Trends in Parasitology
UpdateResearch FocusFrom clonal to sexual: a step in T. congolense evolution?
Section snippets
Sexuality in African trypanosomes?
African trypanosomes are kinetoplastid Protozoa transmitted by invertebrate vectors (mostly tsetse flies); they are important in both human (Trypanosoma brucei gambiense and T. b. rhodesiense) and in veterinary medicine (T. b. brucei, T. congolense, T. evansi, T. equiperdum and T. vivax). The question as to whether or when African trypanosomes are clonal or undergo genetic exchange has been discussed over many years (Box 1). Until now, T. brucei brucei, T. b. rhodesiense and Type 2 T. b.
Mating or clonality in T. congolense: where, when and how?
Among the subgenus Nannomonas, pioneering studies on population diversity subdivided T. congolense into three genetically distinct subgroups designated as Savannah, Forest and Kilifi; the parasite populations exhibited different molecular and genetic properties [11], but they were considered to expand clonaly [12]. It is only recently that Morrison and co-workers evidenced recombination through population genetics data analysis, a strong argument in favour of mating events [13].
In T. congolense,
Use and pitfalls of population genetics tools
A currently debated point raised in Ref. [13] is the use of population genetics tools to argue for either clonal or sexual reproduction in African trypanosomes. For diploids such as trypanosomes, if markers and sampling are suitable, FIS is the most appropriate parameter to be measured; this is because of its well-documented behaviour when reproduction is purely or almost purely clonal 18, 19, 20. Especially in small subpopulations such as those expected for pathogens, linkage disequilibrium
Concluding remarks
The discovery of mating in T. congolense is a fundamental step towards a new consideration of this complex group of parasites. A comparative analysis between the three genetic subgroups (Savannah, Forest and Kilifi) of this parasite would be highly valuable. Similarly, a meta-analysis of data concerning trypanosome models that undergo mating (T. brucei brucei, T. b. rhodesiense and T.congolense) and others that expand clonally (T. b. gambiense and T. vivax), using the same tools, could serve as
Acknowledgments
We first would like to apologise to the many authors who have contributed to the understanding of trypanosomes genetics, but who are not cited in this article. We also would like to thank three anonymous referees for their helpful comments, and V. Jamonneau, P. Grébaut, P. Solano and B. Bucheton for valuable discussion and advice during the initial preparation of the manuscript. PH is supported by CIRAD, SH and GC by IRD and TDM by CNRS.
Glossary
- Clonal reproduction
- Reproduction with no sex. The descent is identical to the parental individual.
- Drift (random genetic)
- Describes the process by which allelic frequencies change from one generation to the other as a result of the random sampling of individuals (adults, zygotes or gametes that will form zygotes) that survive to form the next generation in a population of finite size.
- Endogamy
- Sexual recombination with genetically related individuals (typically selfing).
- FIS
- Inbreeding coefficient of
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