Filarial parasites cause a wide variety human and animal diseases, including human elephantiasis and river blindness, and dog heartworm. All filarial species require an anthropod vector for transmission to new hosts. The parasite stage that infects the vector is the first larval stage, or microfilaria, which must persist in the blood or tissues of the vertebrate host for successful transmission. This application proposes to study processes in filarial embryogenesis leading to the production of microfilariae that can successfully effect transmission. The development of the first stage larva in utero follows either of two patterns in all filariae: one pattern produces sheathed microfilariae, and the other produces microfilariae that hatch free of the eggshell and are nonsheathed. Examples of each pattern will be studied to understand common and distinct patterns of gene expression. The metabolism of a complex polysaccharide, chitin, will be analyzed, to understand the regulation of its synthesis, modification, and degradation during microfilarial morphogenesis. In particular, the relationship of chitin metabolism with eggshell growth, sheath restructuring, and hatching will be studied by biochemical and molecular analyses. The use of protein crosslinking to produce protective structures such as the eggshell, sheath and cuticle will be studied as it relates to embryonic growth and shape change for the two different patterns of development. Specific substrates for crosslinking reactions will be compared between sheathed and nonsheathed microfilarie as they develop in utero. Finally, the patterns of gene expression will be compared by RNA differential display between early and late stages of embryos in two different species, in order to identify new therapeutic targets. The ability to disrupt filarial embryogenesis would be significant for controlling transmission of all filarial infections, and for controlling disease in Onchocercal infections. The biochemical basis of embryonic growth and morphogenesis will be analyzed by these studies in order to assess the potential for interrupting microfilarial production.