Evolution of Hox protein repression function controls the differences in limb number between an insect and a crustacean. M.R. Ronshaugen , W. McGinnis. Dept. Cell and Dev. Biol., Univ California, San Diego, La Jolla, CA.

   We are interested in the molecular mechanisms that account for major innovations in morphological evolution. Molecules in the Hox patterning system are excellent candidates to serve this role. Indeed, changes in Hox expression patterns among different species have been correlated with minor or major changes in morphology. We wished to test whether naturally evolved changes in Hox protein sequences between different species alter morphology.
   Hexapods like Drosophila possess six limbs. Crustaceans like Artemia , have limbs on all segments in the trunk. In Drosophila melanogaster, the Hox proteins Abdominal A (AbdA) and Ultrabithorax (Ubx) repress abdominal limbs, in part through the direct repression of Distal-less (Dll) transcription. In the crustacean Artemia franciscana AbdA /Ubx proteins do not repress Dll expression in the limb-bearing segments. This might be due to variation in cis regulatory regions of Dll (and other genes) so that they no longer respond to Ubx/AbdA. Or, variations in Artemia Ubx/AbdA protein sequences may account for the inability of the Artemia proteins to repress limbs.
   We find that in contrast to the Drosophila abdominal promoting proteins, ectopic expression of the Artemia Ubx and AbdA proteins in the thorax of Drosophila embryos does not repress Dll expression or the formation of larval limbs. There are no detectable changes in DNA binding properties between the Artemia and Drosophila proteins. Experimental results using chimeric Ubx/AbdA proteins and in vivo functional assays indicate that Drosophila has evolved novel transcriptional repression domains that reorganize the body plan.