Genetic exchange in trypanosomes

Sexual reproduction in trypanosomes

For many years it was thought that trypanosomes reproduced asexually by binary fission. Evidence for some form of genetic exchange in Trypanosoma brucei was first published in 1986 (Jenni et al. 1986, Nature 322: 173-175). Hybrid trypanosomes were produced after co-transmission of a mixture of 2 parental strains via the tsetse vector. Genetic exchange was also demonstrated in the South American human pathogenic trypanosome, Trypanosoma cruzi, but this occurs in the mammalian host rather than the bug vector (Gaunt et al. 2003, Nature 421: 936-939). More recently, genetic exchange was shown to occur in Leishmania major, another important trypanosomatid parasite, as it undergoes cyclical development in the sand fly vector (Akopyants et al. 2009, Science 324: 265-268).

Experiments to elucidate the mechanism of genetic exchange in these parasites are driven by their importance as human and animal pathogens. Genetic exchange enables the spread of genes for crucial traits such as virulence or drug resistance and can lead to the creation of new pathogen strains. The genetics of these early branching eukaryotes will also provide insights into the evolution of sex and meiosis in eukaryotes.

Genetic exchange in T. brucei

The process of genetic exchange in T. brucei has not yet been observed directly, so there is much that we still don't know. Genetic exchange occurs in the tsetse vector, which makes it difficult to observe. These flies are not easy to rear in the lab and are refractory to trypanosome infection. The trypanosomes undergo a complicated cycle of development in the vector and follow a tortuous route, starting in the midgut and ending in the salivary glands where the infective forms develop. In genetic crosses of different trypanosome strains, few of the infective forms are hybrids and genetic exchange appears to be a rare and non-obligatory event in the trypanosome's life cycle.

Fluorescent red and green trypanosomes in a tsetse flyWe are currently using genetically modified fluorescent trypanosomes to track the occurrence of hybrid trypanosomes within the tsetse fly. When red and green fluorescent trypanosomes are mated, some of the hybrid progeny are yellow fluorescent and thus are easily distinguished from the parents. We were able to show that yellow hybrids were only found in the tsetse salivary glands and appeared as early as 13 days after fly infection. The salivary glands are therefore the site of genetic exchange in T. brucei.

We have also used genetically engineered fluorescent trypanosomes to identify the meiotic stage in T. brucei. Three meiosis-specific genes were tagged with YFP and expression tracked during trypanosome development in the fly. Trypanosomes expressing the meiosis-specific proteins in the nucleus were found in the salivary glands. Morphologically they were dividing epimastigotes with a single posterior nucleus, 2 kinetoplasts and 2 flagella. This dividing cell is presumed to be in meiosis I, but should eventually result in production of haploid nuclei, possibly as individual cells (gametes), at the end of meiosis II. We therefore searched for haploids and identified a population of haploid trypanosomes with a distinctive morphology that co-occur with the meiotic stage in the salivary glands. The putative haploid gametes have a short, pear-shaped body and a long free flagellum. We called these peculiar cells "tadpoles" in the lab, but more scientifically we refer to them as promastigote-like, because of their resemblance to the promastigote stages found in e.g. Leishmania.

Genetic exchange is particularly important in pathogens because it allows transfer of harmful traits between strains. We demonstrated that mating between the human pathogen Trypanosoma brucei rhodesiense and the animal infective subspecies T. b. brucei enables the transfer of the SRA gene, which confers human infectivity. Hybrid progeny from these crosses that inherited the SRA gene were new strains of the human pathogen, T. b. rhodesiense.


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