|STEPIEN, CAROL - University Of Toledo|
|PIERCE, LINDSEY - University Of Toledo|
|LEAMAN, DOUGLAS - University Of Toledo|
|NINER, MEGAN - University Of Toledo|
Submitted to: PLoS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/18/2015
Publication Date: 8/27/2015
Citation: Stepien, C.A., Pierce, L.R., Leaman, D.W., Niner, M.D., Shepherd, B.S. 2015. Genetic diversification of an emerging pathogen: A decade of mutation by the fish Viral Hemorrhagic Septicemia (VHS) virus in the Laurentian Great Lakes [online serial]. PLoS One. https://doi.org/10.1371/journal.pone.0135146.
Interpretive Summary: Viral Hemorrhagic Septicemia virus (VHSv) causes one of the world's most serious finfish diseases, infecting >80 marine and freshwater species across the Northern Hemisphere. A new and especially virulent substrain, IVb, first appeared in the Great Lakes a decade ago and spread rapidly, causing massive fish kills in 2005 and 2006; IVb also diversified genetically through the years. The various genes that make up the VHS pathogen are thought to contribute to its virulence or ability to evade and suppress the immune response of infected fish. The present study evaluates mutational changes in genes of the VHSv-IVb pathogen across its decade of emergence in the Great Lakes. Using sequences from existing and new viral isolates, we compared relative substitution patterns and rates for the VHSv G-, Nv-, P-, and M- genes to determine their evolutionary diversification. Analysis shows that the Nv-gene evolves the most rapidly, but changes were also seen in the G-, P- and M- genes. Rapid evolutionary diversification of VHS may allow new viral variants to evade fish host recognition and immune responses, enabling long-time persistence in populations along with expansion to new geographic areas. Understanding how VHSv changes over time and space may enable prediction of its future spread, increase our understanding of pathogen virulence, augment our understanding of rapid evolution in various fish species, and aid in the rational design of vaccines to combat spread of this pathogen.
Technical Abstract: Viral Hemorrhagic Septicemia virus (VHSv) is an RNA rhabdovirus, which causes one of the world's most serious fish diseases, infecting >80 freshwater and marine species across the Northern Hemisphere. A new, novel, and especially virulent substrain - VHSv-IVb - first appeared in the Laurentian Great Lakes about a decade ago, resulting in massive fish kills. It rapidly spread and has genetically diversified. This study analyzes temporal and spatial mutational patterns of VHSv-IVb across the Great Lakes for the novel non-virion (Nv) gene that is unique to this group of novirhabdoviruses, in relation to its glycoprotein (G), phosphoprotein (P), and matrix (M) genes. Results show that the Nv-gene has been evolving the fastest (k=2.0x10^-3 substitutions/site/year), with the G-gene at ~1/7 that rate (k=2.8x10^-4). Most (all but one) of the 12 unique Nv- haplotypes identified encode different amino acids, totaling 26 changes. Among the 12 corresponding G-gene haplotypes, seven vary in amino acids with eight total changes. The P- and M- genes are more evolutionarily conserved, evolving at just ~1/15 (k=1.2x10^-4) of the Nv-gene's rate. The 12 isolates contained four P-gene haplotypes with three amino acid changes, and six M-gene haplotypes with two amino acid differences. Patterns of evolutionary changes coincided among the genes for some of the isolates, but appeared independent in others. New viral variants were discovered following the large 2006 outbreak; such differentiation may have been in response to fish populations developing resistance, meriting further investigation. Two 2012 variants were isolated by us from central Lake Erie fish that lacked classic VHSv symptoms, having genetically distinctive Nv-, G-, and M-gene sequences (with one of them also differing in its P-gene); they differ from each other by a G-gene amino acid change and also differ from all other isolates by a shared Nv-gene amino acid change. Such rapid evolutionary differentiation may allow new viral variants to evade fish host recognition and immune responses, facilitating long-time persistence along with expansion to new geographic areas.