Strain-Specific Differences in Cerebellar Anatomy between Laboratory and Wild Rats

Domestication and subsequent breed selection has significantly changed the phenotype of most domesticated animal species. Not only has their external appearance changed, in many species, the brain and individual brain regions often differ in size in domesticated strains compared with their wild ancestors. Although the majority of studies on mammals focus on cortical regions, the cerebellum often differs in relative and absolute size between domestic and wild strains, but more specific data on cell sizes and numbers are often lacking.

We quantified cerebellar anatomy in two domesticated strains (Long-Evans and Sprague-Dawley) and one wild type of brown rat (Rattus norvegicus). Using unbiased stereology, we measured the total cerebellum and its layers' volumes, as well as the number and size of Purkinje cells.

Long-Evans rats had a larger total cerebellum volume, in both absolute and relative terms, than Sprague-Dawley and wild rats, but no other significant differences were detected. Significant differences in the absolute and relative sizes of the molecular, granule cell, and white matter layers were also found, but the differences were inconsistent among strains such that the largest values alternated between the two laboratory strains. The absolute number of Purkinje cells did not differ among strains, but one population of Sprague-Dawley rats and the wild rats had more Purkinje cells relative to cerebellar volume. Last, Long-Evans rats had significantly smaller Purkinje cells than the other strains in both absolute and relative terms.

Only one of the two domesticated strains differed from wild rats in cerebellar anatomy. Our results therefore demonstrate that changes in the brains of domesticated animals do not necessarily follow a universal rule; they can vary between different strains. This highlights the importance of including more than one strain in wild-domesticate comparisons in brain anatomy and avoiding the oversimplification of the effects of domestication on the brain.