The location of Ultrabithorax transcripts in Drosophila tissue sections.

I have used in situ hybridization to map the distribution of transcripts of the Ultrabithorax unit in tissue sections of Drosophila embryos and larvae. The results confirm the prediction of Lewis that genes of the bithorax complex will show segmentally regulated patterns of expression. Transcripts of the Ultrabithorax unit are detected in derivatives of the ectoderm and in some derivatives of the mesoderm, but not in the endoderm. In each of these tissues the transcripts are found in the derivatives of some segments but not of others. In third instar larvae they are most abundant in the imaginal discs of the third thoracic segment, and in a region of the central nervous system that includes parts of the metathoracic and first abdominal neuromeres. They are also detected in the nuclei of polytene cells of the larval epidermis, principally in the third thoracic and first abdominal segments, and in the nuclei of larval muscles in the first six abdominal segments. In late stages of embryogenesis the central nervous system is the most prominently labelled tissue; within it transcripts are found only in the neuromeres of the thoracic and abdominal segments. They are most abundant in a region which includes parts of the neuromeres of the metathorax and the first abdominal segment.