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The appendages of arthropods and vertebrates are not homologous structures, although the underlying genetic mechanisms that pattern them are highly conserved. Members of the evolutionarily conserved Sp family of transcription factors control appendage development from flies to humans. In Drosophila, the two Sp family members Sp1 and Btd act redundantly in the specification of leg fates and Distalless (Dll) activation. We have previously shown that Sp1 also controls the growth of the leg, in part, though the regulation of Notch activity. This family of Sp transcription factors is characterized by the presence of the BTD-box, a conserved 10 aa long domain next to the tripled-C2H2 zinc finger which function is unknown. We have analyzed in vivo the role of this conserved domain during appendage development by generating specific Sp1 and btd mutant alleles. In addition, we have studied a new role for Sp1 in the specification of leg vs antennal fates. Although the leg and the antenna are morphologically distinct appendages they have been considered serial homologous structures that share a similar developmental program. Thus, variations in this developmental program are responsible for the morphological differences between these two appendages. Our results suggest an important role of Sp1 in the specification of leg vs antennal fates.

Appendages are external projections of the body that serve the animal for locomotion, feeding, or environment exploration. The appendages of the fruit fly Drosophila melanogaster are derived from the imaginal discs, epithelial sac-like structures specified in the embryo that grow and pattern during larva development. In the last decades, genetic and developmental studies in the fruit fly have provided extensive knowledge regarding the mechanisms that direct the formation of the appendages. Importantly, many of the signaling pathways and patterning genes identified and characterized in Drosophila have similar functions during vertebrate appendage development. In this review, we will summarize the genetic and molecular mechanisms that lead to the specification of appendage primordia in the embryo and their posterior patterning during imaginal disc development. The identification of the regulatory logic underlying appendage specification in Drosophila suggests that the evolutionary origin of the insect wing is, in part, related to the development of ventral appendages.