Avian Infectious Bronchitis Coronavirus Variation and Selection

Abstract

Infectious bronchitis virus (IBV) is a highly variable virus, its genetic variation is generated by nucleotide insertions, deletions, or point mutations made by the viral polymerase during virus replication. Another mechanism for IBV variability is RNA recombination. Selection of IBV in the host has also been previously demonstrated. Specific objectives during this three-part investigation were to assess IBV intraspatial variation in chickens, determine the IBV evolutionary pathway in viral immunodeficient hosts, and evaluate IBV variation and effects on testes of young and adult roosters. Variation inside the chicken (Intraspatial variation) was assessed, we inoculated chickens with an Ark-type IBV commercial vaccine and characterized the sequences of the spike (S) 1 gene of IBV contained in tear fluid, trachea, and reproductive tract. The predominant IBV phenotype contained in the vaccine (prior to inoculation), became a minor or non-detectable population at all times in all tissues after replication in the chickens. Five new predominant populations designated as component (C) 1 through C5, showing distinct non-synonymous changes, were detected in the tissues or fluids of individual vaccinated chickens. Significant differences were detected in the incidence of some distinct predominant IBV populations in tissues and fluids. These results indicate for the first time that IBV undergoes intraspatial variation during the host invasion. We evaluated the same portion of the S1 gene sequence of the dominant phenotype of an IBV Ark vaccine strain during continued passages in chickens infected iii with the immunosuppressive viruses (CAV and/or IBDV) as well as in immunocompetent chickens. The dominant genotype of the vaccine strain was rapidly negatively selected in all chicken groups. Based on S1 geno/phenotype, exactly the same IBV subpopulations detected in the previous experiment emerged. During the first passage several subpopulations emerged in each group followed by establishment of one predominant population after further passages. Only subpopulation C2 successfully became established in either CAV or IBDV infected chickens. These results indicate that selection does not cease in immunodeficient chickens. Subpopulations C1 or C4 became established in immunocompetent birds but became extinct after only a few succeeding passages. A similar result was observed in chickens co-infected with CAV+IBDV. This finding constitutes further evidence for phenotypic drift occurring mainly as a result of selection. Finally, we assessed IBV virulent strain variation and its effect on the chicken testes. Pre-puber males were inoculated with two IBV virulent strains (M41 and Ark). IBV RNA was detected in most of the testicles of both inoculated male groups. Marginal non-synonymous variation was detected in the S gene of the predominant population of IBV replicating in the testes. IBV M41 and Ark were detected in spermatogonia and Sertoli cells of testicles of infected roosters by immunofluorescence, without histopathological changes. Venereal transmission of IBV was demonstrated by artificially inseminating hens. IBV RNA was detected in the trachea of all hens inseminated with IBV-spiked semen and in 50% of hens inseminated with semen from IBV-infected males providing experimental evidence for IBV venereal transmission. iv These results show high genetic variability of IBV populations resulting from mutation and selection, following the same evolutionary mechanisms originally described by Darwin for more complex species.