The size of different brain areas in mice are regulated independently of each other, and independently of body size, reports a study published in Nature Communications this week. These findings add to the debate over whether developmental constraints prevent individual parts of the brain to freely evolve.
Similar pathways are known to control the development of the entire mammalian brain, suggesting a master plan for the proportions of the brain. However, evidence exists for variations in the size of different brain regions. To resolve the apparent discrepancy, Reinmar Hager and colleagues carry out a 15-year genetic analysis involving over 10,000 inbred mice. They identify genetic loci responsible for regulating the sizes of seven different brain regions. Except for one locus which modulates two regions, each identified locus only controls one brain region. They also observe a distinct set of three loci which influence overall brain size, together implying that microevolution of the brain could be uncoupled from total brain volume. Finally, they report genetic loci affecting body size but not brain size, and show a very low correlation between overall brain size and body size.
While the genetic loci reported in this work represent an important addition to brain evolution theories, the genes within the loci remain to be identified and functionally characterized. Due to the use of artificial mice strains, it is also yet to be tested whether these conclusions can be extrapolated to human populations.