Does IBV change slowly despite the capacity of the spike protein to vary greatly?

We have sequenced that part of the spike protein (S) gene which encodes the aminoterminal and most variable quarter (hypervariable region, HVR) of the S1 subunit of 28 isolates of the 793/B (also known as CR88 and 4/91) serotype of infectious bronchitis virus (IBV) and the whole of S1 for nine of them. The isolates were from France and Britain between the years 1985 (first isolation) and 1996. The maximum nucleotide and amino acid differences between the first isolate and the others were 4.1% and 7.6%, respectively, for the whole of S1 and 7.1% and 14.6%, respectively, in the HVR. Analysis within clearly recognisable subgroups suggested that even in the HVR the nucleotide mutation rate was only 0.3 to 0.6% per year. However, there was no evidence that mutations had become fixed in a progressive manner; this serotype did not appear to be evolving. Strains isolated several years apart could be more similar than those isolated in a given year. It is likely that the amino acid changes are largely at positions where amino acid differences are tolerated rather than as a consequence of immune pressure. Reasons for this conclusion are discussed.