Adaptive plasticity in the gametocyte conversion rate of malaria parasites

Abstract

Sexually reproducing parasites, such as malaria parasites, experience a trade-off between the allocation of resources to asexual replication and the production of sexual forms. Allocation by malaria parasites to sexual forms (the conversion rate) is variable but the evolutionary drivers of this plasticity are poorly understood. We use evolutionary theory for life histories to combine a mathematical model and experiments to reveal that parasites adjust conversion rate according to the dynamics of asexual densities in the blood of the host. Our model predicts the direction of change in conversion rates that returns the greatest fitness after perturbation of asexual densities by different doses of antimalarial drugs. The loss of a high proportion of asexuals is predicted to elicit increased conversion (terminal investment), while smaller losses are managed by reducing conversion (reproductive restraint) to facilitate within-host survival and future transmission. This non-linear pattern of allocation is consistent with adaptive reproductive strategies observed in multicellular organisms. We then empirically estimate conversion rates of the rodent malaria parasite Plasmodium chabaudi in response to the killing of asexual stages by different doses of antimalarial drugs and forecast the short-term fitness consequences of these responses. Our data reveal the predicted non-linear pattern, and this is further supported by analyses of previous experiments that perturb asexual stage densities using drugs or within-host competition, across multiple parasite genotypes. Whilst conversion rates, across all datasets, are most strongly influenced by changes in asexual density, parasites also modulate conversion according to the availability of red blood cell resources. In summary, increasing conversion maximises short-term transmission and reducing conversion facilitates in-host survival and thus, future transmission. Understanding patterns of parasite allocation to reproduction matters because within-host replication is responsible for disease symptoms and between-host transmission determines disease spread.

Fig 1. Predicted reaction norm for plasticity in gametocyte conversion rate in response to perturbations of state.

State refers to a combination of physiological condition and environmental factors. Evolutionary theory for life histories as applied to malaria parasites suggests that genotypes in good state can afford to invest in gametocytes (solid line), but should adopt reproductive restraint if state deteriorates (dashed line), and switch to making a terminal investment (dotted line) if state deteriorates so much that in-host survival becomes unlikely. Note that the x axis is not synonymous with time post infection and that parasite state may improve and deteriorate multiple times during an infection. Most studies in Table 1 can be qualitatively interpreted according to this pattern.

Fig 2. Conversion rate is adaptively adjusted in silico in response to a loss of state in a non-linear manner.

Predicted optimal strategies and their fitness consequences across drug doses that reduce parasite state; strategies highlighted for untreated infections (green: 0 mg/kg), low dose (purple: 5 mg/kg) and high dose (blue: 15 mg/kg) drug treatment. (A) Compared to untreated infections, parasites should reduce conversion in response to low doses (reproductive restraint) but increase conversion in response to high doses (terminal investment). (B) Fitness, measured as cumulative transmission potential post-drug treatment, is highest in untreated infections (green) and reduced by drug treatment (purple and blue). Altering conversion rates allows parasites to partially compensate for the fitness costs of drugs. When treated with a low dose, reproductive restraint (solid purple line) allows parasites to recover to the same rate of transmission as untreated infections (as indicated by the slope of the solid green and purple lines) within 6 days. This also yields greater fitness than if parasites do not adjust conversion and follow the optimal strategy for untreated infections (dashed purple line). Although too small to visualise due to the scale of y-axis, changing conversion in response to a high dose yields a 1% fitness benefit (solid blue line) compared to parasites that do not adjust conversion and follow the optimal strategy for untreated infections (dashed blue line). (C) Optimal strategies are dose-specific. Fitness is calculated for parasites that follow the optimal strategies for low (purple) or high (blue) doses in each of the untreated, low dose, and high dose environments, relative to the optimal strategy for untreated infections. Adopting reproductive restraint (purple) or terminal investment (blue) costs fitness in untreated infections. At low doses, reproductive restraint (purple) is the optimal strategy and terminal investment is the poorest strategy. At high doses, terminal investment (blue) is marginally better than reproductive restraint (purple).

Fig 3. P. chabaudi parasites in vivo adjust conversion rate in response to a loss of state in a non-linear manner.

(A) Dose-response curve for the proportional change in asexual density (“state”) after drug treatment. Note, at the highest drug doses, >99% but <100% of asexuals are lost and because infections were treated post peak, densities also decline in the untreated control infections. (B) Parasites reduce conversion in response to losing up to 85% of their number (reproductive restraint) and then switch to increasing conversion (terminal investment; grey shaded area). Following [54], conversion rate is estimated for each infection based on gametocyte, asexual and RBC dynamics (see S2 Fig). Mean ±SEM conversion rates are presented in black. The observed conversion rate strategies have short-term fitness consequences in terms of within-host survival (C) and between host transmission (D). Specifically, relative to untreated infections, reproductive restraint increases replication rate (C) but reduces the estimated probability of between-host transmission (translated from gametocyte densities according to [61]) (D). All green lines and green shaded areas are the predicted relationships ±SEM between parasite strategies and the loss of state from a generalised additive model, and grey shaded areas represent the region in which parasites make a terminal investment.

Fig 4. Other P. chabaudi genotypes adjust conversion in response changes in state.

The densities of asexual stages were perturbed by drug treatment for genotype CWvir (blue) and by within-host competition with one or two other P. chabaudi genotypes for AS and AJ (combined, red). For these data we use replication rate as a proxy for state, calculated as the density of the asexual cohort making the conversion rate decision relative to the density of its asexual progeny. Replication rates are >1 for expanding infections and <1 in declining infections. Following [25, 66], conversion rate is estimated for each infection based on gametocyte, asexual and RBC dynamics. Solid lines and shaded areas are predictions ± SEM from generalised additive models for conversion as a function of replication rate.

Fig 5. In addition to state, changes in red blood cell (RBC) density modulate conversion rates.

The reproductive restraint region of reaction norms for (A) ER and (B) CWvir are steeper when RBC density is increasing (solid lines; 90^th percentile) compared to remaining constant (dashed lines; 50^th percentile) or decreasing (dotted lines; 10^th percentile). Percentiles for RBC change are used to normalise for the differing dynamics between the experiments and are calculated as the difference in RBC density between the decision-making and parental cohorts of asexuals. Solid lines and shaded areas are predictions ± SEM from generalised additive models for conversion as a function of both replication rate (state) and changes in RBC density.

Fig 6. Parasites in good state increase conversion rates as RBC densities increase.

RBC density was perturbed independently of parasite state for P. chabaudi parasites exposed to EPO (A, days 4–14 PI) or phenylhydrazine (B, day 1 PI). Change in RBC refers to the difference in RBC density between the decision-making and parental cohorts of asexuals. Parasites were in good state during treatments in both experiments, with >90% exceeding a replication rate of 1.61 (A) or 3.3 (B). Note, conversion in (A) is estimated following [54], while a simpler metric had to be used for (B) (see ‘Experiments’ in Materials and methods). Lines and shaded areas are predictions ± SEM from generalised additive models for conversion as a function of changes in RBC density.