Dr. Tyler A. Elliott (@transposableman.bsky.social)

Elevated retrocopy burden and sloth-specific expansions illuminate mammalian genome evolution - BMC Biology Background Xenarthrans, comprising sloths, anteaters, and armadillos, represent one of the most morphologically and physiologically specialized mammalian clades, yet the genomic basis of their adaptations remains poorly understood. Here, we present chromosome-level genomes for the two-toed sloth (Choloepus didactylus) and the southern anteater (Tamandua tetradactyla) and investigate how retrotransposon-mediated gene duplications (retrocopies) have shaped genome evolution in these and other species in Xenarthra. Results Comparative analyses revealed that the xenarthran genomes analyzed here harbour the highest number of retrocopies reported among mammals, with lineage-specific insertion dynamics. Anteater and armadillo genomes contain older LINE1 repertoires and species-specific older retrocopy insertions. In contrast, sloths retain both an abundance of young LINE1s and thousands of young retrocopies, alongside a large shared set that originated from an evolutionary burst of retroduplication in the branch leading to their last common ancestor (~ 30 Mya). In C. didactylus, 49% of retrocopies were found to be expressed in five tissues, compared with 27% in Dasypus novemcinctus in three tissues. Evolutionary analyses identified 38 retrocopies with strong hallmarks of domestication in C. didactylus. Many of these retrocopies derive from parental genes involved in mitochondrial and metabolic processes, suggesting a potential genomic contribution to the physiological specializations of sloths. Conclusions Altogether, our findings identify retrotransposition as a major contributor to the genomic architecture of the xenarthrans presented here and highlight retrocopy origination as a mechanism for generating lineage-specific novelty and, possibly, distinctive biological specializations. https://doi.org/10.1186/s12915-026-02632-5