Excess of retrogene traffic in pig X chromosome

Despite the insights into retrogene evolution in multiple species provided by trans-traffic retrogenes (trans-retrogenes), cis-traffic retrogenes (cis-retrogenes) are hitherto understudied and thus limit our understanding of genetic novelty. Here we used the pig genome as a model to compare cis- and trans-retrogenes. We found that cis- and trans-retrogenes have similar patterns in terms of excessive X-chromosome representation, testis expression preference, and metabolic functional enrichment. Despite these similarities, lower levels of selective pressure were found in cis- than in trans-retrogenes. The evolutionary dating evidence revealed that more cis-retrogenes have originated after pig-cattle divergence than trans-retrogenes, suggesting a more recent origination of cis-retrogenes. The gene family clustering confirmed that excesses of X-derived cis-retrogenes may be driven by their rapid expansion in the recent times. Thus, the different selective constraints between cis- and trans-retrogenes may be due to different evolutionary time-scales rather than any differential functional importance. Thus, this study highlights the ongoing functional contributions from cis- and trans-traffic retrogenes during the reshaping of pig genome.