Birget Philip L G, Repton Charlotte, O'Donnell Aidan J, Schneider Petra, Reece Sarah E
Institutes of Evolutionary Biology, and Immunology and Infection Research, University of Edinburgh, Edinburgh EH9 3FL, UK
Institutes of Evolutionary Biology, and Immunology and Infection Research, University of Edinburgh, Edinburgh EH9 3FL, UK.
Proc Biol Sci. 2017 Aug 16;284(1860). doi: 10.1098/rspb.2017.1229.
The trade-off between survival and reproduction is fundamental in the life history of all sexually reproducing organisms. This includes malaria parasites, which rely on asexually replicating stages for within-host survival and on sexually reproducing stages (gametocytes) for between-host transmission. The proportion of asexual stages that form gametocytes (reproductive effort) varies during infections-i.e. is phenotypically plastic-in response to changes in a number of within-host factors, including anaemia. However, how the density and age structure of red blood cell (RBC) resources shape plasticity in reproductive effort and impacts upon parasite fitness is controversial. Here, we examine how and why the rodent malaria parasite alters its reproductive effort in response to experimental perturbations of the density and age structure of RBCs. We show that all four of the genotypes studied increase reproductive effort when the proportion of RBCs that are immature is elevated during host anaemia, and that the responses of the genotypes differ. We propose that anaemia (counterintuitively) generates a resource-rich environment in which parasites can afford to allocate more energy to reproduction (i.e. transmission) and that anaemia also exposes genetic variation to selection. From an applied perspective, adaptive plasticity in parasite reproductive effort could explain the maintenance of genetic variation for virulence and why anaemia is often observed as a risk factor for transmission in human infections.
在所有有性生殖生物的生命历程中，生存与繁殖之间的权衡是至关重要的。这其中包括疟原虫，疟原虫在宿主体内的生存依赖于无性繁殖阶段，而在宿主之间的传播则依赖于有性繁殖阶段（配子体）。在感染过程中，形成配子体的无性阶段比例（繁殖投入）会发生变化——即表型可塑性——以响应包括贫血在内的多种宿主体内因素的变化。然而，红细胞（RBC）资源的密度和年龄结构如何塑造繁殖投入的可塑性以及对寄生虫适应性产生何种影响仍存在争议。在此，我们研究了啮齿动物疟原虫如何以及为何会响应红细胞密度和年龄结构的实验性扰动而改变其繁殖投入。我们发现，当宿主贫血期间未成熟红细胞的比例升高时，所研究的所有四种基因型都会增加繁殖投入，并且各基因型的反应有所不同。我们提出，贫血（与直觉相反）会产生一个资源丰富的环境，在这个环境中寄生虫能够负担得起将更多能量分配给繁殖（即传播），而且贫血还会使遗传变异暴露于选择之下。从应用角度来看，寄生虫繁殖投入的适应性可塑性可以解释毒力遗传变异的维持，以及为何在人类感染中贫血常常被视为传播的一个风险因素。
