August 22, 2008

ASPM and cerebral cortex evolution in primates

ASPM was initially identified in human-chimp comparisons. Since it was expressed in the brain, it was thought that it played a role in making us different from our closest relatives. This new paper shows that adaptive evolution of ASPM is not limited in the human-chimp split, but occurred in many different primate lineages. Moreover, the target of its evolution was the cerebral cortex.

Note that these results are not directly applicable to the recently selected variant within the human lineage (some recent discussion).

Molecular Biology and Evolution, doi:10.1093/molbev/msn184

Positive selection in ASPM is correlated with cerebral cortex evolution across primates but not with whole brain size

Farhan Ali, and Rudolf Meier

The rapid increase of brain size is a key event in human evolution. ASPM (abnormal spindle-like microcephaly associated) is discussed as a major candidate gene for explaining the exceptionally large brain in humans but ASPM’s role remains controversial. Here we use codon-specific models and a comparative approach to test this candidate gene that was initially identified in Homo-chimp comparisons. We demonstrate that accelerated evolution of ASPM ({omega} = 4.7) at 16 amino acid sites occurred in nine primate lineages with major changes in relative cerebral cortex size. However, ASPM’s evolution is not correlated with major changes in relative whole brain or cerebellum sizes. Our results suggest that a single candidate gene such as ASPM can influence a specific component of the brain across large clades through changes in a few amino acid sites. We furthermore illustrate the power of using continuous phenotypic variability across primates to rigorously test candidate genes that have been implicated in the evolution of key human traits.


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