Differential expression of bithorax complex genes in the absence of the extra sex combs and trithorax genes.

Each body segment of Drosophila follows a unique developmental pathway, controlled by the selective expression of homoeotic genes such as Sex combs reduced (Scr)and the bithorax complex (BX-C). Little is known about the regulation of these genes, though several potential activators or repressers have been described. For instance, absence of the extra sex combs (esc) gene product apparently causes adventitious expression of all the BX-C genes in most or all larval body segments. Absence of the trithorax (trx) gene appears to prevent Scr and BX-C expression but only in adult cells; differentiation of the larval segments is only slightly affected. I show here that the correct segmental differentiation of the larva does not require maternally deposited trx+ product, but that the esc mutant phenotype is suppressed by the removal of the trx gene, which implies that the BX-C can be differentially expressed in the absence of both the trx gene and the esc gene product.