A Genetic Analysis of Two Strains of Plasmodium chabaudi adami that Differ in Growth and Pathogenicity

摘要

Malaria is still a significant public health problem in the Tropics, with an estimatedud200 million cases a year and more than 1 million deaths, mostly in young children inudsub-Saharan Africa. Plasmodium falciparum is the parasite responsible for theudmajority of the morbidity and mortality due to malaria. We know from the historicaluduse of malaria to treat neurosyphilis that there were several different strains of P.udfalciparum, some of which were more pathogenic and had higher multiplication ratesudthan others. High multiplication rates of P. falciparum isolates have been associatedudwith severe disease in Thailand, but not in Kenya or Mali. In determining whatuddifferences exist between fast- and slow-growing malaria parasites, andudunderstanding their relationship with clinical outcome, we may discover a way ofudtargeting those parasites that cause most disease.udThis thesis describes a genetic analysis of the determinants of growth andudpathogenicity in the rodent malaria parasite, Plasmodium chabaudi. The use ofudrodent malaria parasite strains for genetic analysis has several experimental, ethicaludand financial advantages over the use of human malaria parasites. In addition, rodentudmalaria parasite strains also vary significantly in their growth and pathogenicity,udmaking them excellent candidates for a genetic analysis of these characteristics. Theudfirst section of this thesis is concerned with the characterisation of the growth,udpathogenicity and transmissibility of two strains, DS and DK, of the rodent malariaudparasite P. c. adami. The DS strain is fast-growing, pathogenic, non-selective in itsudinvasion of red blood cells and a poor transmitter to mosquitoes. The DK strain isudslow-growing, non-pathogenic, selective in its invasion of red blood cells and a goodudtransmitter to mosquitoes. In the second section of this thesis is a detailed study ofudthe growth characteristics of DS and DK in mixed infections, relative to their growthudin single infections. Both sections provide information relevant for the mainudobjective of this thesis, but also contribute to the body of work on pathogenicity andudtransmissibility, and pathogenicity and strain behaviour in mixed infections, whichudhas been carried out in rodent malaria parasites to-date.udThe third section of the thesis contains the results of a genetic analysis of theuddifference in growth between P. c. adami strains DS and DK, using the LinkageudGroup Selection (LGS) technique. On several occasions, DS and DK were crossed inudthe mosquito vector and, following selection for fast growth in mice, the crossudprogeny were initially screened with genome-wide, quantitative AFLP markers.udMarkers specific to the slow-growing parent DK which were greatly reduced inudintensity after selection were found on P. chabaudi chromosomes 6, 7 and 9. Thisudresult suggests that the difference in growth between the two strains is determined byudmultiple genetic loci. The selection on chromosomes 7 and 9 was then looked at inudgreater detail, using SNP-based markers quantified by Pyrosequencing™. It wasudfound, consistently, that a region at one end of DS chromosome 9 was inherited as audsingle, non-recombining unit in cross progeny selected for fast growth. As this wasudthe region most strongly selected against, it suggests that a gene (or genes) in thisudregion has a major role in the determination of growth characteristics, and thereforeudpathogenicity, in P. c. adami. Narrowing down this region further, in order toudidentify the candidate gene(s), remains a key future objective.