Linkage Group Selection to Investigate Genetic Determinants of Complex Traits of Malaria Parasites
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Malaria parasites of the species infecting humans and animal hosts exhibit genetic and phenotypic diversity. Some of this diversity, including the responses to anti-malarial drugs, growth rate and virulence and antigenic variability, is medically significant. This is because these phenotypes may determine the existence and survival of the parasites in the host and, in turn, contribute to the clinical outcome of infection. Understanding of the biological characteristics and the genetic basis underlying these complex phenotypes can thus lead to the development of effective control strategies against the disease, such as anti-malarial drugs and vaccines. Genetic studies in rodent malaria parasites have proved useful in providing insights into the genetic determinants of these complex traits and thus can be used to complement the study of human malaria. The present studies aim to investigate genetic determinants underlying two major medically important phenotypes, Strain Specific Protective Immunity (SSPI) and Growth rate, using the newly devised genetic method of Linkage Group Selection (LGS). The results presented here relate to the accomplishment of these aims. LGS analysis of SSPI using a genetic cross between clones AJ and CB-pyr10 of Plasmodium chabaudi chabaudi has identified a single region on chromosome 8 containing the gene for the Merozoite Surface Protein-1 as encoding a major target of SSPI. A similar finding was also obtained in a previous LGS study using a different genetic cross between clones AS-pyr1 and CB of P. c. chabaudi (Martinelli et al., 2005). Hence, the results of two independent studies strongly indicate that a single locus within the parasite genome contains a major target antigen, or antigens, of SSPI against P. c. chabaudi malaria. These results have particular relevance for research on SSPI in human malaria and the choice of candidate antigens for malaria vaccine development. LGS analysis of growth rate conducted upon a genetic cross between a fast-growing line, 17XYM, and a slow-growing line, 33XC, of Plasmodium yoelii yoelii has identified a ~ 1 megabase pair region on P. y. yoelii chromosome 13 as containing a major genetic determinant(s) of growth rate in these malaria parasites. This is consistent with the finding of the classical linkage analysis by Walliker et al., (1976), that growth rate in P. y. yoelii is mainly determined at a single genetic locus. Because the fast-growing line 17XYM arose spontaneously during infection with a mild strain of P. y. yoelii 17X, identification of parasites with a slow growth rate phenotype derived from the same genetic stock as 17XYM can be useful in determining genes underlying growth rate in these malaria parasites. It has been shown here that parasites of the P. y. yoelii lines 17X consist of two completely distinct genotypes. One is represented by the fast-growing line, 17XYM, and a slow-growing line of P. y. yoelii, 17XNIMR. The other is represented by another slow-growing line 17XA. Comparing the region of P. y. yoelii chromosome 13 under strong growth selection between the two congenic lines, 17XYM and 17XNIMR, could lead to the identification of the gene(s) controlling growth rate differences in these two parasite lines. Such findings could be relevant to the location of genetic determinants of growth rate in human malaria.