Genetic diversity, clonality and sexuality in Toxoplasma gondii☆
Introduction
Toxoplasma gondii is a protozoan parasite that infects virtually all warm-blooded animal species worldwide. It has a two-host life cycle with felids as definitive hosts in which the sexual replication of the parasite occurs, while mammals and birds are intermediate hosts in which the asexual replication occurs. Even though there is no geographic boundary and host specificity, molecular genotyping studies have showed a fundamental clonal population structure with three clonal lineages, namely types I, II and III (Tibayrenc et al., 1991, Dardé et al., 1992, Sibley and Boothroyd, 1992, Howe and Sibley, 1995, Dardé, 1996, Ajzenberg et al., 2002a). Difference at DNA sequence level among the predominant clonal lineages is less than 2% (Grigg et al., 2001a). There are several explanations for the existence of clonal population structure in T. gondii. First, this parasite is able to transmit among intermediate hosts through carnivorism and scavenging, bypassing sexual recombination events in definitive host cats (Howe and Sibley, 1995, Su et al., 2003). Second, many macrogametes of the parasite remain unfertilised but are capable of forming oocysts in the small intestine of cats by parthenogenesis (Ferguson, 2002). Third, cats simultaneously infected with different strains of T. gondii are likely to be very rare events in nature, therefore there is a temporal barrier for recombination to occur. Though a largely clonal population structure predominates, genetic diversity does exist in T. gondii (Howe and Sibley, 1995, Dardé, 1996, Bossi et al., 1998, Dardé et al., 1998, Cole et al., 2000, Lehmann et al., 2000, Grigg et al., 2001b, Carme et al., 2002b, Ghosn et al., 2003, Miller et al., 2004). Moreover, recombination phenomena have been described as occasional events (Howe and Sibley, 1995). To better estimate the genetic diversity and sexual recombination of T. gondii, we analysed a substantial set of strains of wild and/or tropical origin, mainly from French Guiana, by multilocus microsatellite sequencing and phylogenetic analysis.
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Toxoplasma and Hammondia strains
A total of 43 T. gondii strains were selected for this study (Table 1). Most strains were selected from a pool of 245 strains mainly collected by the BRC ToxoBS group and categorised based on length polymorphism of five microsatellite (MS) markers. Eighty-eight percent (216/245) of these strains belong to the three classical type I, II and III lineages, with a clear predominance of type II strains (188/245, 77%). For sequence analysis, 14 strains were arbitrarily selected to represent the three
Molecular analysis
All 43 T. gondii strains were successfully sequenced at the five MS loci and the sequences were submitted to GenBank (see Table 2 for accession numbers). Based on the polymorphisms of dinucleotide repeats in microsatellite regions and the single nucleotide polymorphisms (SNPs) in flanking regions, a number of total alleles were identified for each marker including four alleles at TUB2, 15 at W35, 10 at TgM-A, seven at B18 and 16 at B17 (Fig. 1 and Table 2).
The number of SNP alleles varies
Discussion
The clonal theory of T. gondii population structure, with three predominant lineages, is based on T. gondii samples collected mainly from the European or the North American domestic cycle either from clinical cases of human toxoplasmosis or from meat producing animals such as pigs, sheep and chicken (Dardé et al., 1992, Howe and Sibley, 1995, Ajzenberg et al., 2002a, Ajzenberg et al., 2002b). This collection may not reflect the true status of T. gondii in remote geographical areas or in
Acknowledgements
This work was supported by French Ministry of Research (support no. 02 g 0418 for the development of Biological Resource Centers). We wish to thank for strain providing the BRC ToxoBS group (French parasitologist network for Toxoplasma strain collection): J.M. Pinon, I. Villena (Reims, RMS), P. Thulliez (Paris Institut de Puériculture, IPP), P. Marty (Nice, LPN), M.H. Bessieres (Toulouse, TOU), J.P. Gangneux (Rennes, REN), L. Paris (Paris Pitié-Salpétrière, PSP), J. Berthonneau (Poitiers), A.
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