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Diversifying selection in a parasitoid's symbiotic virus among genes involved in inhibiting host immunity

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Abstract

During parasitization of their hosts some insect parasitoids deliver resident viruses which encode genes that must be expressed in the host for successful parasitization. Among these viruses the Campoletis sonorensis Ichnovirus has been well studied and encodes a cys-motif gene family implicated in disruption of host immunity and other physiological systems. Members of this gene family encode one or more intercystine-knot structural motifs in which the non-cysteine residues of the motif are variable. We analyzed patterns of synonymous and non-synonymous substitution within the cys-motif to investigate the evolution of this gene family and the likelihood of virus-host gene coevolution. Maximum likelihood techniques suggest positive selection acts on 8 of 51 codons in the aligned cysteine-rich region. Although the detected positive selection was not strong, it likely contributes to the diversification of this gene family. Comparison of selection pressure relative to tertiary structure of the VHv1.1 cys-motif protein suggests that the hypervariable sites are exposed. Furthermore, invariant residues in the motif exhibit a region-specific pattern of codon bias, suggesting there are unusual mechanisms of effecting selection pressure at work in this system, though the mechanism has yet to be studied. The positive selection and duplication of both the gene family and the cys-motif implies either selection is driving the molecular radiation of immune suppressive genes toward novel hosts, or molecular coevolution with host targets.

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Acknowledgements

We thank Dr. Austin Hughes for his communications and SCR3 software. We also thank Prof. Yves Carton and Dr. Mike Sharkey for their constructive discussions, Dr. David Wahl for supplying most identified specimens, Prof. Ziheng Yang for his pioneering work on the subject and Dr. Valerie Copie for donation of the coordinates for the VHv1.1 structure. This work was partially supported by an NSF grant to BAW (MCB 0094403) and is publication 03-08-079 of the Kentucky Experiment Station.

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Authors and Affiliations

  1. Department of Entomology, University of Kentucky, S-225 Agricultural Science Building North, Lexington, KY 40546-0091, USA

    Stéphane Dupas, Matthew W. Turnbull & Bruce A. Webb

  2. Laboratoire Populations Génétique et Evolution, CNRS, 91198, Gif-sur-Yvette, Cedex, France

    Stéphane Dupas

  3. Department of Entomology, Soils and Plant Sciences, Clemson University, Clemson, SC 29634-0364, USA

    Matthew W. Turnbull

Authors
  1. Stéphane Dupas
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  2. Matthew W. Turnbull
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  3. Bruce A. Webb
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Corresponding author

Correspondence to Bruce A. Webb.

Additional information

Novel nucleotide sequence data reported are available in the DDBJ/EMBL/GenBank databases under the accession numbers AY033945, AY197489, AY197490, AY197491, AY197492, AY197493 and AY197494

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Dupas, S., Turnbull, M.W. & Webb, B.A. Diversifying selection in a parasitoid's symbiotic virus among genes involved in inhibiting host immunity. Immunogenetics 55, 351–361 (2003). https://doi.org/10.1007/s00251-003-0595-4

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  • DOI: https://doi.org/10.1007/s00251-003-0595-4

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