A single chromosome unexpectedly links highly divergent isolates of Toxoplasma gondii

Abstract

Toxoplasma gondii is an obligate intracellular parasite that can cause disease in all warm-blooded animals studied to date, including humans. Over a billion people have been infected with this parasite worldwide. In Europe and North America, Toxoplasma has a clonal population structure, where only three lineages are highly dominant (strain types I, II, and III). Khan et al. [mBio 2(6): e00228-11, 2011] have carried out phylogenetic analyses on a large number of diverse strains from outside of these lineages and found evidence for a significant split between the clonal North American/European lineages and those in South America. In contrast to most of the genome, nearly all North American/European strains sampled, and the majority of South American strains sampled, harbored at least portions of a monomorphic chromosome Ia (Ia*). In contrast to previous models, these data suggest that the monomorphic haplotype originated in South America and migrated to the North. These authors propose that South American haplotype 12 was a precursor to modern-day type II, while South American haplotypes 6 and 9 crossed with haplotype 12 to give rise to the type I and III lineages, respectively. However, the findings reported by Khan et al. complicate the origin of chromosome Ia, since there are members of haplotypes 9 and 12 with nearly complete versions of Ia* and members of haplotypes 6 and 12 with over 50% of Ia*. This unexpected finding raises exciting new questions about how an entire common chromosome can be found within strains that are highly divergent at most other genomic loci.

FIG 1

One possible model for the migration of strains harboring monomorphic chromosome Ia (Ia*) from South America to North America and hypothetical crosses that could have led to the modern-day North American and European clonal lineages (I, II, and III). South American strains exhibit a comparatively high level of recombination, and Ia* could have spread throughout this interbreeding population due to selection and/or segregation bias. Human activity and/or avian migration could have then facilitated the movement of South American strains harboring Ia* to North America. These strains then recombined to produce the ancestors to the modern-day clonal lineages that are dominant in Europe and North America. The genealogy proposed for the origins of clonal types I, II, and III is based on the number of strains harboring Ia* in each haplotype group. For example, most of the sampled strains belonging to haplotype 12 harbor Ia chromosomes that are only partially monomorphic, as is the case for haplotype 6. Haplotype 12* (H12*) is a hypothetical haplotype with a fully monomorphic Ia, similar to strain B41 in the work of Khan et al. (7).