The distribution of Ultrabithorax transcripts in Drosophila embryos.

We have used in situ hybridization to monitor the distribution of Ultrabithorax (Ubx) transcripts during the early stages of Drosophila embryogenesis. When first detectable, in the late syncytial blastoderm, Ubx transcripts are broadly distributed from 20 to 50% egg length (measured from the posterior pole). By the completion of cellular blastoderm formation, a precisely bounded zone one segment wide is defined by the accumulation of high levels of Ubx transcripts. This zone is probably the primordium for parasegment 6, that is, the posterior compartment of the third thoracic segment (T3) and the anterior compartment of the first abdominal segment (A1). Following gastrulation, seven metameric units of the germ band express Ubx transcripts in most or all cells of both the ectoderm and the mesoderm. This principal domain of Ubx expression is sharply bounded at superficial grooves in the embryo which probably mark the A/P compartment boundaries in T3 and A7, and so consists of parasegments 6-12. Outside this region, Ubx transcripts can be detected in only a few cells of the ectoderm of parasegments 5 and 13, and in the amnioserosa. This distribution suggests that the major role of Ubx in the early embryo lies in the morphological domain generally ascribed to the control of the bithoraxoid gene. Later in embryogenesis, the pattern of Ubx transcript accumulation is different in each of the major germ layers, suggesting that different controls regulate Ubx in each. In the ectoderm, but not in the mesoderm, Ubx transcripts accumulate differentially in anterior and posterior compartments. Probes prepared from different regions of the Ubx transcription unit show related but different patterns of hybridization. These patterns suggest that the 3.2-kb Ubx transcript is distributed throughout the wider domain of Ubx expression, but that the 4.7-kb transcript accumulates preferentially in parasegment 6.