Re: In the antennapedia state of drosophilla do the legs fuction(on the head)?

Hello Pj!

Thanks for your question!

You've touched on a very interesting point. Unfortunately, there is very little information regarding your question in the scientific literature. However, I have contacted some of the people who work with these mutants and, luckily, they were able to provide me with some answers.

The homeotic, (also called homeobox, or Hox for short) genes (one of which is the Antennapedia gene) comprise a family of evolutionarily conserved genes that specify segment identity in Drosophila. When these genes are mutated, they often result in intriguing phenotypes in which one organ or segment is changed into another. The mutant you refer to in your question harbors a dominant gain-of-function mutation in the Antennapedia (Antp) gene, which transforms the antennae of the fly into legs. In a wildtype (normal) fly, Antp is expressed in the thorax and is thought to specify the identity of the second thoracic segment. However, in the dominant mutants, it is expressed in head as well as the thorax. The result is that the imaginal discs of the head "think" they are in the thorax, so they become legs (which are the appropriate appendages for a morphological thorax) rather than antennae. (Imaginal discs are nests of cells that are present in the Drosophila larva that will give rise to adult structures such as the eyes, legs, wings, and antennae.) Incidentally, in the recessive loss-of-function Antp mutant, the gene is not expressed in the second thoracic segment, and antennae are formed in the place of legs.

One of the reasons for such a lack of published information regarding your question may be that it is difficult to assess leg function in these mutant flies. In a wildtype fly, one could easily monitor general leg function by watching the flies walk. However, given the position of these ectopic legs, it is not possible to make such observations. Therefore, one must conduct more intricate analyses, such as detailed morphological studies and/or physiological measurements of leg function.

In the late 1970's, it was noted that the "homeotic legs" (the legs that are present in place of antennae) in the dominant Antp mutants can function in a reflex response that is characteristic of normal legs. When the homeotic legs are subjected to gustatory stimulation, the fly extends its proboscis just as it would if its normal thoracic legs were stimulated. This led some investigators to examine the neuronal wiring of the homeotic legs. They found some of the neural connections in the homeotic leg to be appropriate for a normal leg. However, it was also noted that the muscles in the homeotic leg are only rudimentary, if present at all. According to the experts I corresponded with, the reality is that these legs are usually quite distorted and probably wouldn't really work even if they did have muscles. A likely reason for this distortion is that mutant flies with ectopic body structures are often somewhat deformed as a result of enclosure in the pupal case during metamorphosis.

I hope this information helps! Please feel free to email me if you have any further questions!

Nikki
nmdavis@fas.harvard.edu