Transgenic malaria-resistant mosquitoes have a fitness advantage when feeding on Plasmodium-infected blood

Abstract

The introduction of genes that impair Plasmodium development into mosquito populations is a strategy being considered for malaria control. The effect of the transgene on mosquito fitness is a crucial parameter influencing the success of this approach. We have previously shown that anopheline mosquitoes expressing the SM1 peptide in the midgut lumen are impaired for transmission of Plasmodium berghei. Moreover, the transgenic mosquitoes had no noticeable fitness load compared with nontransgenic mosquitoes when fed on noninfected mice. Here we show that when fed on mice infected with P. berghei, these transgenic mosquitoes are more fit (higher fecundity and lower mortality) than sibling nontransgenic mosquitoes. In cage experiments, transgenic mosquitoes gradually replaced nontransgenics when mosquitoes were maintained on mice infected with gametocyte-producing parasites (strain ANKA 2.34) but not when maintained on mice infected with gametocyte-deficient parasites (strain ANKA 2.33). These findings suggest that when feeding on Plasmodium-infected blood, transgenic malaria-resistant mosquitoes have a selective advantage over nontransgenic mosquitoes. This fitness advantage has important implications for devising malaria control strategies by means of genetic modification of mosquitoes.

Fig. 1.

Changes in the prevalence of transgenic mosquitoes in populations maintained with Plasmodium-infected blood. Experiment 1 consisted of a cage started with 250 transgenic virgin females (CCBF6 line) and 250 WT males. Experiment 2 (reverse) was 250 WT virgin females and 250 transgenic males (CCBF6). Experiment 3 consisted of 250 transgenic virgin females (CCBF3 line) with 250 WT males. Experiment 4 was 250 transgenic virgin females (CCBF3) and 250 WT males. Mosquitoes fed on ANKA 2.34-infected mice up to generation 4 and on ANKA 2.33 from generation 5 onward (arrow). Experiment 5 (reverse) consisted of 250 WT females and 250 transgenic males (CCBF3). Mosquitoes fed on ANKA 2.34-infected mice up to generation 3 and on ANKA 2.33 from generation 4 onward (arrow). Neutral indicates expected outcome if there were no cost or benefit to being transgenic (Hardy–Weinberg equilibrium). Model indicates the predicted outcome if there were a 50% benefit to being transgenic and a 35% cost to being homozygous for the transgene (see text). Significant deviation from the neutral frequency of 56% WT and 44% transgenic (Hardy–Weinberg equilibrium) was observed after the second generation in experiment 1 and after the third generation in experiments 2 and 3 (χ² test, P < 0.05).

Fig. 2.

Fecundity of transgenic and WT mosquitoes fed on a parasite-containing blood meal. Hemizygous transgenic males were crossed with virgin nontransgenic females to yield a population of 50% hemizygous transgenic and 50% nontransgenic sibling mosquitoes originating from the same rearing pans. Mosquitoes were fed on mice infected with gametocyte-producing (ANKA 2.34) or gametocyte-deficient (ANKA 2.33) parasites after which single engorged females were kept in separate containers for egg collection. The interaction line chart shows eggs produced by transgenic and WT females. ANOVA indicated a significant effect of parasite type on number of eggs oviposited per female and a significant interaction between parasite type and transgenic type (Parasite: F = 25.1, d.f. = 1, P < 0.0001; Transgenic: F = 2.54, d.f. = 1, P = 0.113; Parasite × Transgenic: F = 6.56, d.f. = 1, P = 0.011).