Discovery of novel non-retroviral endogenous viral elements reveals their long-term co-evolution with spiders

Endogenous viral elements (EVEs) are widespread in the genomes of various organisms and have played a crucial role in evolution. Historically, research on EVEs primarily focused on those derived from retroviruses; however, the significance of non-retroviral EVEs (nrEVEs) has gradually gained recognition. In this study, we employed a novel approach that combines protein structure prediction with sequence analysis to identify a large group of previously unrecognized nrEVEs across spider genomes. Additionally, we identified nrEVE-related messenger RNAs, microRNAs, and PIWI-interacting RNAs in spiders, suggesting that these nrEVEs may be functionally active. We also experimentally confirmed the presence of spider nrEVEs and their transcripts in individual spiders. Evolutionary analysis suggests that these spider nrEVEs originated from an ancestral nuclear arthropod large DNA virus (NALDV) belonging to the order Lefavirales , class Naldaviricetes , approximately 270 million years ago. The integration of these nrEVEs occurred prior to the last common ancestor of the Araneae, indicating a long-term co-evolutionary relationship between these nrEVEs and spiders. Our findings reveal a novel group of nrEVEs and provide valuable insights into their evolutionary relationship with arthropods.