Compartmentalization of the precheliceral neuroectoderm in the spider Cupiennius salei: development of the arcuate body, optic ganglia, and mushroom body.

Similarly to vertebrates, arthropod brains are compartmentalized into centers with specific neurological functions such as cognition, behavior, and memory. The centers can be further subdivided into smaller functional units. This raises the question of how these compartments are formed during development and how they are integrated into brain centers. We show here for the first time how the precheliceral neuroectoderm of the spider Cupiennius salei is compartmentalized to form the distinct brain centers of the visual system: the optic ganglia, the mushroom bodies, and the arcuate body. The areas of the visual brain centers are defined by the formation of grooves and vesicles and express the proneural gene CsASH1, followed by expression of the neural differentiation marker Prospero. Furthermore, the transcription factor dachshund, which is strongly enriched in the mushroom bodies and the outer optic ganglion of Drosophila, is expressed in the optic anlagen and the mushroom bodies of the spider. The developing brain centers are further subdivided into single neural precursor groups, which become incorporated into the grooves and vesicles but remain distinguishable throughout development, suggesting that they encode spatial information for neural subtype identity. Several molecular and morphological aspects of the development of the optic ganglia and the mushroom bodies are similar in the spider and in insects. Furthermore, we show that the primary engrailed head spot contributes neurons to the optic ganglia of the median eyes, whereas the secondary head spot, which has been associated with the optic ganglia in insects and crustaceans, is absent.