Conserved patterns of axogenesis in the panarthropod brain

• Comparative studies reveal a wide spectrum of neuroarchitectures in the brain midline of Panarthropoda.
• In insects a simple orthogonal axon scaffold is formed initially and subsequently transformed into the mature chiasmal form.
• Transformation involves axons from identified lineages decussating at stereotypic locations across the brain midline.
• Topographic decussation probably evolved early and so represents a conserved developmental program in panarthropods.
• Neuropilar elaboration therefore reflects lifestyle and the extent to which decussation has progressed during development.

Neuropils in the cerebral midline of Panarthropoda exhibit a wide spectrum of neuroarchitectures – from rudimentary to highly elaborated – and which at first sight defy a unifying neuroarchitectural principle. Developmental approaches have shown that in model arthropods such as insects, conserved cellular and molecular mechanisms first establish a simple axon scaffold in the brain. However, to be adapted for adult life, this immature ground plan is transformed by a developmental process – known in the grasshopper as “fascicle switching” – in which subsets of neurons systematically redirect their growth cones at stereotypic locations across the brain midline. A topographic system of choice points along the transverse brain axis where axons decussate features in all panarthropods studied even though different modes of neurogenesis and varying degrees of neuropilar elaboration are involved. This suggests that the molecular mechanisms regulating choice point selection may be conserved. In combination with recent cladistic interpretations of arthropod phylogeny based on nuclear protein-coding sequences the data argue for this topographic decussation as having evolved early and being a conserved feature of the Panarthropoda. Differences in elaboration likely reflect both the extent to which neuropilar reorganization has progressed during development and the lifestyle of the individual organism.