A domesticated totivirus-like tandem array undergoes interspecific transfer and asymmetric evolution.

STORM, a tandem array of four totivirus-derived ORFs (TLC1–TLC4) in wood- and insect-associated yeasts, evolved about seven-fold faster than related exogenous totiviruses over at least 15 million years.

• Why it matters: Integrated nonretroviral viral genes are expected to evolve slowly in host genomes, yet STORM’s long-term maintenance and divergence raise questions about how such paleoviral arrays persist and change.
• What they did: The study characterized STORM across yeast lineages, used host-calibrated rate analyses to compare evolutionary speeds with exogenous totiviruses, and applied RELAX and BUSTED to test for relaxation and episodic positive selection.
• The result: Evolution was asymmetric: RELAX found strong gene-wide relaxation in TLC4 (K = 0.29) with purifying selection at several predicted decapping-associated positions, and RNA-seq showed paralog-specific expression.

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