Theileria parva ribosomal internal transcribed spacer sequences exhibit extensive polymorphism and mosaic evolution: application to the characterization of parasites from cattle and buffalo

We sequenced the rRNA genes and internal transcribed spacers (ITS) of sever al Theileria parva isolates in an attempt to distinguish between the causat ive agents of East coast fever and Corridor disease. The small subunit (SSU ) and large subunit (LSU) rRNA genes from a cloned T. p. lawrencei parasite were sequenced; the former was identical to that of T. p. parva Muguga, an d there were minor heterogeneities in the latter. The 5.8S gene sequences o f 11 T. parva isolates were identical, but major differences were found in the ITS. Six characterization oligonucleotides were designed to hybridize w ithin the variable ITS1 region; 93.5% of T. p. parva isolates examined were detected by probe TPP1 and 81.8% of T. p. lawrencei isolates were detected by TPL2 and/or TPL3a. There was no absolute distinction between T. p. parv a and T. p. lawrencei and the former hybridized with fewer of the probes th an did the latter. It therefore seems that a relatively homogenous subpopul ation of T. parva has been selected in cattle from a more diverse gene pool in buffalo. The ITSs of both T. p. parva and T. p. lawrencei contained dif ferent combinations of identifiable sequence segments, resulting in a mosai c of segments in any one isolate, suggesting that the two populations under go genetic recombination and that their gene pools are not completely separ ate.