We argue that the mode of reproduction of microorganisms in nature can only be decided by population genetic information. The evidence available indicates that many parasitic protozoa and unicellular fungi have clonal rather than sexual population structures, which has major consequences for medical research and practice. Plasmodium falciparum, the agent of malaria, is a special case: the scarce evidence available is contradictory, some suggesting that uniparental lineages may exist in nature. This is puzzling (because P. falciparum is known to have a sexual stage) and poses a challenge that can be readily settled by ascertaining the frequency distribution of genotypes in natural populations.Sexual reproduction is generally assumed to be a common mode of reproduction of eukaryotes. In the case of parasitic protozoa, the assumption of sexual reproduction relies largely on the presumption that these organisms are diploid, as well as on the occurrence of sexual recombination in the laboratory under appropriate circumstances (review in ref. 1), rather than on relevant evidence obtained from nature. Yet, whether or not sexual reproduction prevails in these organisms is of considerable medical and agronomic consequence as well as of scientific interest. These eukaryotic microorganisms include the agents of malaria, sleeping sickness, Chagas disease, and other parasitic diseases that affect more than 10% of the world population. The strategies for developing vaccines or curative drugs as well as for diagnosis and treatment are different for clonal and for sexual organisms.That sexual reproduction may occur in laboratory cultures or even occasionally in nature does not by itself settle the issue, since that simply manifests that the potentiality for sexual reproduction has not been lost. What remains to be determined is the prevailing mode of reproduction of these organisms in natural circumstances. The evidence to settle the matter exists for some of these organisms and could be obtained for others without massive investment of resources or new scientific or medical advances. We herein advance a sustained argument to show that population genetic evidence and population genetic theory is all that is needed to ascertain the extent to which, if at all, these (or any other) organisms reproduce sexually in nature. We have already reviewed the evidence for Trypanosoma cruzi, the agent of Chagas disease (2), and some other protozoa (3). Here we develop further the argument and present the results of a survey of the available evidence for parasitic protozoa and unicellular fungi. CLONALITY IN MICROBIAL EUKARIOTESThe two genetic consequences of sexual reproduction are segregation and recombination. Population genetic methods make it possible to ascertain whether or not the distribution of genotypes in natural populations is consistent with the occurrence of segregation and recombination. The kind of evidence that is needed is the frequency distribution ofgenotypes rather than the direct observation of sexual or clonal r...

Trypanosoma cruzi, the causative agent of Chagas disease, presents wide genetic diversity. Currently, six discrete typing units (DTUs), named TcI to TcVI, and a seventh one called TcBat are used for strain typing. Beyond the debate concerning this classification, this systematic review has attempted to provide an inventory by compiling the results of 137 articles that have used it. A total of 6,343 DTU identifications were analyzed according to the geographical and host origins. Ninety-one percent of the data available is linked to South America. This sample, although not free of potential bias, nevertheless provides today’s picture of T. cruzi genetic diversity that is closest to reality. DTUs were genotyped from 158 species, including 42 vector species. Remarkably, TcI predominated in the overall sample (around 60%), in both sylvatic and domestic cycles. This DTU known to present a high genetic diversity, is very widely distributed geographically, compatible with a long-term evolution. The marsupial is thought to be its most ancestral host and the Gran Chaco region the place of its putative origin. TcII was rarely sampled (9.6%), absent, or extremely rare in North and Central America, and more frequently identified in domestic cycles than in sylvatic cycles. It has a low genetic diversity and has probably found refuge in some mammal species. It is thought to originate in the south-Amazon area. TcIII and TcIV were also rarely sampled. They showed substantial genetic diversity and are thought to be composed of possible polyphyletic subgroups. Even if they are mostly associated with sylvatic transmission cycles, a total of 150 human infections with these DTUs have been reported. TcV and TcVI are clearly associated with domestic transmission cycles. Less than 10% of these DTUs were identified together in sylvatic hosts. They are thought to originate in the Gran Chaco region, where they are predominant and where putative parents exist (TcII and TcIII). Trends in host-DTU specificities exist, but generally it seems that the complexity of the cycles and the participation of numerous vectors and mammal hosts in a shared area, maintains DTU diversity.

Randomly amplified polymorphic DNA (RAPD) has emerged as an effective genetic marker for analysis of Trypanosoma cruzi population variability. This method has been used to study the genetic variability of Mexican T. cruzi stocks and to relate these results to previous classifications. High clonal diversity was observed among the Mexican populations: 24 RAPD types were scored among 56 stocks analyzed. Only two stocks (3.6%) belonged to the T. cruzi II lineage, while all others belonged to T. cruzi I. The robustness of these clusters was statistically highly significant. Mexican T. cruzi I stocks formed a homogeneous group with reduced genetic distances among its members. Parasites from this group were isolated from both domestic and sylvatic cycles over a broad geographic area in Mexico. The two Mexican stocks classified as T. cruzi II (isolated from sylvatic cycles) were of the same RAPD type, although they were not closely related to the three reference T. cruzi II stocks circulating in domestic cycles in Argentina, Brazil, Bolivia, and Chile. These stocks were also unrelated to the formerly named Zymodeme III.Trypanosoma cruzi, the agent of Chagas' disease, which is widely distributed in Latin America, shows considerable genetic polymorphism (14). Extensive studies carried out by using multilocus enzyme electrophoresis have demonstrated that natural populations of T. cruzi have a clonal structure (21), which has led to subdivision of the taxon into two major lineages, T. cruzi I and T. cruzi II (1,13,18,20). Each lineage is genetically heterogeneous, and despite failure to identify subdivisions within T. cruzi I, five subgroups of T. cruzi II have been proposed as discrete typing units (DTU) on the basis of randomly amplified polymorphic DNA (RAPD) analysis (1, 5). These six DTU have been proposed as a reference framework for genetic variability and for biological characterization studies of T. cruzi stocks.A few studies have attempted to characterize Mexican T. cruzi stocks by genetic methods. Seventeen stocks were analyzed using HindIII restriction fragment length polymorphisms associated with rRNA gene spacers and kinetoplast DNA maxicircle polymorphisms (9,12,22). These studies identified two groups of Mexican stocks, which correlate with geographic origin, although assignment to a T. cruzi lineage was not possible. The same stocks were also analyzed by multilocus enzyme electrophoresis, demonstrating a high degree of heterogeneity that was not related to geographic origin (12). Analysis of minicircle kDNA variability among Mexican stocks showed no correlation between the sequence homologies of the hypervariable region of minicircles and clustering (2), contrasting with results obtained for South American stocks (3, 4).In the present study, 56 Mexican stocks, isolated from diverse hosts and with diverse geographic origins, were analyzed by RAPD. Relevant phylogenetic analyses were used to assess population variability and structure, and to evaluate the taxonomic position of Mexican stocks and their corresp...

Eight natural Bolivian populations of two closely related species of Triatominae, Triatoma sordida and T. guasayana, were analysed by multilocus enzyme electrophoresis at 20 loci. Both species were readily separated and no natural hybrid was observed. Among the silvatic sample of T. sordida, strong departure from panmixia within and between loci was detected in two sites of the Chaco, suggesting two reproductively separate populations easily recognized at Idh2 and Mdh2 loci. Genetic distance between them was in agreement with the hypothesis of distinct species. However, the detection of 3% of putative hybrids suggested a recent evolutionary divergence.

A cross section of a human population (501 individuals)

No abstract

No abstract

No abstract