Convergent reduction of olfactory genes and olfactory bulb size in mammalian species at altitude.

The invasion of specialized ecological niches can cause drastic changes to selection regimes, resulting in genomic and phenotypic transformation.¹ High-altitude habitats offer an excellent opportunity to investigate the genetic basis of local adaptation,^2,3 as the repeated specialization of multiple lineages for high altitude has produced striking examples of convergent evolution, adaptation, and changes in their underlying genes.^4,5,6 Although enlightening, this focus on adaptation has left aspects of evolution in high-altitude locations understudied-including the role of gene loss and pseudogenization, maladaptation and trait loss, and physiological aspects outside of respiration and gas exchange. To characterize how mammals responded to high altitude in a new, unbiased way, we screened the genomes of 27 species living exclusively at high altitude (>1,000-1,500 m) and their lowland relatives for inactivated pseudogenes or lost genes.⁷ Genes that convergently lost function in high-altitude species were highly enriched for olfactory receptor (OR) genes, with an average reduction of ∼23% of OR repertoire in high-altitude species. No such trend was found for genes involved in pheromone detection and taste perception. In addition to OR loss, cranial endocasts show the brains of high-altitude species have on average ∼18% smaller olfactory bulbs relative to lowland relatives. Together, these repeated evolutionary outcomes suggest a general relaxation of constraint on olfaction at altitude, perhaps due to reduced odorant diversity in high-altitude environments or reduced effectiveness of mammalian olfactory physiology in thin, dry, or cold air.