Gene copy number is differentially regulated in a multipartite virus.

Multipartite viruses have a genome divided into several nucleic acid segments, each encapsidated separately. An evident cost for these viral systems, particularly if some segments are rare, is the difficulty of gathering one copy of each segment to ensure infection. Here, we investigate the segment frequency-related cost by monitoring the copy number of the eight single-gene segments composing the genome of a plant nanovirus. We show that some viral genes accumulate at low frequency, whereas others dominate. We further show that the relative frequency of viral genes impacts both viral accumulation and symptom expression, and changes specifically in different hosts. Earlier proposed benefits of viral genome segmentation do not depend on the segments' frequency and cannot explain our observations. We propose that the differential control of gene/segment copy number may represent an unforeseen benefit for multipartite viruses, which may compensate for the extra costs induced by the low-frequency segments.