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| Author(s): |
Michelle Klein Sercundes
Total Authors: 1
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| Document type: | Master's Dissertation |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Faculdade de Medicina Veterinária e Zootecnia (FMVZ/SBD) |
| Defense date: | 2010-02-23 |
| Examining board members: |
Rodrigo Martins Soares;
Paulo Eduardo Brandão;
Silvio Luís Pereira de Souza
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| Advisor: | Rodrigo Martins Soares |
| Abstract | |
The known members of the sub-family Toxoplasmatinae are Hammondia hammondi, Toxoplasma gondii, Neospora hughesi, Neospora caninum, Hammondia heydorni and Besnoitia spp. Dogs (and probably other species of dogs) are definitive hosts of N. caninum and H. heydorni. The oocysts of coccidia of these species are morphologically indistinguishable and the coprological differential diagnosis between the two agents is virtually impossible if used conventional methods of diagnosis. Similar situation is observed with the cats (and other species of felids) with respect to T. gondii and H. hammondi. The objective of this study was to propose a phylogenetic reconstruction of protozoa belonging to the subfamily Toxoplasmatinae by analyzing nucleotide sequences of mitochondrial genes and apicoplast. We used gene sequences of cytochrome b and two apicoplast genes, the gene encoding the beta subunit of DNA dependent RNA polymerase (RpoB) and the gene encoding caseinolitic protein (ClpC). From the phylogenetic analysis and the analysis of nucleotide and amino acids variability, was shown that the species H. heydorni is equidistant from all other species of toxoplasmatineos. The relative positions of the genera Toxoplasma, Neospora and Hammondia in the phylogenetic trees were not congruent in all reconstructions, because the topologies of the reconstructions varies according to the taxons that are used as outgroups and clades are poorly supported statistically. Thus, reconstructions of topologies with short branches that derive to poorly statistical supported nodes, coupled with the evolutionary equidistance between taxa the assessed (Neospora spp. H. heydorni. and T. gondii) suggests that a consistent polytomous evolution would explain the evolution within this group of organisms, namely the the relative placement of these taxa could be the result of a radiated evolution. The genes of organelles were more conserved than nuclear genes. Although the apicoplast genes may be more conserved than nuclear genes, they have the ratio between non-synonymous substitutions and synonymous substitutions considerably higher than those of nuclear and mitochondrial genes, which may indicate that the gene products are being subjected to positive selective pressure. In the case of nuclear and mitochondrial genes, it is possible to assume that they are subject to negative selective pressure, indicating that the substitutions are likely to be harmful to organisms and therefore changes in gene products to be less frequently recorded. Still, the variability in non-synonymous sites is considerably higher for sequences of apicoplast in relation to others loci, particularly in the case of RpoB sequences. Also in terms of variability in non-synonymous sites, it is observed that the sequences of apicoplast genes of H. heydorni are as different from those of T. gondii as the N. caninum. The analyzes of apicoplast genes revealed a striking divergence between the two strains of H. heydorni. It is noteworthy that the genotypic differences between the two strains of H. heydorni are greater than the differences between the two species of Neospora, indicating that the two strains of H. heydorni could be classified as two distinct species; if solely criteria of molecular evolution were considered. (AU) | |