A planarian nidovirus expands the limits of RNA genome size

Amir Saberi; Anastasia A. Gulyaeva; John L. Brubacher; Phillip A. Newmark; Alexander E. Gorbalenya

PSCNV likely derived from multi-ORF nidoviruses and expanded disproportionately in the ORF1b-like region. To learn about the evolution of PSCNV, we inferred the rooted phylogeny of PSCNV based on the RdRp protein domain of nidoviruses and two astroviruses ( Fig. 1F ; SI Text). PSCNV predominantly clustered with other invertebrate nidoviruses in the Bayesian sample, basal to either mesoni-and roniviruses (54.7% of the trees), roniviruses (20.6%), or mesoniviruses (13.4%). Ancestral state reconstruction analysis indicated that the most recent common ancestor of nidoviruses contained multiple ORFs (Log Bayes Factor 6.06) (Fig. S17) . Thus, the single-ORF organization of PSCNV likely emerged through loss of signals separating the three ORF regions of ancestral nidoviruses.