An ancient alpharetrovirus lineage in bats: Evolutionary insights and possible roles in reproduction

Alpharetroviruses are an important group of pathogens known to cause leukemias and tumors, and were historically considered to be restricted to avian hosts. The identification of alpharetrovirus-like envelopes in bat genomes has hinted at a potentially wider host range, although their relations to modern alpharetroviruses and distribution remains unclear. Through a paleovirological screening of 818 vertebrate genomes we identified CHIRalphaEnv, a lineage that belongs firmly within alpharetroviruses, and emerged from a cross-class transmission from saurian hosts. We determine that CHIRalphaEnv envelope genes have been co-opted across bats on eight separate occasions between 43.8 and 18.9 million years ago and are preserved in most bat genomes screened. CHIRalphaEnv elements encode full-length envelope proteins and have been maintained under purifying selection, demonstrating multiple instances of exaptation by their bat hosts and a likely ubiquitous function. We observe high expression levels of CHIRalphaEnv envelopes in endometrium tissue from Carollia perspicillata, suggesting an involvement in reproductive function. We find CHIRalphaEnv sequence relatives in multiple mammalian clades (Afrotherians, rodents and bats), expanding the host range and extending origins of alpharetroviruses beyond 43 million years. We also propose the first mammalian co-opted Endogenous retrovirus (ERV) derived from an Alpharetrovirus envelope and explore the convergent functional recruitment of CHIRalphaEnv in hemochorial placentation in bats, elephant shrews and spiny mice. These findings highlight alpharetroviruses as a previously underappreciated source of functional exaptation in mammals.