SELECTION OF DIVERSITY AT PUTATIVE GLYCOSYLATION SITES IN THE IMMUNODOMINANT MEROZOITE PIROPLASM SURFACE-ANTIGEN OF THEILERIA PARASITES

The immunodominant merozoite/piroplasm surface antigen of Theileria pa rasites has potential as a diagnostic reagent and as a component of a sub-unit vaccine. This molecule is known to be antigenically diverse, and it is important to determine the nature and extent of this heterog eneity. In the present study nucleotide sequences, representing allele s of the gene (Tams1) encoding this molecule in Theileria annulata wer e compared to each other and to sequences of homologous genes in Theil eria sergenti, Theileria buffeli and Theileria parva. This analysis re vealed that a region of the polypeptide which contains putative N-link ed glycosylation sites is particularly diverse and, in analogy to retr oviral systems, may indicate selection of variable glycosylation sites or amino acid epitopes to evade the bovine immune response. This conc lusion was also made from the results of a phylogenetic analysis which compared the variable region of the genes with a second region, which appeared to show no bias for diversity or functional constraint. The results indicated that the variable sequence encoding putative glycosy lation sites has diverged, both within and between Theileria species, at a much faster rate than the rest of the molecule. Southern blot ana lysis of T. annulata populations from within a single geographical reg ion detected six possible variant Tams1 alleles. However, a correlatio n between restriction-fragment-length polymorphism (RFLP) patterns det ected by the Tams1-1 gene probe and geographical location could not be made. Tn addition, although a high prevalence of one particular RFLP was found, this is unlikely to be the result of a clonal population st ructure, as we present evidence for significant parasite genotypic var iability within a single endemic region.