The role of position in determining homoeotic gene function in Drosophila.

Homoeotic mutations of Drosophila lead to the replacement of one structure by another, for example, Antennapedia replaces the antenna with a mesothoracic leg and bithorax produces an anterior wing instead of the anterior haltere. The transformed structures differentiated by the homoeotic mutants are essentially normal--only the position in which they appear is abnormal. The mutant phenotypes suggest that in normal development homoeotic genes are involved in developmental alternatives and contribute to a genetic address that defines the particular developmental pathway taken by a primordial group of cells. Thus, in the absence of homoeotic gene function, primordia in different positions should follow the same basic developmental pathway. This is indeed the case for embryos that show no activity of bithorax genes; thoracic and abdominal segments develop as mesothorax. The simplest view on homoeotic gene function is that the genes act selectively on primordia depending on their position in the embryo. To test this hypothesis, we used a mutation at the Antennapedia locus, Antp, which transforms the antenna into a mesothoracic leg, and we observed the function of the homoeotic genes engrailed and Ultrabithorax in two apparently morphologically identical appendages which develop from primordia in different positions. Our results indicate that position is the relevant factor in the function of these two homoeotic genes.