MIXED-GENOTYPE INFECTIONS OF MALARIA PARASITES - WITHIN-HOST DYNAMICSAND TRANSMISSION SUCCESS OF COMPETING CLONES

Mixed-genotype infections of microparasites are common, but almost not hing is known about how competitive interactions within hosts affect t he subsequent transmission success of individual genotypes. We investi gated changes in the composition of mixed-genotype infections of the r odent malaria Plasmodium chabaudi clones CR and ER by monoclonal antib ody analysis of the asexual infection in mice, and by PCR amplificatio n of clone-specific alleles in oocysts sampled from mosquitoes which h ad fed on these mice. Mixed-clone infections were initiated with a 9:1 ratio of the two clones, with ER as the minority in the first experim ent and CR as the minority in the second experiment. When beginning as the majority, clones achieved parasite densities in mice comparable t o those achieved in control (single-clone) infections. When they began as the minority, clones were suppressed to less than 10 % of control parasitaemias during the early part of the infections. However, in mos quitoes, the frequency of the initially rare clone was substantially g reater than it was in mice at the start of the infection or four days prior to the feed. In both experiments, the minority clone in the inoc ula produced as many, or more, oocysts than it did as a single-clone i nfection. These experiments show that asexual dominance during most of the infection is poorly correlated to transmission probability, and t herefore that the assumption that within-host population size correlat es to transmission probability may not be warranted. They also raise t he fundamental question of why transmission rates of individual genoty pes are often higher from mixed than single-clone infections.