GENETIC EXCHANGE

As yet, genetic exchange in trypanosomes has only been proven for the African trypanosome, Trypanosoma brucei. Extensive experimental crosses have been performed, with virtually all combinations of T. brucei subspecies, by copassage of parental isolates through tsetse files. The inheritance of drug resistant markers has greatly facilitated the selection of hybrid progeny from these experiments (see Gibson & Stevens 1999 for review).

Typical triple-banded heterozygous isenzyme patterns for dimeric enzymes, such as glucose phosphate isomerase, were an early sign that led to the suggestion that genetic exchange might occur in T. cruzi. Nevertheless, most population genetics analyses of allozyme frequencies among T. cruzi populations have found no evidence of randomly mating (panmictic) populations. Departures from Hardy-Weinberg equilibrium, and linkage disequilibrium, have consistently been found using either isoenzyme or RAPD characters (Tibayrenc 1998). It should be noted, however, that such studies have largely involved T. cruzi strains from widely dispersed geographical regions, rather than multiple samples from sympatric T. cruzi populations in a single locality and they might therefore have missed heterogeneity through lack of sampling. Thus, although clonal propagation may predominate, genetic exchange in T. cruzi cannot be entirely excluded. Two studies of multiple T. cruzi isolates from single localities have found some evidence of genetic exchange. In both studies putative homozygotes and heterozygotes were found circulating sympatrically (Bogliolo et al. 1996, Carrasco et al. 1996). Among 36 isolates of T. cruzi Z1 from the Amazon basin Carrasco et al. (1996) observed parental and hybrid RAPD profiles, and although Hardy-Weinberg equilibrium analysis was extremely limited, allozyme frequencies for phosphoglucomutase did accord with expectations for panmixia. Taken together these observations are highly indicative that T. cruzi has the capacity for genetic exchange, and that genetic exchange may well be a current phenomenon in undisturbed sylvatic transmission cycles.