Sexual reproduction in the livestock pathogen Trypanosoma congolense

Trypanosomiasis is a major livestock disease that historically crippled the development of agriculture in sub-Saharan Africa by destroying both draught and production animals. This tsetse fly-transmitted disease continues to severely constrain livestock production in many countries in Africa, because it is very widespread and affects most of the major livestock species, including cattle, sheep, goats, pigs, horses, donkeys and camels. Vets have few drugs available for prophylaxis or treatment, and drug resistance is increasing and widespread; there is no vaccine. Climate change is likely to exacerbate the problem of animal trypanosomiasis, which already affects temperate as well as tropical and subtropical regions.

The disease is caused by microscopic, single-celled organisms called trypanosomes, which are found in the blood of affected livestock. The goal of our research is to find out how trypanosomes swap their genetic material among themselves and whether this generates novel types of parasite with new features. Genetic exchange enables the rapid spread of genes through a cell population by transfer from cell to cell. This is particularly important in the case of pathogenic microbes, because once a gene for drug resistance has arisen, it can quickly spread, rendering a previously effective drug useless. A microbe with a sexual cycle is therefore potentially more difficult to control than one that reproduces asexually.

Here we will apply the knowledge and skills we have gained from our studies of Trypanosoma brucei, which is responsible for human trypanosomiasis, to investigate sexual reproduction in T. congolense, a related livestock pathogen. Both these trypanosomes are carried by tsetse flies, but with significant differences: T. brucei develops in the fly salivary glands and this is where it is known to undergo sexual reproduction with formation of novel hybrid strains; T. congolense develops in the fly's biting and feeding apparatus, but as yet we have no knowledge of whether it can also undergo sexual reproduction there. There is, however, evidence that T. congolense does undergo genetic exchange in natural populations from genetic comparisons of strains collected in Africa.

We previously developed an experimental setup to identify hybrid trypanosomes directly in tsetse flies by mixing trypanosome strains that fluoresced with different colours, red and green. The combination of red and green fluorescence yields yellow fluorescence, so that any hybrid trypanosome can be distinguished from both parents. We plan to use this system to investigate sexual reproduction in T. congolense in the laboratory. For T. brucei, this was crucial in identifying hybrids and where they occurred in the fly, so we could then search for the cells that actually mate - the gametes. Eggs and sperm are the gametes in humans, but in T. brucei the gametes were all a single kind of cell. The gametes are produced by a special form of cell division called meiosis. To identify these meiotic cells, we will use fluorescent reporter genes to track expression of meiosis-specific proteins; we already know that T. congolense has the genes for these proteins. Identification of the meiotic cells will lead us to the gametes; during mating, these cells fuse together and exchange DNA and cytoplasm. We expect to find major differences in the meiotic cells and gametes of T. congolense and T. brucei, based on their known developmental cycles.

We developed all the tools and approaches we need for this project for the related parasite, T. brucei, so it is relatively straightforward to apply these to T. congolense, and our project thus has a high chance of success. By the end of the project we will have established whether T. congolense can undergo mating and how it does it. This has profound implications for the spread of harmful genes among different strains of this important livestock parasite.

Trypanosomiasis is a major livestock disease that causes vast economic losses and is widespread in tsetse-infested regions of sub-Saharan Africa. It is caused by trypanosomes, protozoan parasites found in the blood of affected livestock and transmitted by tsetse. There is no vaccine and few drugs are available for treatment, while reports of drug resistance are increasing. Climate change is likely to exacerbate the problem of animal trypanosomiasis, which already affects temperate as well as tropical and subtropical regions. The goal of our research is to elucidate the sexual cycles of trypanosomes. Sexual reproduction is particularly important in pathogenic microbes, because it mobilizes genes, e.g. for virulence or drug resistance, and potentially generates novel pathogen genotypes. Here we will apply the knowledge and skills gained from our studies of the model organism Trypanosoma brucei to investigate sexual reproduction in T. congolense, a related livestock pathogen of major economic importance to African agriculture. Population genetics studies suggest that T. congolense undergoes sexual reproduction in natural populations in Africa, and here we seek experimental proof and understanding of the mechanism. T. congolense and T. brucei have similar fly developmental cycles, but with significantly different life cycle stages, none obviously analogous to the sexual stages of T. brucei. We previously developed an experimental setup to identify hybrid trypanosomes directly in tsetse flies by mixing genetically modified lines of T. brucei expressing different fluorochromes and will use this approach to identify T. congolense hybrids. Tracking expression of meiosis-specific genes present in the T. congolense genome will lead to identification of meiotic stages and gametes. These tools and approaches have been successfully used by us to demonstrate meiosis and haploid gametes in T. brucei, and can be readily adapted for this study of sex in T. congolense.

Who might benefit?
The results of the research will be disseminated to veterinary practitioners and personnel of government departments and non-governmental organisations involved in control of livestock diseases, and to those with affected or at risk livestock such as farmers, ranchers and small-holders in subSaharan Africa. To reach this audience, we will report our work in both academic and non-academic publications, and on specialist animal health websites. The World Organization for Animal Health (OIE; http://www.oie.int/) acts as a central authority and source of information on animal diseases worldwide. The annual ISCTRC (International Scientific Council for Trypanosomiasis Research and Control) meeting held in subSaharan Africa is an important forum to reach those engaged in research and control of animal trypanosomiasis in the endemic region.

We will engage with the general public and school children about research in parasitology via events co-ordinated by the Centre for Public Engagement (CPE) at University of Bristol, such as the annual Festival of Nature and Bristol Open Doors day. CPE organizes science discovery displays at both events and we will develop a parasitology-themed display relevant to this project.

How might they benefit?
These beneficiaries will gain increased awareness and understanding of animal trypanosomiasis and parasitology in general, and find out about ways to avoid or control the disease.