Adaptive Evolution of Four Microcephaly Genes and the Evolution of Brain Size in Anthropoid Primates

Adaptive Evolution of Four Microcephaly Genes and the Evolution of Brain Size in Anthropoid Primates Stephen H. Montgomery,1 Isabella Capellini,2 Chris Venditti,3 Robert A. Barton,2 and Nicholas I. Mundy*,1 1 Department of Zoology, University of Cambridge, Cambridge, United Kingdom Evolutionary Anthropology Research Group, Department of Anthropology, Durham University, Durham, United Kingdom 3 School of Biological Sciences, University of Reading, Reading, United Kingdom *Corresponding author: E-mail: . Associate editor: Anne Stone 2 Abstract Key words: ASPM, MCPH1, CDK5RAP2, CENPJ, brain, neurogenesis, primates. Introduction The expansion of the brain, and in particular the neocortex, is a major hallmark of primate evolution (Jerison 1973; Martin 1990). After correcting for allometric scaling with body mass, primates have larger brains than most other mammals (Martin 1990; Barton 2006b) and both absolute and relative brain size have increased along multiple, independent primate lineages (Montgomery et al. 2010). The adaptive significance and anatomical basis of the diversity of primate brains has long been studied using comparative methods (for review, see Falk and Gibson 2001; Finlay et al. 2001; Barton 2006a), but the investigation of the genetic basis of primate brain expansion has only begun relatively recently and is currently a topic of intense interest. The convergent evolution of increased brain size in different lineages provides an opportunity to study whether the independent evolution of complex traits involves convergence at the molecular level (Arendt and Reznick 2007) and may provide insights into lineage-specific evolution, for example, on the human lineage. Both scans of brainexpressed genes in published primate genomes (Dorus, Vallender, et al. 2004; Shi et al. 2006; Yu et al. 2006; Wang et al. 2007) and studies of candidate genes (e.g., Enard et al. 2002; Burki and Kaessmann 2004; Wang et al. 2005) have mostly focused on identifying changes along the lineage leading to humans and have largely ignored convergent increase in brain size in multiple primate lineages. One group of genes of particular interest in relation to the evolution of gross brain size is the microcephaly genes. Autosomal recessive primary microcephaly is a congenital disorder characterized by reduced growth of the cerebral cortex in the absence of environmental, metabolic, or cytogenetic etiologies (Bond and Woods 2006; Cox et al. 2006). In humans, it is inherited as a recessive Mendelian trait involving at least eight loci, of which five have now been identified at the molecular level: ASPM, MCPH1, CDK5RAP2, CENPJ (Jackson et al. 1998; Bond et al. 2002, 2005; Thornton and Woods 2009) and the more recently identified STIL (Kumar et al. 2009). The five genes are expressed in the fetal brain during neurogenesis (Bond et al. 2002, 2005; Jackson et al. 2002; Kouprina et al. 2005; Kumar et al. 2009). ASPM, CDK5RAP2, © The Author 2010. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: Mol. Biol. Evol. 28(1):625–638. 2011 doi:10.1093/molbev/msq237 Advance Access publication October 20, 2010 625 Downloaded fromarticle https://academic.oup.com/mbe/article/28/1/625/985033 by guest on 08 June 2024 Research The anatomical basis and adaptive function of the expansion in primate brain size have long been studied; however, we are only beginning to understand the genetic basis of these evolutionary changes. Genes linked to human primary microcephaly have received much attention as they have accelerated evolutionary rates along lineages leading to humans. However, these studies focus narrowly on apes, and the link between microcephaly gene evolution and brain evolution is disputed. We analyzed the molecular evolution of four genes associated with microcephaly (ASPM, CDK5RAP2, CENPJ, MCPH1) across 21 species representing all major clades of anthropoid primates. Contrary to prevailing assumptions, positive selection was not limited to or intensified along the lineage leading to humans. In fact we show that all four loci were subject to positive selection across the anthropoid primate phylogeny. We developed clearly defined hypotheses to explicitly test if selection on these loci was associated with the evolution of brain size. We found positive relationships between both CDK5RAP2 and ASPM and neonatal brain mass and somewhat weaker relationships between these genes and adult brain size. In contrast, there is no evidence linking CENPJ and MCPH1 to brain size evolution. The stronger association of ASPM and CDK5RAP2 evolution with neonatal brain size than with adult brain size is consistent with these loci having a direct effect on prenatal neuronal proliferation. These results suggest that primate brain size may have at least a partially conserved genetic basis. Our results contradict a previous study that linked adaptive evolution of ASPM to changes in relative cortex size; however, our analysis indicates that this conclusion is not robust. Our finding that the coding regions of two widely expressed loci has experienced pervasive positive selection in relation to a complex, quantitative developmental phenotype provides a notable counterexample to the commonly asserted hypothesis that cisregulatory regions play a dominant role in phenotypic evolution. Montgomery et al. · doi:10.1093/molbev/msq237 626 between brain and body mass (Barton 2006b). However, given the implied functions of the four microcephaly genes in regulating the proliferation and survival of neurons, absolute brain mass may be a more relevant phenotypic measure as in primates it increases linearly with the total number of neurons (Herculano-Houzel et al. 2007). In agreement with quantitative genetic analysis of brain and body size allometry (Lande 1979), it has recently been shown that primate brain and body size differ in their evolutionary trajectories (Montgomery et al. 2010) suggesting that these two traits must be developmentally and genetically decoupled to some extent despite their closely correlated evolution. Crucially, because primate neocortical neurogenesis is largely restricted to prenatal development (Rakic 1988, 2002; Bhardwaj et al. 2006) and microcephaly is primarily a disorder of fetal brain growth (Cox et al. 2006), microcephaly gene evolution should be more closely related to neonatal brain size than to adult brain size. Postnatal brain growth is largely driven by gliogenesis (Low and Cheng 2006), axon growth (Sauvageot and Stiles 2002), and myelination (Sowell et al. 2001) rather than by production of new neurons. There are only two known sites in the primate brain, which are small and noncortical, in which substantial postnatal neurogenesis occurs (Jabes et al. 2010). Indeed, apoptosis eliminates large numbers of neurons (Buss et al. 2006). Variation in these and other nonneurogenic processes will re (...truncated)