Neurotrophins are growth factor proteins that induce the survival and function of neurons, in large part prenatally but maintained to an extent through the rest of the lifespan as well. Major neurotrophins in vertebrates are nerve growth factor (sensory and sympathetic neurons), brain derived neurotrophic factor (mainly found in the PNS but also the hippocampus and cortex), neurotrophin-3 (activates a diverse set of neurons because of its action on receptor tyrosine kinase in the cell), and neurotrophin-4.
Until recently a fruit fly homologue had not been identified, but Zhu and colleagues recently accomplished just this, which they called drosophila neurotrophin 1 (DNT-1). Just as in vertebrate neurotrophins, DNT-1 promotes cell survival, as its expression in the midline prevents naturally occurring cell death. It has orthologues in many other insects, including mosquitoes, jewel wasp, and honeybees, meaning that it is well-conserved. The authors suggest that there must have been a neurotrophin present in a common ancestor of invertebrates and vertebrates, thus supporting the theory of a common origin for the centralization of the nervous system.
Zhu B, Pennack JA, McQuilton P, Forero MG, Mizuguchi K, et al. 2008 Drosophila neurotrophins reveal a common mechanism for nervous system formation. PLoS Biol 6(11): e284. doi:10.1371/journal.pbio.0060284