DISTRIBUTION OF GABA-LIKE IMMUNOREACTIVE NEURONS IN INSECTS SUGGESTS LINEAGE HOMOLOGY

gamma-Aminobutyric acid (GABA) is an important inhibitory neurotransmi tter in vertebrates and invertebrates (Sattelle [1990] Adv. Insect Phy siol. 22:1-113). The GABA phenotype is lineally determined in postembr yonic neurons in the tobacco hawkmoth, Manduca sexta (Witten and Truma n, [1991] J. Neurosci. 11:1980-1989) and is restricted to six identifi able postembryonic lineages in the moth's thoracic hemiganglia. We use d a comparative approach to determine whether this distinct clustering of GABAergic neurons is conserved in Insecta. In the nine orders of i nsects surveyed (Thysanura, Odonata, Orthoptera, Isoptera, Hemiptera, Coleoptera, Diptera, Lepidoptera, and Hymenoptera), GABA-like immunore active neurons within a thoracic hemiganglion were clustered into six distinct groups that occupied positions similar to the six postembryon ic lineages in Manduca. On the basis of cell body position and axon tr ajectories, we suggest that these are indeed homologous lineage groups and that the lineal origins of the GABAergic cells have been very con servative through insect evolution. The distinctive clustering of GABA -positive cells is shared with crustaceans (Mulloney and Hall [1990] J . Comp. Neurol. 291:383-394; Homberg et al. [1993] Cell Tissue Res. 27 1:279-288) but is not found in the centipede Lithobius forficulatus. T here is a two- to threefold increase in numbers of thoracic neurons be tween the-flightless Thysanura and the most advanced orders of insects . Using the GABA clusters as indicators of specific lineages, we find that only selected lineages have significantly contributed to this inc rease in neuronal numbers. J. Comp. Neurol. 398:515-528, 1998. (C) 199 8 Wiley-Liss, Inc.