Genomic approaches to understanding host resistance and parasite virulence in Trypanosoma parasites

Roughly one-third of cattle in sub-Saharan Africa are at risk of contracting "Nagana" - a disease caused by Trypanosoma parasites similar to those that cause human "Sleeping Sickness". Laboratory mice can also be infected by trypanosomes, and different mouse breeds show varying levels of susceptibility to infection, similar to what is seen between breeds of cattle. We have applied next-generation technologies to identify shared polymorphisms between susceptible mice, and annotated these for potential function alongside publicly available SNP data sets. By so doing, short lists of genes at the QTL have been created to aid functional testing in cattle. This includes two promising 'candidate genes': Pram1 and Cd244, which can now be tested to confirm their effect on response to trypanosome infection. The human-infective parasite Trypanosoma brucei rhodesiense generally causes an acute form of "sleeping sickness" across Eastern Africa, compared to the more chronic T b. gambiense infections found in Western Africa. The 1988-1993 Ugandan T b. rhodesiense outbreak constituted infections by parasites with differences in their clinical manifestation. Two such sub types, termed Busoga 17 (B 17) and Zambesi 310 (Z31O), caused more acute, and more chronic infections, respectively. In order to investigate whether the major QTL that regulates survival in T congolense infections (Tir 1) does so in a similar manner in T b. rhodesiense, mice congenic for the C5 7BL/ 6 allele (Tirl CC) at Tir 1 were infected with Z310 and B 1 7 zymodeme T b. rhodesiense parasites. Whilst Tir 1 was not found to have a significant effect on survival, all mice had a significantly shorter mean survival time when infected with B 17 (~1O. 7 days) than those infected with Z31 0 (~15.6 days), in line with previous observations of human infections. In order to identify genetic loci that might underlie differences in virulence between T b. rhodesiense zymodemes, cluster analysis was performed on the microsatellite genotypes of 31 T b. rhodesiense isolates that represented nine different zymodemes. Despite STRUCTURE identifying three population clusters, the Z310 and B 17 parasite populations could not be distinguished, suggesting that either multiple genes control virulence, that there is gene flow between similar parasite populations, or that the microsatellite genotyping is insufficient to distinguish between different parasite populations. Finally, we present the first whole-genome sequences of T b. rhodesiense field isolates, one each of Z310 and B 1 7. Genomic analysis of east African T b. rhodesiense and west African T b. gambiense has suggested that recombination may be occurring between them. SNP genotyping of 32 T b. rhodesiense isolates showed that differences in clinical phenotypes were associated with differences in alleles on chromosome 8. The genome sequence suggests that chromosome 8 is heterozygous for alleles of west African origin in the more virulent strain, suggesting that recombination may be associated with parasite virulence. This suggests that the human subspecies of T brucei are not genetically distinct, which has major implications for the control of the parasite, the spread of drug resistance and understanding the variation in virulence and the emergence of human infectivity. Further genetic analysis of T b. brucei populations from Western, central and Eastern Africa may be necessary to ascertain whether recombination is occurring directly between human-infective subspecies, or in the underlying animal-infective population.EThOS - Electronic Theses Online ServiceGBUnited Kingdo