Structural, functional and developmental convergence of the insect mushroom bodies with higher brain centers of vertebrates.

Convergence of higher processing centers has been proposed for insects and vertebrates, but the extent of these similarities remains controversial. The present study demonstrates that one higher brain center of insects, the mushroom bodies, displays a number of similarities with mammalian higher brain centers that are arguably the products of adaptation to common behavioral ecologies, despite their deeply divergent origins. Quantitative neuroanatomy, immunohistochemistry, fluorescent tract tracing and BrdU labeling are employed to investigate the relationships among behavioral ecology and mushroom body size, sensory input and mode of development in one taxon, the scarab beetles (Coleoptera: Scarabaeidae). Comparisons are extended to a taxon in which similar mushroom body architectures have arisen independently, the cockroaches (Dictyoptera), and to published accounts of vertebrate brain evolution. This study demonstrates that evolutionary increases in higher brain center size and intrinsic neuron number are associated with flexibility in food acquisition behaviors in both vertebrates and insects. These evolutionarily expanded higher brain centers are divided into novel structural subcompartments that acquire novel processing functions. Increased numbers of neurons comprising enlarged higher brain centers are generated by expanded neural precursor pools, and the time for development of these brain centers is protracted. Taken together, these findings extend our understanding of how evolutionarily constrained neural substrates might converge under shared adaptive landscapes, even after 600 million years of divergence, and even at the level of higher brain centers that generate complex behaviors. Copyright 2008 S. Karger AG, Basel.