A conserved plan for wiring up the fan-shaped body in the grasshopper and Drosophila

Abstract

The central complex comprises an elaborate system of modular neuropils which mediate spatial orientation and sensory-motor integration in insects such as the grasshopper and Drosophila. The neuroarchitecture of the largest of these modules, the fan-shaped body, is characterized by its stereotypic set of decussating fiber bundles. These are generated during development by axons from four homologous protocerebral lineages which enter the commissural system and subsequently decussate at stereotypic locations across the brain midline. Since the commissural organization prior to fan-shaped body formation has not been previously analysed in either species, it was not clear how the decussating bundles relate to individual lineages, or if the projection pattern is conserved across species. In this study, we trace the axonal projections from the homologous central complex lineages into the commissural system of the embryonic and larval brains of both the grasshopper and Drosophila. Projections into the primordial commissures of both species are found to be lineage-specific and allow putatively equivalent fascicles to be identified. Comparison of the projection pattern before and after the commencement of axon decussation in both species reveals that equivalent commissural fascicles are involved in generating the columnar neuroarchitecture of the fan-shaped body. Further, the tract-specific columns in both the grasshopper and Drosophila can be shown to contain axons from identical combinations of central complex lineages, suggesting that this columnar neuroarchitecture is also conserved.

Developmentally Arrested Precursors of Pontine Neurons Establish an Embryonic Blueprint of the Drosophila Central Complex

“…Remarkably, the central complex, as defined anatomically for the adult, is the one major set of compartments of the fly brain that lacks a larval counterpart. Thus, the large majority of tangential and columnar neurons with their highly ordered connections outlined above are secondary neurons born in the larva [20], even though the four DM lineages have been distinguished in the embryo [21][22][23]. This prompts the question of what guidance mechanisms control central complex connectivity and what part primary neurons play during this process.…”

Developmentally arrested precursors of pontine neurons establish an embryonic blueprint of theDrosophilacentral complex

“…Also secondary tracts of uni-glomerular neurons, including the 83H12-positive PB-FB/EB-NO neurons and 19G02-positive PB-EB/FB gall neurons, cross in the posterior plexus, forming a system of chiasms that establish the characteristic, modular connectivity of the fan-shaped body and ellipsoid body (Fig.6N, P, Q, S, T). Tracts of DM1 and DM2 cross the midline to terminate in the lateral and medial half of the contralateral prFB, respectively; those of DM3 and DM4 remain ipsilateral, targeting the medial and lateral half of the ipsilateral prFB, respectively (Fig.6Q, T; see also Boyan et al, 2017).…”

Neuronal constituents and putative interactions within the Drosophila ellipsoid body neuropil

“…into lineage-based modules (Ito and Awasaki, 2008;Yang et al, 2013), a ground plan that is likely 103 conserved across insects (Boyan et al, 2017). A lineage refers to the set of sibling neurons derived 104…”