Sexual recombination punctuated by outbreaks and clonal expansions predicts Toxoplasma gondii population genetics

Abstract

The cosmopolitan parasitic pathogen Toxoplasma gondii is capable of infecting essentially any warm-blooded vertebrate worldwide, including most birds and mammals, and establishes chronic infections in one-third of the globe's human population. The success of this highly prevalent zoonosis is largely the result of its ability to propagate both sexually and clonally. Frequent genetic exchanges via sexual recombination among extant parasite lineages that mix in the definitive felid host produces new lines that emerge to expand the parasite's host range and cause outbreaks. Highly successful lines spread clonally via carnivorism and in some cases sweep to pandemic levels. The extent to which sexual reproduction versus clonal expansion shapes Toxoplasma's current, global population genetic structure is the central question this review will attempt to answer.

Fig. 1

Allele inheritance patterns among Toxoplasma gondii strains. Multi-locus sequence analysis of strains from 11 haplogroups revealed different combinations of archetypal and unique alleles randomly inherited among seven mostly unlinked intron loci. Consensus is defined as the nucleotide sequence common to at least two of the three archetypal strains. For Archetypal single nucleotide polymorphisms (SNPs), each subscript identifies the number of unique polymorphisms relative to consensus. For archetypal strains I, II and III, only two allelic classes exist. The “A” allele is an open circle ○ and is defined as the allelic class shared by at least two of the archetypal strains. The “E” allele is an open square □. Where variation exists within the A allelic class, hatches through the circle ; defines the number of unique nucleotide polymorphisms. Shaded circles ; and squares indicate a drifted archetypal allele, where polymorphism is less than 0.4% from archetypal. Shaded diamond represents a unique allele with >0.4% polymorphism from the A or E alleles. “-” indicates no sequence available.