Homologs of Drosophila appendage genes in the patterning of arthropod limbs

Studies of the genes involved in patterning the appendages of Drosophila me lanogaster have revealed a system of signaling and transcriptional regulati on that is responsible for specifying the proximo-distal limb axis. Here we report the expression patterns of presumptive homologs of the Drosophila g enes extradenticle, dachshund, nubbin, ventral veins lacking (a.k.a. Cf1-a) , and DII in the limbs of the woodlouse Porcellio scaber and the spider Ste atoda triangulosa. Although the expression domains of the appendage genes r oughly correspond to those of Drosophila, their relative positions and segm ental affiliation are distinct. In addition, the expression patterns of the appendage genes allows a resolution of the segmental composition of differ ent appendages within crustacean and spider embryos. We conclude that certa in limb types, e.g., mouthparts, appear to be derived from a leg-like groun d-plan via the elimination/fusion of the intermediate and distal podomeres. Moreover, we observe lust such a modification during the transformation of the anterior legs into mouthparts in P. scaber. Although our data do not u nequivocally resolve the question of homology of the arthropod leg segments , they do provide evidence for a single conserved proximo-distal patterning system in the development of noninsect arthropod limbs. (C) 2000 Academic Press.