A homoeotic mutation transforming leg to antenna in Drosophila.

Abstract

Insects are thought to have evolved from millipede-like ancestors composed largely of a series of identical, leg-bearing segments. This view of insect evolution is supported by the existence of homoeotic mutations which transform particular abdominal and head segments into thoracic segments1–3. Such transformations are described as atavistic3, because they return specialized segments to a more primitive condition. In Drosophila, several dominant mutations of the homoeotic locus Antennapedia (Antp) lead to a transformation of the antenna to the second leg4–8. Here, I describe the isolation and characterization of apparent null alleles of the Antp locus. These mutations lead to a homoeotic phenotype which is the reverse of the dominant Antennapedia phenotype, namely, they result in the transformation of the second leg into an antenna but do not alter the development of the normal antenna itself. This result indicates that (1) the product of the wild-type Antp gene is normally active in the mesothorax where it promotes a mesothoracic pathway of development instead of an antennal pathway, (2) the Antp+ gene product is normally absent or inactive in the antenna, and (3) dominant mutations of the locus result in the inappropriate activity of the wild-type gene product in the antenna, and hence in the transformation of antenna to leg. Thus, unlike most other homoeotic gene products, the product of the Antp+ gene seems to promote, not to repress or modify, an atavistic condition.