Evolution of Human-Specific Neural SRGAP2 Genes by Incomplete Segmental Duplication

Department of Genome Sciences, University of Washington School of Medicine, Seattle, 98195, USA.
Cell (Impact Factor: 32.24). 05/2012; 149(4):912-22. DOI: 10.1016/j.cell.2012.03.033
Source: PubMed


Gene duplication is an important source of phenotypic change and adaptive evolution. We leverage a haploid hydatidiform mole to identify highly identical sequences missing from the reference genome, confirming that the cortical development gene Slit-Robo Rho GTPase-activating protein 2 (SRGAP2) duplicated three times exclusively in humans. We show that the promoter and first nine exons of SRGAP2 duplicated from 1q32.1 (SRGAP2A) to 1q21.1 (SRGAP2B) ∼3.4 million years ago (mya). Two larger duplications later copied SRGAP2B to chromosome 1p12 (SRGAP2C) and to proximal 1q21.1 (SRGAP2D) ∼2.4 and ∼1 mya, respectively. Sequence and expression analyses show that SRGAP2C is the most likely duplicate to encode a functional protein and is among the most fixed human-specific duplicate genes. Our data suggest a mechanism where incomplete duplication created a novel gene function-antagonizing parental SRGAP2 function-immediately "at birth" 2-3 mya, which is a time corresponding to the transition from Australopithecus to Homo and the beginning of neocortex expansion.

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    • "Interestingly, both gene families, FAM72 and SRGAP2, were composed of four human paralogues located as unique pairs on chr 1 (Fig. 1A). However, all other species investigated contained only one orthologue (Fig. 3) [31]. Thus, FAM72 and SRGAP2 appear to represent a unique gene couple that characterizes the emergence of vertebrates with a notochord (with one gene pair) and defines the human species containing four gene pairs (Fig. 3). "
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    • "To date, the genetic basis of human-specific phenotypes remains largely unknown, complicated by the difficulties in distinguishing between phenotypically significant and benign variation. Thus, evolutionary changes in protein-coding sequences have received considerable attention, as the phenotypic consequences of these mutations have historically been easier to interpret (Clark et al. 2003; Stedman et al. 2004; Chimpanzee Sequencing and Analysis Consortium 2005; Nielsen et al. 2005; Arbiza et al. 2006; Dennis et al. 2012; Sudmant et al. 2013). Although protein-coding evolution has clearly played a role in human evolution, proteins account for only ∼1.5% of the human genome, most of which exhibit high sequence similarity between humans and chimpanzees (Chimpanzee Sequencing and Analysis Consortium 2005). "
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