Meeting Abstract
While there are clear differences in relative brain region size across vertebrates, it is unclear how these patterns evolved. The concerted hypothesis predicts brain region size has a predictable relationship with total brain size across species, as a consequence of developmental timing. The mosaic hypothesis predicts the sizes of brain regions can evolve independently due to selection acting on specific functions or behaviors. We addressed these hypotheses in mormyrids, a group of fishes that generate and detect weak electric fields. Mormyrids have a large brain and gigantocerebellum. However, no study has addressed how mosaic and concerted changes have contributed to the evolution of mormyrid brains. We did micro-computed tomography scans of brains of 6 mormyrid and 3 outgroup species. We measured the volumes of 8 brain regions. Within clades, the size of each brain region scaled with brain size, as predicted by concerted evolution. However, we also found evidence for mosaic evolution, with mormyrids having a disproportionately large cerebellum compared to outgroups. Further, the concerted hypothesis predicts the cerebellum and telencephalon should show disproportionately large growth as brain size increases, since they are generated latest in mammals. However, we found the torus semicircularus showed the greatest growth with increasing brain size, suggesting either the sequence of neurogenesis is different from mammals, or it is not responsible for the differential scaling of brain regions. Finally, the outgroup species Xenomystus nigri is electroreceptive but does not actively generate electric fields. Like mormyrids, its cerebellum was disproportionately large compared to other outgroup species. Thus, our results suggest the evolution of electroreception has driven mosaic expansion of the cerebellum.
