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.