Life history focus on a malaria parasite: linked traits and variation among genetic clones

Abstract

Life history theory has long been a major campaign in evolutionary ecology, but has typically focused only on animals and plants. Life history research on single-celled eukaryotic protists such as malaria parasites (Plasmodium) will offer new insights into the theory’s general utility as well as the parasite’s basic biology. For example, parasitologists have described the Plasmodium life cycle and cell types in exquisite detail, with little discussion of evolutionary issues such as developmental links between traits. We measured 10 life history traits of replicate single-genotype experimental Plasmodium mexicanum infections in its natural lizard host to identify groups of linked traits. These 10 traits formed 4 trait groups: “Rate/Peak” merges measures of growth rate and maximum parasitemia of infections; “Timing” combines time to patency and maximum parasitemia; “Growth Shape” describes the fit to an exponential growth curve; and “Sex Ratio” includes only the gametocyte sex ratio. Parasite genotype (clone) showed no effect on the life history trait groups, with the exception of gametocyte sex ratio. Therefore, variation in most life history traits among infections appears to be driven by environmental (individual host) effects. The findings support the model that life history traits are often linked by developmental constraints. Understanding why life history traits of Plasmodium are linked in this way would offer a new window into the evolution of the parasites, and also should inform public health efforts to control infection prevalence.