Bottlenecks and the Maintenance of Minor Genotypes during the Life Cycle of Trypanosoma brucei

Abstract

African trypanosomes are digenetic parasites that undergo part of their developmental cycle in mammals and part in tsetse flies. We established a novel technique to monitor the population dynamics of Trypanosoma brucei throughout its life cycle while minimising the confounding factors of strain differences or variation in fitness. Clones derived from a single trypanosome were tagged with short synthetic DNA sequences in a non-transcribed region of the genome. Infections were initiated with mixtures of tagged parasites and a combination of polymerase chain reaction and deep sequencing were used to monitor the composition of populations throughout the life cycle. This revealed that a minimum of several hundred parasites survived transmission from a tsetse fly to a mouse, or vice versa, and contributed to the infection in the new host. In contrast, the parasites experienced a pronounced bottleneck during differentiation and migration from the midgut to the salivary glands of tsetse. In two cases a single tag accounted for ≥99% of the population in the glands, although minor tags could be also detected. Minor tags were transmitted to mice together with the dominant tag(s), persisted during a chronic infection, and survived transmission to a new insect host. An important outcome of the bottleneck within the tsetse is that rare variants can be amplified in individual flies and disseminated by them. This is compatible with the epidemic population structure of T. brucei, in which clonal expansion of a few genotypes in a region occurs against a background of frequent recombination between strains.

The Social Life of African Trypanosomes

“…By this time most of these parasites are late procyclic forms [12]. Studies with tagged trypanosomes indicate that the ectoperitrophic space can be colonised by a founding population of several hundred trypanosomes [14], but it is not known whether these migrate individually or in groups. Even when colonisation of the midgut is successful, many infections fail to progress beyond this stage [15].…”

“…To complete the life cycle, parasites must move forward to the anterior midgut, gain access to the proventriculus and then invade the salivary glands. This migration represents a major bottleneck with only a few founder cells colonising the glands [14]. The transmission cycle is completed in the salivary glands with the formation of infectious metacyclic forms that are transferred to a new mammalian host.…”

A new asymmetric division contributes to the continuous production of infective trypanosomes in the tsetse fly

“…To complete their life cycle, trypanosomes need to reach the tsetse salivary glands and to transform into infective metacyclic parasites that are found free in the saliva. This is not direct and requires several migration, proliferation and differentiation steps that take place in a strictly defined chronological order in specific fly tissues (up to 3 weeks) (Oberle et al, 2010;Rotureau et al, 2011;Sharma et al, 2008;Van Den Abbeele et al, 1999;Vickerman, 1985). Once trypanosomes are present in the salivary glands, a fly can produce hundreds of metacyclic parasites per day (Otieno and Darji, 1979) and remains infective for its whole life (~3 months), a meaningful fact considering that a rather low proportion of flies is infected (< 0.1%) (Aksoy et al, 2003;Brun et al, 2009).…”

Forward motility is essential for trypanosome infection in the tsetse fly

“…Again, it could be hypothesized that the nature of the microenvironment (e.g. lower viscosity around the proventriculus), and/or the reduction in parasites density (Oberle et al, 2010). …”

“…Trypanosomes are transmitted by the bite of the tsetse flies, which become vectors when they consume a blood meal from an infected mammal. To complete their life cycle, trypanosomes initially present in the posterior midgut of the fly have to reach the tsetse salivary glands in order to transform into infective parasites (Vickerman, 1985;Peacock et al, 2007;Oberle et al, 2010;Rotureau et al, 2012). This is not direct and requires several proliferation, differentiation and migration steps that take place for 2-3 weeks in a strictly defined chronological order in five distinct fly tissues (Vickerman, 1985;Van Den Abbeele et al, 1999;Sharma et al, 2009;Oberle et al, 2010).…”