Ecological influences on the behaviour and fertility of malaria parasites

Abstract

Background: Sexual reproduction in the mosquito is essential for the transmission of malaria parasites and a major target for transmission-blocking interventions. Male gametes need to locate and fertilize females in the challenging environment of the mosquito blood meal, but remarkably little is known about the ecology and behaviour of male gametes.

Methods: Here, a series of experiments explores how some aspects of the chemical and physical environment experienced during mating impacts upon the production, motility, and fertility of male gametes.

Results and conclusions: Specifically, the data confirm that: (a) rates of male gametogenesis vary when induced by the family of compounds (tryptophan metabolites) thought to trigger gamete differentiation in nature; and (b) complex relationships between gametogenesis and mating success exist across parasite species. In addition, the data reveal that (c) microparticles of the same size as red blood cells negatively affect mating success; and (d) instead of swimming in random directions, male gametes may be attracted by female gametes. Understanding the mating ecology of malaria parasites, may offer novel approaches for blocking transmission and explain adaptation to different species of mosquito vectors.

Keywords: Blood meal; Fertilization; Malaria; Microgamete; Transmission.

Fig. 1

Chamber design and assay set up for the chemotaxis experiment. Three coverslips (1 large rectangular and 2 small and square) were fixed on to each slide to create a chamber at the centre. For each assay, 7 µl microgamete culture was placed into the chamber, followed immediately by 2 μl of the treatment to create an interface. “A” represents the area counted for the away location, and “I” for the interface location. a View from above, b view from the side (not to scale)

Fig. 2

Dose response to GAFs. Mean ± SEM of log2 transformed densities of exflagellating males (a) and ookinetes (b) relative to the pH 8 control, when exposed to 10⁻⁶ to 10⁻² M xanthurenic acid (XA), kyneurenic acid (KA), or tryptophan (Tryp). n = 10–11 (independent infections) for each GAF and dose combination

Fig. 3

Variation in response to GAFs of Plasmodium yoelii subspecies. Responses of P. yoelii subspecies (Pyn: P. yoelii nigeriensis; Pys: P. yoelii subspecies; and Pyy: P. yoelii yoelii) to GAFs at 10⁻⁴ M (XA: xanthurenic acid; KA: kynurenic acid; and Tryp: tryptophan). a Mean ± SEM log2 + 0.001 transformed densities of exflagellating males relative to the pH 8 control. b Mean ± SEM log2 transformed densities of ookinetes. Zeros indicate combinations where no ookinetes were observed. N ranges from 3 to 5 independent infections for each GAF

Fig. 4

The density of microparticles affects mating success as measured by ookinete density. Mean ± SEM log10 transformed ookinetes/ml blood for each P. berghei line (Pb820 and PbANKA) in cultures in which media was replenished post-fertilization (a), or not replenished (b). N ranges from 3 to 6 independent infections for each mean

Fig. 5

Microgametes move towards a female cue. The mean proportion of microgametes (±95 % confidence interval) at the interface with either a control treatments (RBC and/or asexual stage material) or female material treatments (live female gametes or lysed female gametes) at the start of the assay (0–6 min), or after 20–26 min in the chamber (End). Note that 95 % CI are given instead of SEM for clarity of the borderline difference. N = 3 independent infections