Double and triple mutant combinations of bithorax complex of Drosophila.

We have constructed double and triple mutant combinations for the Ubx, abd-A and Abd-B genes of the bithorax complex and have examined the homeotic transformations they produce in the larval and adult patterns. Embryos hemizygous for the triple combination exhibit a metameric pattern consisting of parasegments 5-12 being transformed into parasegment 4. In addition, parasegment 13 develops like a mixture of parasegment 3 and 4, and parasegment 14 is abnormal. The same phenotype is displayed by embryos homozygous for DfP9, lacking all the BX-C DNA, >300 kb. This result strongly supports the notion that the BX-C contains only three genes which account for all the developmental functions of the complex. The phenotypes of the different double combinations also support the same view; the Ubx abd-a comthoracic and several abdominal functions. The abd-A Abd-B combination exhibits the same phenotype of DpP10 DfP9, lacking all the abdominal functions except those specific for A1. Our results also indicate that each BX-C gene becomes active autonomously regardless of the presence or functional state of the other BX-C genes.