Abstract
During periods of reduced O2 supply, the most profound changes in gene expression are mediated by hypoxia-inducible factor (HIF) transcription factors that play a key role in cellular responses to low-O2 tension. Using target-enrichment sequencing, we tested whether variation in 26 genes in the HIF signaling pathway was associated with high altitude and therefore corresponding O2 availability in three duck species that colonized the Andes from ancestral low-altitude habitats in South America. We found strong support for convergent evolution in the case of two of the three duck species with the same genes (EGLN1, EPAS1), and even the same exons (exon 12, EPAS1), exhibiting extreme outliers with a high probability of directional selection in the high-altitude populations. These results mirror patterns of adaptation seen in human populations, which showed mutations in EPAS1, and transcriptional regulation differences in EGLN1, causing changes in downstream target transactivation, associated with a blunted hypoxic response.
Highlights
Convergent evolution of adaptive traits in distantly related organisms under the same selective pressures is common, at the genomic, cellular, and phenotypic level (Conte et al 2012; Gompel and Prud’homme 2009; Losos 2011; Stern and Orgogozo 2009)
There were a total of 16,339 hypoxia-inducible factor (HIF) pathway SNPs for the cinnamon teal, 21,674 SNPs for speckled teal, and 26,484 SNPs for the yellow-billed pintail, which likely reflects apparent differences in the effective population sizes (Ne) of these three species (Graham et al 2018; McCracken et al 2009c; Wilson et al 2013)
The cinnamon teal showed evidence of outliers in seven HIF pathway genes, none were significant in the corresponding Bayesian analysis; the evidence for convergence on the HIF pathway as a means of adaptation to high-altitude environments was much stronger in the speckled teal and yellow-billed pintail —our results suggest that selection acted on the same genes (EPAS1, EGLN1; Figs 2 and 3) and even the same gene regions, though not at the same amino acid positions
Summary
Convergent evolution of adaptive traits in distantly related organisms under the same selective pressures is common, at the genomic, cellular, and phenotypic level (Conte et al 2012; Gompel and Prud’homme 2009; Losos 2011; Stern and Orgogozo 2009). Under hypoxic conditions, such degradation of the HIFA (HIF1α, HIF2α/EPAS1) subunits does not occur, allowing the HIF heterodimers to enter the nucleus where they recognize hypoxia-response elements (HREs) within the promoters of a large number of genes, effecting changes in transcriptional activity (Fig. 1) (Wenger et al 2005). Many of these target genes increase O2 transport to hypoxic tissues by promoting red blood cell maturation and angiogenesis/vasomotor control (Haase 2013; Majmundar et al 2010)
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