Hammondia

FIGURE 3.2 Phylogram of 43 Toxoplasma gondii strains as determined by analysis of the sequences of five microsatellite markers (TUB2, W35, TgM-A, B18, B17). Hammondia hammondii was used as the outgroup. The tree was built by Wagner analysis after bootstrapping with 100 repetitions. FG indicates a French Guianan strain. Adapted from Ajzenberg et al. (2004).

Type II and related isolates

Type III and related isolates

FIGURE 3.2 Phylogram of 43 Toxoplasma gondii strains as determined by analysis of the sequences of five microsatellite markers (TUB2, W35, TgM-A, B18, B17). Hammondia hammondii was used as the outgroup. The tree was built by Wagner analysis after bootstrapping with 100 repetitions. FG indicates a French Guianan strain. Adapted from Ajzenberg et al. (2004).

from the 'domestic' population of Toxoplasma in Europe or North America is very uncommon; they include four isolates from North American pigs, P62, P80, P89, and P105 (Mondragon et al., 1998), five DNA samples (including four identical described as type IV) from human ocular toxoplasmosis in North America (Grigg et al., 2001b), two isolates from AIDS patients in the USA (Howe and Sibley, 1995), and four isolates from human congenital toxoplasmosis in France (Ajzenberg et al., 2002b, 2004). The number of recombinant strains in the literature will probably dramatically increase when chicken isolates sampled all over the world by Dubey's laboratory (references already cited) will be characterized by multilocus analyses.

Atypical genotypes More rarely, 'atypical' or 'exotic' strains are discovered (Grigg et al., 2001a; Su et al., 2003; Ajzenberg et al., 2004). These have many unique polymorphisms and 'novel' alleles (Table 3.4). Phylogenetic analysis shows that the atypical genotypes are distributed throughout the tree without any obvious structuration, and cannot be related to the three main lineages (Ajzenberg et al., 2004) (Figure 3.2). However, global observation of the genetic diversity indices showed that although allelic diversity was high, the level of genetic polymorphism of sequences remained very low in agreement with previous observations of diversity in Toxoplasma strains (Grigg et al., 2001a; Su et al., 2003; Ajzenberg et al., 2004). At some loci, atypical strains show evidence of the dimorphic allele patterns that typify the clonal lineages (Su et al., 2003). This strongly suggests that they have introgressed with the clonal lineages through subsequent crosses (Sibley, 2003).

Only 14 strains with atypical genotypes are described in the literature. The first, MAS, isolated from a case of human congenital toxoplasmosis in France, and the second, CASTELLS, taken from an aborted sheep in Uruguay, have been well characterized with many markers in many studies (Dardé, 1996; Grigg et al., 2001a; Su et al., 2003; Ajzenberg et al., 2004). Additional atypical strains include a cougar isolate from Canada (Lehmann et al., 2000; Grigg et al., 2001a; Su et al., 2003), an atypical genotype (named type X) from marine mammals (Miller et al., 2004; Conrad et al., 2005), a second isolate (IPP-2002-URB) from a human congenital case in France (Ajzenberg et al., 2004), and nine strains from French Guiana (Carme et al., 2002; Ajzenberg et al., 2004).

The French Guianan (FG) strains were isolated from human patients with severe disseminated toxoplasmosis, except for one strain (IPP-2002-BAT) taken from a case of asymptomatic congenital toxoplasmosis. Since infections occurred in individuals living in the rainforest, eating under-cooked game and drinking untreated river water (Carme et al., 2002), the strains are probably transmitted from wildlife species. Variation in the FG strains (RUB, VAND) was first detected by isoenzyme analysis (Bossi et al., 1998; Dardé et al., 1998), then multiple polymorphisms were reported via sequencing of antigen genes SAG1, SAG2A, SAG3, SAG4, BSR4 (Grigg et al., 2001a) and GRA6 (Fazaeli et al., 2000a). The nine FG strains have also been genetically characterized by multi-locus microsatellite sequencing together with 34 strains from diverse host and geographical origins (Ajzenberg et al., 2004). Each French Guianan strain had a unique multilocus genotype, the genotypic diversity was maximal (number of different genotypes on the total number of genotypes, G = 1), and the majority of atypical alleles were found in FG strains and, for some alleles, only in FG strains (Table 3.5). The level of heterogeneity between the FG strains contrasts markedly with 'domestic' isolates from human congenital toxoplasmosis in France, where, in 57 consecutive cases, 96.5 percent of isolates had only one genotype, type II (Ajzenberg et al., 2002b).

3.3.3.3 New data on population structure

Genetic exchange can have many effects: it may lower linkage disequilibria, shuffle genotypes and reduce phylogenetic divergence (Tibayrenc and Ayala, 2002). There is evidence that, as with many parasites, the use of sexual recombination may vary in Toxoplasma. Microsatellite analysis found lower linkage disequilibrium in a Brazilian chicken sample than in a North American domestic animal

TABLE 3.5 Genotypic diversity of nine French Guianan strains in comparison with three archetypal strains (BK, BEV, and NED)

Isolate

Host

Origin

type

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