Tomer et al. have combined gene expression profiling with image registration to find that the mushroom body of the segmented annelid worm Platynereis dumerilii shares many features with the mammalian cerebral cortex. They suggest that the mushroom body and cortex evolved from the same structure in the common ancestor of vertebrates and invertebrates, before the appearance of bilateral symmetry in animals. Here is their summary:
The evolution of the highest-order human brain center, the “pallium” or “cortex,” remains enigmatic. To elucidate its origins, we set out to identify related brain parts in phylogenetically distant animals, to then unravel common aspects in cellular composition and molecular architecture. Here, we compare vertebrate pallium development to that of the mushroom bodies, sensory-associative brain centers, in an annelid. Using a newly developed protocol for cellular profiling by image registration (PrImR), we obtain a high-resolution gene expression map for the developing annelid brain. Comparison to the vertebrate pallium reveals that the annelid mushroom bodies develop from similar molecular coordinates within a conserved overall molecular brain topology and that their development involves conserved patterning mechanisms and produces conserved neuron types that existed already in the protostome-deuterostome ancestors. These data indicate deep homology of pallium and mushroom bodies and date back the origin of higher brain centers to prebilaterian times.
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