Extensive Genotypic Diversity in a Recombining Population of the Apicomplexan ParasiteTheileria parva

Abstract

We evaluated sexual recombination in the apicomplexan parasite Theileria parva using genome-wide marker analysis of haploid sporozoite populations obtained from infected Rhipicephalus appendiculatus ticks. Analysis of 231 parasite clones derived by in vitro infection of bovine lymphocytes revealed 48 distinct combinations of 64 polymorphic marker loci. One genotype accounted for more than 75% of the clones, and the population was highly inbred with respect to this. The occurrence of frequent recombination was evident from reassortment of contiguous markers in blocks, with some recombination occurring within blocks. Analysis of four polymorphic loci encoding antigens targeted by protective cytotoxic-T-lymphocyte responses confirmed that these loci reassort, both within and between chromosomes, suggesting that recombination may influence immune recognition. Marker analysis of a panel of 142 clones derived from the population after an additional passage through a calf and the same tick colony revealed 18 genotypes, with the original dominant genotype accounting for 75% of the population and a higher level of inbreeding with respect to it in the remaining clones. Selected marker analysis of genomic DNA from these stabilates and the two preceding generations of the isolate, each derived from distinct tick colonies, revealed shifts in population structure with each generation, suggesting that the tick vector may impose nonrandom selective pressure on the parasite.

Influence of Host Immunity on Parasite Diversity inTheileria parva

“…Using a genome-wide panel of polymorphic markers, we have recently provided evidence for extensive genotypic diversity in T. parva populations derived from ticks fed on cattle infected with the Marikebuni stock of the parasite (13). Substantial reassortment of alleles at marker loci was evident on additional calf/tick passage, which is consistent with frequent recombination and the capacity to respond to selective pressures imposed by host or vector.…”

“…The CTVM St72 stabilate was used and represents the fifth passage of the isolate from the field. St72 is dominated by a genotype designated 72-01, which accounts for approximately 75% of the population (13). A third stabilate, St103, was used that was derived from R. appendiculatus ticks fed on an animal infected by inoculation with 5 ϫ 10 6 autologous parasitized cells carrying a 72-01 clone.…”

“…For example, while there is ample evidence for immune selection against Plasmodium falciparum in humans, substantial attrition of malaria parasites also occurs during differentiation in the mosquito vector (28). Considerable attrition (in the region of 4 to 5 orders of magnitude) is also observed during passage of Theileria parva, a related parasite of cattle, through its tick vector, Rhipicephalus appendiculatus (13). However, little is known about the impact of immune selection in the bovine host on the population structure of this parasite, which causes an often fatal lymphoproliferative disease known as East Coast fever in eastern, central, and southern Africa (12).…”

Selection of Genetic Markers to Determine Diversity in Theileria annulata Populations after Recombination

“…Vektörler arasında soy düzeyinde ve bulundukları coğrafik bölgelere adaptasyonları bakımından gözlenen farklılıklar ile etkenlerin vektörlere aktarım yoğunluğu ve vektörlerdeki enfeksiyon düzeyindeki farklılıklar, bölgeler arasında genetik çeşitliliklere neden olabilmektedirler (4,5). Plasmodium falciparum ile ilgili yapılan çalışmalarda, vektöre aktarılma oranlarının düşük ya da yoğun olmasının, farklı bölgelerdeki popülasyon yapısında değişimlere yol açtığı belirlenmiştir (6)(7)(8).…”

“…Deneysel olarak, T. parva'da seksüel rekombinasyonun varlığı moleküler düzeyde kanıtlanmıştır (5). T. annulata popülasyonlarında ise seksüel rekombinasyonu destekler nitelikte veriler elde edilse de bu veriler deneysel olarak kanıtlanmamıştır (18,19).…”