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Conclusion: Trypanosoma cruzi should be a star in the field of pathogen population genetics

This is unexpected, since the community of scientists working on Chagas disease is very limited by comparison with those working on AIDS, tuberculosis, and malaria. Nevertheless, it can be said that T. cruzi is by far the pathogenic microorganism that population genetics knows best and can compete in this domain with the bacterium Escherichia coli. In spite of the obstacle that it is a pathogenic agent, it is our hope now to “sell” Trypanosoma cruzi as a pet model for basic evolutionary research, together with Escherichia coli, Drosophila melanogaster, Mus musculus, and Caenorhabditis elegans.99

As far as applied research is concerned, it is our hope that T. cruzi’s genetic variability is far from having revealed all its secrets and will continue to make great contributions to the survey, control, and cure of Chagas disease.

Glossary of specialized terms

Clade Evolutionary lineage defined by cladistic analysis. A clade is monophyletic (it has only one ancestor) and is genetically isolated (which means that it evolves independently) from other clades.

Cladistic analysis A specific method of phylogenetic analysis based on the polarization of characters, that are divided into ancestral (plesiomorphic) and derived (apomorphic) characteristics.

Genetic distance Various statistical measures inferred from genetic data, estimating the genetic dissimilarities among individuals or populations. Genetic distances can be based on the percentage of band mismatches on gels (as for markers such as MLEE or RAPD) or allelic frequency differences or the percentage of sequence divergence.

Genotype Genetic constitution of a given organism; cf. phenotype.

Homoplasy Possession shared by distinct phylogenetic lines of identical characteristics that do not come from a common ancestry. The origin of homoplasic characteristics can be: (a) convergence (possession of identical characteristics derived from different ancestral characteristics, due to convergent evolutionary pressure; f), (b) parallelism (possession of identical characteristics derived from a single ancestral characteristic, and generated independently in different phylogenetic lines), and (c) reversion (restoration of an ancestral characteristic from a derived characteristic).

Molecular clock In its strict, original sense (more appropriately called the DNA clock hypothesis), the concept that the rate of nucleotide substitutions in DNA is constant. In a broader sense, it is simply the evolutionary speed of the part of the genome that encodes the variability of a given marker. This speed is commended by the rate of substitution/ mutation. It may be regular or irregular.

Multilocus genotype The combined genotype of a given strain or a given individual established with several genetic loci.

Outgroup In cladistics or phylogenetics, an outgroup is a (monophyletic) group of organisms that is used as a reference group for determining the phylogenetic relationship in a set of monophyletic groups of organisms. The outgroup is selected to be closely related to the groups under study, but less closely than any single one of the groups under study is to another.

Phenotype All observable properties of a given individual or a given population apart from the genotype. The phenotype is not limited to morphological characteristics and can include, e.g., physiological or biochemical parameters. The pathogenicity of a microorganism is a phenotypic property. The phenotype is produced by the interaction between the genotype and the environment.

Phylogenetics A branch of genetics that aims at reconstructing the evolutionary past and genetic relationships of taxa, species, strains, or of separate evolutionary lines.

Phylogeny Evolutionary relationships between taxa, species, organisms, genes, or molecules.

Population genetics Analysis of allele and genotype frequency distribution and modifications under the influence of genetic drift, natural selection, mutation, and gene flow. It also takes into account the factors of population subdivision and population structure.

Sympatry Living in the same geographical location.

Acknowledgment

We thank Jenny Telleria for designing Fig. 21.2.

 
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