The neurons of the sensory receptors on the wing of Drosophila melanogaster have highly characteristic axon

The establishment of correct neuronal connections during development of the nervous system requires information both to guide growing axons and to ensure proper connections. In both vertebrates and invertebrates, it has been found that necessary guidance cues mediating interactions between axons and the environment can be provided by cell surface molecules (reviewed in Jessell, 1990; Hortsch and Goodman, 1991). In Drosophila melanogaster genetic techniques have led to the identification of putative neuronal guidance molecules (Grenningloh et al., 1991). In the work presented here, the role of four proteins, those encoded by the fasciclinI (fasI), fasciclinII (fasII), fasci clinIII (fasIII) and neurally altered carbohydrate (nac) genes, was examined in the developing wing sensory system. The fasciclin genes, fasI, fasII and fasIII, encode cell surface glycoproteins thought to be involved in the fasciculation of axon bundles during development (Grenningloh and Goodman, 1992; Grenningloh et al., 1990). Additionally, the fasciclin molecules have been shown to function as homophilic cell adhesion molecules mediating cell sorting in vitro (Snow et al., 1989; Elkins et al., 1990; Grenningloh et al., 1990). Null mutants of the fasI and fasIII genes show no gross morphologic abnormalities and are homozygous viable (Elkins et al., 1990b; Snow et al., 1989; T. Elkins, D. Ferres-Marco, C.S. Goodman, personal communication), while null fasII mutations are lethal (Grenningloh et al., 1991). The nac mutation affects an enzymatic process required for the addition of a carbohydrate moiety common to many proteins expressed on the surface of neurons and other tissues. One effect of the nac mutation is the loss of epitopes recognized by the antibodies to horseradish peroxidase (Katz et al, 1988; Snow et al., 1987). The axons of wing sensory neurons in animals mutant for these genes were examined both in their development within the wing and in their final adult projection pattern in the central nervous system (CNS). The axons of the adult wing sensory neurons follow highly characteristic, easily identifiable and spatially separate pathways in the CNS, (Ghysen, 1978, 1980; Palka et al., 1979, 1986). The sequence of differentiation of neurons in the periphery and the establishment of axonal trajectories in the wing tissue are known (Murray et al., 1984; Jan et al., 1985), as is the developmental profile of the wing axonal projection pattern in the CNS during metamorphosis (Whitlock and Palka, 1989). Thus, the wing sensory 1251 Development 117, 1251-1260 (1993) Printed in Great Britain © The Company of Biologists Limited 1993