Segment-specific organization of leg motoneurones is transformed in bithorax mutants of Drosophila

Abstract

In Drosophila, genes controlling segmentation in the thorax and abdomen are clustered in one region of the genome known as the bithorax complex. Studies of the genetics of this complex suggest that loss of activity of a gene causes transformation of a particular segment to a more anterior one, mesothorax representing the ultimate transformation¹. This transformation is well described for the epidermis, but it is not clear whether other segmentally arranged tissues are also transformed. The segmental ganglia are fused in Drosophila into a single compact mass termed the thoracic ganglion but the segmental organization of the nervous system is still apparent. There are discrete regions of neuropil, termed neuromeres, corresponding to the three thoracic segments: pro thorax, mesothorax and meta-thorax. A small terminal neuromere corresponds to the abdominal segments. Evidence is presented here that the leg motoneurones of each of the three thoracic segments are arranged in a segment-specific pattern in the thoracic ganglion. In mutant flies which have the metathoracic cuticle transformed to mesothoracic, the arrangement of the metathoracic leg motoneurones can be altered to resemble that of the mesothoracic leg motoneurones.