THE TOLL PATHWAY IS REQUIRED IN THE EPIDERMIS FOR MUSCLE DEVELOPMENT IN THE DROSOPHILA EMBRYO

The Toll signaling pathway functions in several Drosophila processes, including dorsal-ventral pattern formation and the immune response. He re, we demonstrate that this pathway is required in the epidermis for proper muscle development. Previously, we showed that the zygotic Toll protein is necessary for normal muscle development; in the absence of zygotic Toll, close to 50% of hemisegments have muscle patterning def ects consisting of missing, duplicated and misinserted muscle fibers ( Halfon, M.S., Hashimoto, C., and Keshishian, H., Dev. Biol. 169, 151-1 67, 1995). We have now also analyzed the requirements for easter, spat zle, tube, and pelle, all of which function in the Toll-mediated dorsa l-ventral patterning pathway. We find that spatzle, tube, and pelle, b ut not easter, are necessary for muscle development. Mutations in thes e genes give a phenotype identical to that seen in Toll mutants, sugge sting that elements of the same pathway used for Toll signaling in dor sal-ventral development are used during muscle development. By express ing the Toll cDNA under the control of distinct Toll enhancer elements in Toll mutant flies, we have examined the spatial requirements for T oll expression during muscle development. Expression of Toll in a subs et of epidermal cells that includes the epidermal muscle attachment ce lls, but not Toll expression in the musculature, is necessary for prop er muscle development. Our results suggest that signals received by th e epidermis early during muscle development are an important part of t he muscle patterning process. (C) 1998 Academic Press.