Evolution of mushroom body inversion and associated gyriform neuropils parallel the evolved transposition of the mammalian hippocampus

Abstract

Neural organization of mushroom bodies is largely consistent across insects, whereas the ancestral ground pattern diverges broadly across crustacean lineages, resulting in successive loss of columns and the acquisition of domed centers retaining ancestral Hebbian-like networks and aminergic connections. We demonstrate here a major departure from this evolutionary trend in Brachyura, the most recent malacostracan lineage. Instead of occupying the rostral surface of the lateral protocerebrum, mushroom body calyces are buried deep within it, with their columns extending outwards to an expansive system of gyri on the brains surface. The organization amongst mushroom body neurons reaches extreme elaboration throughout its constituent neuropils. The calyces, columns, and especially the gyri show DC0 immunoreactivity, an indicator of extensive circuits involved in learning and memory.