Tumorous-head-type mutants of the distal bithorax complex cause dominant gain and recessive loss of function in Drosophila melanogaster.

The homeotic mutants tuh-3, SGA62, I127, and R17.32 are located in the abdominal portion of the bithorax complex in Drosophila melanogaster. The genes act as semidominants in the head, causing a gain of expression of leg, genitalia, tergite, or combinations of the structures. SGA62, tuh-3, and I127 additionally act as recessives in the posterior abdomen, causing a loss of these functions. Each mutant has its own focus of expression in the head region. I127 and tuh-3 can cause head defects only in the presence of the recessive maternal effect allele tuh-1h, while SGA62 and R17.32 cause head defects in the absence of this maternal effect. In the presence of the dominant tuh-1g maternal effect allele, the tuh-3 and I127 heads are normal. However, these flies now show a genital disc defect. SGA62 homozygotes have abdominal segments 6 and 7 transformed into a more anterior segment irrespective of the maternal effect in force. R17.32 appears to map to the abd-A domain, while SGA62, I127, and tuh-3 belong to Abd-B. SGA62 is located in the morphogenetic element of Abd-B, while I127 has characteristics of mutants affecting both the morphogenetic and regulatory elements of Abd-B. The tuh-3 mutant is within the regulatory element. The maternal effect alleles appear to interact specifically with mutants in the regulatory region of Abd-B.