A double-edged sword: The multiple roles of endogenous retroviruses in chickens

Chicken endogenous retroviruses (ChERVs), a class of retrovirus-derived elements, originated from ancient retroviral infections that became stably integrated into the avian genome through germline transmission. ChERVs account for 3∼5 % of the chicken genome, with sequences and insertion sites that uniquely illuminate the evolution of avian species and their interactions with viruses. Over their long evolution, ChERVs have undergone mutations and deletions that have widely disabled their autonomous replicability, and only a portion now exhibit transcriptional activity. Recent research has additionally demonstrated that the insertion of ChERVs can result in changes in the traits of chickens, including the production of blue-shelled eggs, the presence of white feathers, and henny feathering. ChERVs also exert a bidirectional regulatory function on the biological functions of chickens. In particular, their long terminal repeats can engage in expressing a host's genes via cis-acting elements, such that they can modulate the expression of genes associated with embryonic development or interferon-stimulated genes. Beyond that, though endogenous retroviruses can generally be transcribed into antisense long non-coding RNAs, thereby activating the antiviral innate immune pathway, ChERVs can also recombine with exogenous viruses, which can give rise to tumors and immunosuppression. Last, ChERVs can cause disease by acting in concert with other avian pathogens, which poses a threat to poultry health. Although studying ChERVs is therefore essential for poultry farming, it remains underexamined in research on numerous avian diseases. In response, in-depth analyses of the epigenetic regulatory mechanisms of ChERVs are imperative, as are studies on the molecular basis of their interactions with hosts, especially ones seeking to provide novel insights into disease-resistant poultry breeding.