Читать книгу Parasitology - Alan Gunn - Страница 96

The population structure of T. gondii is highly clonal (Galal et al. 2019). In Europe and North America, most strains of T. gondii belong to one of three distinct genotypes: Types I, II, and III. However, other genetically distinct strains exist in Brazil and French Guiana. There is limited variability within genotypes I, II, and III, and this suggests that they probably arose relatively recently. Furthermore, this polymorphism appears limited to just two alleles called A and E – short for Adam and Eve – on the basis that all extant strains derive from mixtures of two ancestral founder strains. The continued existence of these separate strains probably results from the parasite’s ability to maintain asexually reproducing populations, possibly indefinitely, by horizontal and vertical transmission between intermediate hosts and the limited opportunity for genetic mixing during sexual reproduction within the cat definitive host. Because T. gondii is haploid, genetic mixing only occurs if a cat has simultaneous infections with two different strains of the parasite. Furthermore, the development of the two strains would have to be synchronous because the production of gametes occurs over a relatively short period. This scenario might occur if the cat consumed a mouse harbouring two or more strains of the parasite or two mice harbouring different strains in a short period. There is limited evidence of intermediate hosts harbouring mixed strain T. gondii infections, but there is a need for more work in this area.