Meeting Abstract
The rate at which the number of neurons increases with the overall size of the brain, and its constituent brain regions, varies across clades. This has led to the development of ‘neuronal scaling rules’, defined as the allometric relationship between neuron numbers and brain region volumes. Although these scaling rules appear to explain several aspects of brain diversification in vertebrates, it is unclear whether these scaling rules apply equally across different neuronal populations or to what extent neuron size follows brain region or clade specific scaling rules. To gain a better understanding of how neuronal scaling rules affect the evolution of species differences in brain region size, we quantified different aspects of the anatomy of the cerebellum, a key region for motor coordination that varies in size and morphology across species. Using unbiased stereology, we quantified the volumes of molecular, granule and white matter layers and the number and size of Purkinje cells across 60 species of birds. Phylogeny-based statistical analyses show that the different layers of the cerebellum evolve in a concerted fashion across all birds. In other words, when one layer increases in size, all the others increase at the same rate. Both the number and size of Purkinje cells, the sole output neurons of the cerebellar cortex, increase with cerebellum size, but Purkinje cells are added at a faster rate than they change in size. Last, more folded cerebella also had more and larger Purkinje cells. Thus, the evolution of larger cerebella is due to coordinated increases across cell layers as well as increases in the number and size of Purkinje cells according to a common set of neuronal scaling rules.
