Fitness of sulfadoxine-resistant Plasmodium berghei harboring a single mutation in dihydropteroate synthase (DHPS).

Genetic changes conferring drug resistance are generally believed to impose fitness costs to pathogens in the absence of the drug. However, the fitness of resistant parasites against sulfadoxine/pyrimethamine has been inconclusive in Plasmodium falciparum. This is because resistance is conferred by the complex combination of mutations in dihydropteroate synthase (dhps) and dihydrofolate reductase (dhfr), which makes it difficult to separately assess the extent and magnitude of the costs imposed by mutations in dhps and dhfr. To assess the fitness costs imposed by sulfadoxine resistance alone, we generated a transgenic rodent malaria parasite, P. berghei clone harboring an A394G mutation in dhps (PbDHPS-A394G), corresponding to the causative mutation for sulfadoxine resistance in P. falciparum (PfDHPS-A437G). A four-day suppressive test confirmed that the PbDHPS-A394G clone was resistant to sulfadoxine. PbDHPS-A394G and wild-type clones showed similar growth rates and gametocyte production. This observation was confirmed in competitive experiments in which PbDHPS-A394G and wild-type clones were co-infected into mice to directly assess the survival competition between them. In the mosquitoes, there were no significant differences in oocyst production between PbDHPS-A394G and wild-type. These results indicate that the PbDHPS-A394G mutation alters the parasites to sulfadoxine resistance but may not impose fitness disadvantages during the blood stages in mice and oocyst formation in mosquitoes. These results partly explain the persistence of the PfDHPS-A437G mutant in the natural parasite populations.