Mutations in enzymes that process complex N-linked glycans alter adult brain morphology in D. melanogaster. (A) The major lobes of the adult brain are shown in cross section. (Hatched areas) γ-lobes; (α,β) α- and β-lobes; (ped) peduncle; (Ca) calyx; (EB) ellipsoid body; (FB) fan-shaped body; (NO) noduli; (PB) protocerebral bridge. (B,C) Symmetrical sets of α- and β-lobes (B) are distinctly separated at the midline (C, arrow) in the wild-type adult. (D) A mutation in the fused lobes (fdl) gene, which encodes for a hexosaminidase that removes the N-acetylglucosamine added by GlcNAcT-I, yields increased expression of complex glycans and midline fusion of the β-lobes (arrow). (E) A loss-of-function mutation in GlcNAcT-I (Mgat1) decreases complex N-linked glycan expression, but it also fuses the β-lobes (arrow). Despite driving glycosylation in opposite directions, these two mutations yield convergent brain phenotypes. (A,B,D, Reprinted, with permission of Wiley-Liss, Inc., from Boquet I, et al. 2000. J Neurobiol 42: 33–48; C,E, reprinted, with permission of the American Society for Biochemistry and Molecular Biology, from Sarkar M, et al. 2006. J Biol Chem 281: 12776–12785.)