Evolution of Genome Size and Complexity in the Rhabdoviridae

Abstract

RNA viruses exhibit substantial structural, ecological and genomic diversity. However, genome size in RNA viruses is likely limited by a high mutation rate, resulting in the evolution of various mechanisms to increase complexity while minimising genome expansion. Here we conduct a large-scale analysis of the genome sequences of 99 animal rhabdoviruses, including 45 genomes which we determined de novo, to identify patterns of genome expansion and the evolution of genome complexity. All but seven of the rhabdoviruses clustered into 17 well-supported monophyletic groups, of which eight corresponded to established genera, seven were assigned as new genera, and two were taxonomically ambiguous. We show that the acquisition and loss of new genes appears to have been a central theme of rhabdovirus evolution, and has been associated with the appearance of alternative, overlapping and consecutive ORFs within the major structural protein genes, and the insertion and loss of additional ORFs in each gene junction in a clade-specific manner. Changes in the lengths of gene junctions accounted for as much as 48.5% of the variation in genome size from the smallest to the largest genome, and the frequency with which new ORFs were observed increased in the 3’ to 5’ direction along the genome. We also identify several new families of accessory genes encoded in these regions, and show that non-canonical expression strategies involving TURBS-like termination-reinitiation, ribosomal frame-shifts and leaky ribosomal scanning appear to be common. We conclude that rhabdoviruses have an unusual capacity for genomic plasticity that may be linked to their discontinuous transcription strategy from the negative-sense single-stranded RNA genome, and propose a model that accounts for the regular occurrence of genome expansion and contraction throughout the evolution of the Rhabdoviridae.

Author Summary

Understanding the patterns and mechanisms of genome evolution is one of the most important, yet least understood, aspects of RNA virus biology. The evolutionary challenge faced by RNA viruses is to maximize functional diversity within severe constraints on genome size. Here we show that rhabdoviruses, a family of RNA viruses that infect hosts as diverse as plants, insects and vertebrates, have an unusual capacity for genomic plasticity. By analysing the complete or near-complete genome sequences of 99 animal rhabdoviruses, we show that genome expansion and contraction has likely occurred frequently throughout the evolution of the family. Genomic plasticity has been associated with the evolution of alternative, overlapping and consecutive ORFs within the major structural protein genes, as well as the insertion and loss of additional ORFs in each gene junction in a clade-specific manner. This has resulted in remarkable diversity in genome organisation and gene expression strategies that is reflective of the broad ecological diversity of rhabdoviruses. We conclude that genomic plasticity in rhabdoviruses may be linked to their discontinuous transcription strategy from the negative-sense single-stranded RNA genome and propose a general model that accounts for both gains and losses in genome size and complexity.