An unusual mosaic structure of the PIM gene of Theileria parva and its relationship to allelic diversity.

Genetic diversity and structural organisation of the polymorphic immunodominant molecule (PIM) gene of the protozoan parasite Theileria parva was studied in isolates from sympatric and allopatric areas. The analyses revealed a mosaic structure consisting of highly conserved regions shared among some of the isolates from geographically different areas and homologous sequence runs shared among isolates from one area. The specific pattern of diversity in which large insertions and deletions were observed, giving a mosaic structure to the PIM locus, is quite exceptional for single-locus genes. The polymorphic middle region of the gene was characterised by large deletions or insertions in many isolates. There was no correlation between the copy number of the tetrapeptide repeats in this region and the total length of the sequence. The gene was highly polymorphic when compared with sequences from other known T. parva antigenic regions. The findings support the concept that as yet unidentified mechanisms are generating extensive diversity and shaping the PIM locus. The relevance of this finding for diagnosis and the relationship between these mechanisms and the possible role of this protein in host immune responses is discussed.