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.