Clonal Population Structure
has a highly unusual population genetic pattern consisting of three
widespread, clonal lineages known as types I, II and III
and Sibley 1995).
Despite the presence of a meiotic stage in the
this clonal pattern persists in Nature, suggesting that much of
transmission is based on mitotic replication. The three lineages are
highly related, differing by only ~2% at the
nucleotide level. Studies of
genetic polymorphism reveal that at each locus there exist only two alleles,
indicating these three lineages arose from a single genetic recombination in the
(Grigg et al.
2001; Su et al. 2003).
The small genetic differences between strains likely underlie important
biological differences. For example, type I strains are acutely lethal in mice
(LD100 = 1 viable organism), and
genetic linkage analysis
has been used to analyze genes controlling this trait
(Su et al. 2002).
It remains uncertain to what extent the genotype of the
parasite contributes to clinical severity in human toxoplasmosis
(Boothroyd and Grigg 2002).
Some studies indicate an increased prevalence of type I strains in human
toxoplasmosis, although samples sizes are relatively small, geographic
coverage is not uniform, and referral bias remains a problem. In most
locations, type II strains are the most prevalent in human
toxoplasmosis, while type III strains are largely found in animals.
Further studies on the genotype of T. gondii strains from human
infections, as well as animals, are needed from a wider geographic
range. DNA polymorphisms can easily be used to
genotype new isolates.