Genetic transfer in trypanosomes. I. Syngamy in Trypanosoma cruzi

Isolation and partial characterization of mutants of the trypanosomatid Crithidia fasciculata and their use in detecting genetic recombination

Crithidia fasciculata was used as a model trypanosomatid to study the possible existence of genetic recombination in this group of protozoa. The approach was based on the ability to select a variety of mutants on agar plates. Following mutagenesis of wild type cells by nitrosoguanidine or ethylmethanesulfonate, stable mutant phenotypes were obtained. These included mutants resistant to the drugs actinomycin D, 6-azauracil, 6-azauridine, and 5-fluorouracil, auxotrophs and colony morphology mutants. Following mixed growth of pairs of drug-resistant mutants on selective media, isolates exhibiting stable recombinant phenotypes were obtained. The data presented suggest that 1) Crithidia undergoes some type of genetic recombination and 2) Crithidia must be diploid at some time during this process.

Sexual processes in the kinetoplastida

The commonly held view that the kinetoplastida, and in particular trypanosomes, are asexual is largely derived from the principle that an organism is asexual until proved sexual. If the basis for this view is examined in detail, it largely arises from the lack of morphologically distinguishable gametes, the difficulties encountered in visualizing chromosomes and a few experiments, using drug-resistant stocks, in which no recombination between stocks could be demonstrated. While it is clear that these organisms are able to reproduce asexually, the existence of a sexual cycle was, until recently, an entirely open question. The early work strongly suggests that any sexual process (in the species examined extensively at the morphological level) does not involve classical well-differentiated gametes and so must involve fusion of morphologically very similar cells. These findings taken together with the inability to visualize chromosomes and thereby identify meiosis, mean that classical methods are unable to detect any sexual process even if it did occur. This review examines the evidence provided by the experimental approaches which have been applied recently to the question of kinetoplastid sexuality. These approaches include isoenzyme studies and the analysis of possible genetic exchange by the use of selective markers (e.g. drug resistance). The results which these techniques have produced make it clear that the kinetoplastid protozoans cannot be regarded as a totally asexual group of organisms.

Evidence for diploidy and mating in trypanosomes

The question of whether any sexual process takes place at some stage of the life cycle of the trypanosome has often been raised and crosses between different drug-resistant strains have provided no convincing answer. Multinucleate forms of trypanosomes have been observed by electron microscopy but their significance and origin remain obscure. As part of a study aimed at examining the speciation and genetics of the Trypanosoma brucei complex of trypanosomes, a series of isolates from a population of T.b. brucei have been screened for electrophoretic variation in 19 enzymes. The results of this survey, reported here, provide strong evidence that trypanosomes are diploid and undergo random mating and recombination.