Patterns of temperature sensitivity in Contrabithorax/Ultrabithorax heterozygotes of Drosophila

Contrabithorax (Cbx) is a dominant homeotic mutant of Drosophila which transforms wings to halteres, while Ultrabithorax (Ubx) is a dominant mutant which transforms halteres to wings. Therefore $\text{Cbx}\text{Ubx}$ flies carry dominant homeotic mutants engaged in opposing transformations. This article reports that $\text{Cbx}\text{TM2 Ubx}{}^{130}$ is temperature sensitive. At 29°C, flies express strong Cbx transformation of wings, and minor Ubx transformation of halteres. Larvae shifted to 17°C prior to 72 hr express strong Ubx transformation of halteres toward wings, and slight Cbx transformation of wings. Seventeen-degree temperature-pulse experiments show that the $\text{Cbx}\text{TM2 Ubx}{}^{130}$ system is temperature sensitive continuously during embryonic and larval life. Expression of the Cbx transformation in left and right wings is highly correlated in all conditions studied, as is expression of the Ubx transformation in left and right halteres, but the Cbx transformation in wings and Ubx transformation in halteres can be negatively correlated or uncorrelated. The temperature sensitivity in $\text{Cbx}\text{TM2 Ubx}{}^{130}$ is not found in $\text{Cbx}\text{Ubx}{}^{\mathrm{61D}}$, and Ubx is only weakly expressed in $\text{Sb}\text{TM2}$Ubx¹³⁰ flies. These results show that Cbx in trans to Ubx can enhance Ubx expression, although Cbx also causes an opposing transformation to Ubx; that the $\text{Cbx}\text{Ubx}$ system acts in both the mesothorax and metathorax to modulate their phenotypes; and that both transformations are broadly temperature sensitive through embryonic and larval life. This suggests that the $\text{Cbx}\text{TM2 Ubx}{}^{130}$ system functions continuously during embryonic and larval development to maintain mesothoracic and metathoracic commitments. The results are interpreted in terms of a $\text{Cbx}\text{Ubx}$ feedback loop which maintains the mesothorax in a state of low $\text{Cbx}\text{Ubx}$ activity, and metathorax in a state of high $\text{Cbx}\text{Ubx}$ activity.