Recent Expansion of Toxoplasma Through Enhanced Oral Transmission

Abstract

The global predominance of three clonal Toxoplasma gondii lineages suggests that they are endowed with an exceptional trait responsible for their current parasitism of nearly all warm-blooded vertebrates. Genetic polymorphism analyses indicate that these clonal lineages emerged within the last 10,000 years after a single genetic cross. Comparison with ancient strains (approximately 1 million years) suggests that the success of the clonal lineages resulted from the concurrent acquisition of direct oral infectivity. This key adaptation circumvented sexual recombination, simultaneously promoting transmission through successive hosts, hence leading to clonal expansion. Thus, changes in complex life cycles can occur rapidly and can profoundly influence pathogenicity.

Adaptive host manipulation by Toxoplasma gondii: fact or fiction?

“…Having multiple routes of transmission and such a wide host range make T. gondii unique and successful [14,22], perhaps eliminating the need for this parasite to enhance transmission by adaptive manipulation.…”

Molecular and functional aspects of parasite invasion

“…Most apicomplexans typically infect a limited range of host organisms, whereas T. gondii can infect virtually all warm-blooded animals and almost any cell type. Despite sexual reproduction occurring exclusively in felids (definitive hosts), the broad range of T. gondii has recently been explained by evidence for a significant change in its life cycle, allowing direct oral transmission between many different intermediate hosts [1].…”

Just one cross appears capable of dramatically altering the population biology of a eukaryotic pathogen like Toxoplasma gondii

“…Remarkably, these three clonotypes, as well as many of the less common strains, appear to represent the recent merging of two distinct gene pools (8,9), because among these strains there are just two major allelic types for each locus (9,10). The success of Toxoplasma as a parasite belies this genetic simplicity: it can infect almost any warm-blooded vertebrate, and it maintains a high prevalence in many host species, including humans.…”

“…The success of Toxoplasma as a parasite belies this genetic simplicity: it can infect almost any warm-blooded vertebrate, and it maintains a high prevalence in many host species, including humans. It appears, then, that much of the ''success'' of Toxoplasma in certain geographical locales is due to the emergence of only a few lines that have dramatically enhanced fitness when compared to other genotypes (8,9). Similar clonal population structures have been described in other parasitic protozoa (1), and although the mechanisms that led to clonal propagation may differ between species (and the clonal line), identifying the genetic basis for their emergence and eventual dominance in a large geographical area has the potential to shed considerable light on the evolution of eukaryotic pathogens.…”