Stress and sex in malaria parasites

Abstract

For vector-borne parasites such as malaria, how within- and between-host processes interact to shape transmission is poorly understood. In the host, malaria parasites replicate asexually but for transmission to occur, specialized sexual stages (gametocytes) must be produced. Despite the central role that gametocytes play in disease transmission, explanations of why parasites adjust gametocyte production in response to in-host factors remain controversial. We propose that evolutionary theory developed to explain variation in reproductive effort in multicellular organisms, provides a framework to understand gametocyte investment strategies. We examine why parasites adjust investment in gametocytes according to the impact of changing conditions on their in-host survival. We then outline experiments required to determine whether plasticity in gametocyte investment enables parasites to maintain fitness in a variable environment. Gametocytes are a target for anti-malarial transmission-blocking interventions so understanding plasticity in investment is central to maximizing the success of control measures in the face of parasite evolution.

Avian malaria: a new lease of life for an old experimental model to study the evolutionary ecology ofPlasmodium

“…We calculated the reproductive effort of malaria parasites as the density of gametocytes relative to the total number of parasites at the peak of the infection. For simplicity, we term this 'conversion ratio', to distinguish it from 'conversion rate' which refers to the investment into gametocytes across parasite cohorts [26]. In our system, gametocytes constituted on average a third (mean + s.e., 33 + 2.4%) of the intracellular parasite population in the blood.…”

Sexual development in Plasmodium parasites: knowing when it's time to commit

“…This makes evolutionary sense for the parasite: if the host is dying (for example, as a result of high parasitaemia) then transmission to the mosquito must occur if the parasite is to survive. However, reproductive restraint also has a role in determining whether commitment occurs, as the parasite faces a trade-off between continued asexual proliferation and transmission to a new host 25,26 . Indeed, in natural infections, drug treatment and high parasitaemia do not necessarily lead to increased numbers of gametocytes 27,28 .…”

Hypothesis: Dynamics of Classical Malaria Epidemics Show Plasmodium falciparum's Survival Strategy

“…28 Anemia and other seasonal effects on parasite growth have been shown to increase gametocytes in Thailand. 29 Sublethal treatment with antimalarial drugs, especially antifolates, such as sulfadoxine/pyrimethamine, and recrudescent infections also increase the number of people with gametocytes.…”