Neuromeric Organization of the Microbat Brain: Conserved and Distinct Regional Features.

Myotis myotis and Tadarida brasiliensis are both microbat species belonging to the Vespertilionidae and Molossidae families, respectively. Our goal is to determine if the 85-million-year evolutionary divergence between microbats and the Muridae family (mice and rats) has led to significant regional variations in the brain. However, the 34-million-year split between M. myotis and T. brasiliensis serves as in-group control to contextualize the larger divergence with rodents. Using the prosomeric framework, main brain derivatives from each neuromeric partition were compared between these two microbats and with rodents. We found that although the fundamental neuromeric organization is conserved across microbats (M. myotis and T. brasiliensis) and rodents (rats, mice, and gerbils), there are significant regional differences within distinct derivatives such that microbats exhibit smaller corpus callosum, isocortex, optic chiasm, and cerebellum when compared to rodents. On the other hand, the overall pattern of tyrosine hydroxylase (TH)-positive processes and tracts in the basal plate of the diencephalon-midbrain-rostral hindbrain in both bats is similar to that found in rodents and primates. However, a key difference was found in the medial habenula (MHb). Although M. myotis showed selective TH expression in the MHb, this was absent in T. brasiliensis. Collectively, these findings suggest that the 85-million-year evolutionary divergence between bats and rodents has led to notable regional variations in brain anatomy, even though their basic modular plan remained the same.