21 - Genetic Exchange in Trypanosomatids and Its Relevance to Epidemiology

Abstract

This chapter discusses the genetic exchange in Trypanosomatids and its relevance to epidemiology. Genetic exchange has now been demonstrated in all three of the so-called TriTryps, the three trypanosomatids for which genome sequences have been published Trypanosoma brucei, T. cruzi, and Leishmania major. The first successful laboratory cross was reported through tsetse flies. The hybrids were found among metacyclics recovered from the salivary glands showing that genetic exchange had occurred some time during the trypanosome's developmental cycle in the tsetse fly. The discovery of green fluorescent protein (GFP) and the development of methods to image the protein in living cells have opened exciting new approaches for studying genetic exchange in trypanosomes. Hybrids were most likely to be found in flies infected for at least 28 days, after sufficient time for salivary gland invasion and colonization. In order to mate, both parental trypanosomes need to reach and colonize the same salivary gland, and it became evident that this is not always the case. In epidemics of human trypanosomiasis caused by T. b. rhodesiense, transmission may well be direct from human to human rather than from an animal reservoir, allowing the clonal expansion of particular trypanosome genotypes. Trypanosoma cruzi is considered to be a single species but comprises six distinct genetic lineages or discrete typing units. The double-drug-resistant hybrids were recovered from the culture supernatant, which contained trypomastigotes released from lysed mammalian cells. Salivary gland infections cannot be considered a prerequisite for genetic exchange to occur in triatomines. This chapter also states that the mechanism of genetic exchange that produced the six experimental hybrid T. cruzi clones is at least partially understood. Molecular markers have provided a more objective means of analyzing genetic diversity and the structure of natural Leishmania populations.