Horizontally acquired mitochondrial alternative oxidase contributes to springtail bioenergetics and life belowground

Springtails are tiny hexapods that occupy habitats ranging from above the soil surface to hypoxia-prone belowground environments, yet the physiological mechanisms enabling this ecological expansion have remained unresolved. Here, I take an evolutionary bioenergetics approach to show that a mitochondrial alternative oxidase (AOX) acquired by horizontal gene transfer in the springtail ancestor became functionally integrated into animal physiology and was preferentially retained in lineages occupying low-oxygen habitats. Surveying 202 springtail genome assemblies, I identified 65 high-confidence AOX loci in 48 species, each embedded within otherwise typical springtail genomic neighborhoods. Phylogenetic and motif-based analyses support an oomycete donor and indicate ancestral acquisition followed by repeated loss, especially in aboveground taxa. High-resolution respirometry and hypoxia-exposure experiments further show that AOX is active only in AOX-positive species and is associated with hypoxia tolerance. These results identify horizontal gene transfer as a source of ecophysiological innovation in animals and suggest that acquired mitochondrial functions can help shape ecological sorting across environmental gradients, with implications for soil ecosystem processes. Significance StatementHorizontal gene transfer can introduce new genes into animal genomes, but few cases connect gene origin to physiology and ecology. Springtails are abundant soil mesofauna that help define ecological systems and nutrient flux yet physiological challenges that subterranean environments impose raise questions regarding the mechanisms of adaptation to life below ground. This study shows that springtails acquired mitochondrial alternative oxidase (AOX), a respiratory enzyme widely associated with stress tolerance in plants, fungi, protists, and some animals, from an oomycete donor early in their evolutionary history and that AOX has been retained in hypoxia-tolerant species and incorporated it into the physiology as a potential mechanism for tolerance of belowground conditions. These findings link macroevolutionary patterns to bioenergetic processes that help shape ecological distribution and tolerance of environmental stressors, with implications for biodiversity and ecosystem function.