Abstract
BackgroundSexual reproduction in Plasmodium falciparum and Trypanosoma brucei occurs in the insect vector and is important in generating hybrid strains with different combinations of parental characteristics. Production of hybrid parasite genotypes depends on the likelihood of co-infection of the vector with multiple strains. In mosquitoes, existing infection with Plasmodium facilitates the establishment of a second infection, although the asynchronicity of gamete production subsequently prevents mating. In the trypanosome/tsetse system, flies become increasingly refractory to infection as they age, so the likelihood of a fly acquiring a second infection also decreases. This effectively restricts opportunities for trypanosome mating to co-infections picked up by the fly on its first feed, unless an existing infection increases the chance of successful second infection as in the Plasmodium/mosquito system.ResultsUsing green and red fluorescent trypanosomes, we compared the rates of trypanosome infection and hybrid production in flies co-infected on the first feed, co-infected on a subsequent feed 18 days after emergence, or fed sequentially with each trypanosome clone 18 days apart. Infection rates were highest in the midguts and salivary glands (SG) of flies that received both trypanosome clones in their first feed, and were halved when the infected feed was delayed to day 18. In flies fed the two trypanosome clones sequentially, the second clone often failed to establish a midgut infection and consequently was not present in the SG. Nevertheless, hybrids were recovered from all three groups of infected flies. Meiotic stages and gametes were produced continuously from day 11 to 42 after the infective feed, and in sequentially infected flies, the co-occurrence of gametes led to hybrid formation.ConclusionsWe found that a second trypanosome strain can establish infection in the tsetse SG 18 days after the first infected feed, with co-mingling of gametes and production of trypanosome hybrids. Establishment of the second strain was severely compromised by the strong immune response of the fly to the existing infection. Although sequential infection provides an opportunity for trypanosome mating, the easiest way for a tsetse fly to acquire a mixed infection is by feeding on a co-infected host.
Highlights
Sexual reproduction in Plasmodium falciparum and Trypanosoma brucei occurs in the insect vector and is important in generating hybrid strains with different combinations of parental characteristics
Midgut infection rates in non-teneral flies We compared infection rates of two distinguishable T. b. brucei strains in different infection regimes: 1) Teneral, co-infection of newly-emerged flies with F1G2 and F1R1 on their first feed; 2) non-teneral, co-infection of 18 day old flies; 3) and 4) sequential, newly-emerged flies fed with F1G2 or FIR1, followed by an infected bloodmeal with the other strain on day 18 (Fig. 1)
The midgut infection rate was greatly reduced in the non-teneral flies, but 83 % of the infected midguts contained both clones (15/18)
Summary
Sexual reproduction in Plasmodium falciparum and Trypanosoma brucei occurs in the insect vector and is important in generating hybrid strains with different combinations of parental characteristics. The bloodmeal triggers physiological changes in the fly including extrusion of the peritrophic matrix that holds the bloodmeal and prevents interaction of pathogens with the gut epithelium [9, 10] and up-regulation of innate immune defences such as secretion of lectins and anti-microbial peptides [8, 10,11,12,13,14] These observations go some way to explain the very low trypanosome infection rates recorded for wild-caught flies, typically < 0.1% of tsetse have SG infected with T. brucei ([15] and examples reviewed therein). A fly could become co-infected with Tbr and Tbb only by feeding on an infected reservoir host with a mixed infection, unless sequential infection resulting from separate feeds on different hosts were possible
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