WILLIAMS, T.A.; NULSEN, C.; NAGY, L.M.: A complex role for Distal-less in the development of branched crustacean limbs

The developing leg of Drosophila is initially patterned by subdivision of the leg into proximal and distal domains by the activity of the homeodomain proteins Extradenticle (Exd) and Distal-less (Dll). These early domains of gene expression are postulated to reflect a scenario of limb evolution in which an undifferentiated appendage outgrowth was subdivided into two functional parts, the coxapodite and telopodite. The legs of most arthropods have a more complex morphology than the simple rod-shaped leg of Drosophila. We document the expression of Dll and Exd in two crustacean species with complex branched limbs. We present evidence that while arthropod legs possess an initial Dll-expressing domain, the morphological structure that corresponds to this domain varies between arthropod clades. Similarly, Exd expression does not define a homologous structure throughout arthropods. In addition, we find extensive overlap in Exd and Dll expression and that Dll expression correlates not only with distal positional values in the leg but also with setal-forming cells throughout the proximal/distal extent of the leg. This may reflect a pleisiomorphic function of Dll in development of the peripheral nervous system. Our results confirm previous observations that branch formation in multi-branched arthropod limbs is not regulated by a simple iteration of the proximal/distal patterning module employed in Drosophila limb development.