Tracing the evolutionary history of small HERCs reveals an unprecedented expansion in fish related to virus infection.

Small HERCs are a class of E3 ubiquitin ligases composed of a N-terminal RLD (RCC1-like domain) and a C-terminal HECT (Homologous to E6-associated protein C-terminus), and there are four members named HERC3-6 in humans. However, two novel members named HERC5/6 and HERC7 appeared in non-mammalian vertebrates instead of HERC5 and HERC6, raising a question of how small HERCs evolve and function alongside species radiation. Here we identified an ancient member (named HERC3/4) by a genome-wide search of 90 major eukaryotic lineages, thus tracing the origination of small HERCs back to unicellular filozoa approximately 1 billion years ago. With the emergence of vertebrates, HERC3/4 was lost, and other members (HERC3, HERC4, HERC7, HERC5/6, HERC5, and HERC6) appeared successively, exhibiting lineage-specific and even species-specific member gain and loss. Strikingly, HERC7, and in rare cases HERC5/6, have undergone gene expansions in fish lineages but not in non-fish vertebrates, while the other members usually harbor a single copy. Taking yellow catfish Tachysurus fulvidraco as an example, we further provided evidence that, unlike catfish HERC3 and HERC4, catfish HERC7 and HERC5/6 were induced by viral infection to shape fish interferon antiviral response through different molecular mechanisms, indicating that fish-specific expansions of HERC7 and/or HERC5/6 might be essential for fish adapting to unique underwater niches, such as to virus infection. Our results provide insights into the evolutionary history, family classification, fish-specific expansion and function diversification of small HERCs.