubx in wing discs strictly coincides with beta-galactosidase expression. Consequently, wing cells are transformed into cells that appear to be intermediates between wing and haltere cells, contesting the view that homeotic genes act as binary switches.