Toxoplasma gondii is among the most prevalent parasites worldwide, infecting many wild and domestic animals and causing zoonotic infections in humans. T. gondii differs substantially in its broad distribution from closely related parasites that typically have narrow, specialized host ranges. To elucidate the genetic basis for these differences, we compared the genomes of 62 globally distributed T. gondii isolates to several closely related coccidian parasites. Our findings reveal that tandem amplification and diversification of secretory pathogenesis determinants is the primary feature that distinguishes the closely related genomes of these biologically diverse parasites. We further show that the unusual population structure of T. gondii is characterized by clade-specific inheritance of large conserved haploblocks that are significantly enriched in tandemly clustered secretory pathogenesis determinants. The shared inheritance of these conserved haploblocks, which show a different ancestry than the genome as a whole, may thus influence transmission, host range and pathogenicity.

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...

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Detection of Toxoplasma gondii oocysts in environmental samples is a great challenge for researchers as this coccidian parasite can be responsible for severe infections in humans and in animals via ingestion of a single oocyst from contaminated water, soil, fruits or vegetables. Despite field investigations, oocysts have been rarely recovered from the environment due to the lack of sensitive methods. Immunomagnetic separation, fluorescence-activated cell sorting, and polymerase chain reaction have recently shown promising use in detection of protozoa from complex matrices. Such procedures could be applied to T. gondii detection, if studies on the antigenic and biochemical composition of the oocyst wall are completed. Using such methods, it will be possible to assess the occurrence, prevalence, viability and virulence of T. gondii oocysts in environmental matrices and specify sources of human and animal contamination.