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ABSTRACT: Despite its intrinsic difficulty, ancestral character state reconstruction is an essential tool for testing evolutionary hypothesis. Two major classes of approaches to this question can be distinguished: parsimony- or likelihood-based approaches. We focus here on the second class of methods, more specifically on approaches based on continuous-time Markov modeling of character evolution. Among them, we consider the most-likely-ancestor reconstruction, the posterior-probability reconstruction, the likelihood-ratio method, and the Bayesian approach. We discuss and compare the above-mentioned methods over several phylogenetic trees, adding the maximum-parsimony method performance in the comparison. Under the assumption that the character evolves according a continuous-time Markov process, we compute and compare the expectations of success of each method for a broad range of model parameter values. Moreover, we show how the knowledge of the evolution model parameters allows to compute upper bounds of reconstruction performances, which are provided as references. The results of all these reconstruction methods are quite close one to another, and the expectations of success are not so far from their theoretical upper bounds. But the performance ranking heavily depends on the topology of the studied tree, on the ancestral node that is to be inferred and on the parameter values. Consequently, we propose a protocol providing for each parameter value the best method in terms of expectation of success, with regard to the phylogenetic tree and the ancestral node to infer.
Mathematical biosciences 12/2012; · 1.30 Impact Factor
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ABSTRACT: Free-living protozoa allow horizontal gene transfer with and between the microorganisms that they host. They host mycobacteria for which the sources of transferred genes remain unknown. Using BLASTp, we searched within the genomes of 15 mycobacteria for homologous genes with 34 amoeba-resistant bacteria and the free-living protozoa Dictyostelium discoideum. Subsequent phylogenetic analysis of these sequences revealed that eight mycobacterial open-reading frames (ORFs) were probably acquired via horizontal transfer from beta- and gamma-Proteobacteria and from Firmicutes, but the transfer histories could not be reliably established in details. One further ORF encoding a pyridine nucleotide disulfide oxidoreductase (pyr-redox) placed non-tuberculous mycobacteria in a clade with Legionella spp., Francisella spp., Coxiella burnetii, the ciliate Tetrahymena thermophila and D. discoideum with a high reliability. Co-culturing Mycobacterium avium and Legionella pneumophila with the amoeba Acanthamoeba polyphaga demonstrated that these two bacteria could live together in amoebae for five days, indicating the biological relevance of intra-amoebal transfer of the pyr-redox gene. In conclusion, the results of this study support the hypothesis that protists can serve as a source and a place for gene transfer in mycobacteria.
PLoS ONE 01/2012; 7(4):e34754. · 4.09 Impact Factor
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ABSTRACT: The increase in huge number of genomic sequences in recent years has contributed to various genetic events such as horizontal gene transfer (HGT), gene duplication and hybridization of species. Among them HGT has played an important role in the genome evolution and was believed to occur only in Bacterial and Archaeal genomes. As a result, genomes were found to be chimeric and the evolution of life was represented in different forms such as forests, networks and species evolution was described more like a rhizome, rather than a tree. However, in the last few years, HGT has also been evidenced in other group such as metazoa (for example in root-knot nematodes, bdelloid rotifers and mammals). In addition to HGT, other genetic events such as transfer by retrotransposons and hybridization between more closely related lineages are also well established. Therefore, in the light of such genetic events, whether the evolution of metazoa exists in the form of a tree, network or rhizome is highly questionable and needs to be determined. In the current review, we will focus on the role of HGT, retrotransposons and hybridization in the metazoan evolution.
Frontiers in cellular and infection microbiology. 01/2012; 2:50.
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ABSTRACT: The intracellular flea symbiont, Rickettsia felis, may meet other organisms intracellularly such as R. typhi. We used a single-gene phylogenetic approach of the 1375 R. felis genes to look for horizontal transfers that occurred as a result of the bacterial promiscuity with other organisms. Our results showed that besides genes that are linked to the Spotted Fever Group, 165 genes have a different history and are linked to other Rickettsia such as R. bellii (107 genes), R. typhi (15 genes), or to other bacteria such as Legionella sp. and Francisella sp. or to eukaryotes. Among these genes, we identified 73 individual genes and 34 spatial clusters containing 2-4 adjacent genes, a total of 79 genes, with evidence of en bloc transfer. We described 13 chimeric genes resulting from gene recombination with sympatric R. typhi. The transferred DNA sequences present different sizes and functions, suggesting that the horizontal transfer in R. felis is random and neutral within its specific host. Our study shows that the strict intracellular bacteria R. felis exhibits a mosaic genome. We therefore developed a new representation for the evolutionary history of R. felis showing its different putative ancestors in the form of a rhizome.
Molecular Biology and Evolution 11/2011; 28(11):3213-23. · 5.55 Impact Factor
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ABSTRACT: Genome degradation is an ongoing process in all members of the Rickettsiales order, which makes these bacterial species an excellent model for studying reductive evolution through interspecies variation in genome size and gene content. In this study, we evaluated the degree to which gene loss shaped the content of some Rickettsiales genomes. We shed light on the role played by horizontal gene transfers in the genome evolution of Rickettsiales.
Our phylogenomic tree, based on whole-genome content, presented a topology distinct from that of the whole core gene concatenated phylogenetic tree, suggesting that the gene repertoires involved have different evolutionary histories. Indeed, we present evidence for 3 possible horizontal gene transfer events from various organisms to Orientia and 6 to Rickettsia spp., while we also identified 3 possible horizontal gene transfer events from Rickettsia and Orientia to other bacteria. We found 17 putative genes in Rickettsia spp. that are probably the result of de novo gene creation; 2 of these genes appear to be functional. On the basis of these results, we were able to reconstruct the gene repertoires of "proto-Rickettsiales" and "proto-Rickettsiaceae", which correspond to the ancestors of Rickettsiales and Rickettsiaceae, respectively. Finally, we found that 2,135 genes were lost during the evolution of the Rickettsiaceae to an intracellular lifestyle.
Our phylogenetic analysis allowed us to track the gene gain and loss events occurring in bacterial genomes during their evolution from a free-living to an intracellular lifestyle. We have shown that the primary mechanism of evolution and specialization in strictly intracellular bacteria is gene loss. Despite the intracellular habitat, we found several horizontal gene transfers between Rickettsiales species and various prokaryotic, viral and eukaryotic species. OPEN PEER REVIEW: Reviewed by Arcady Mushegian, Eugene V. Koonin and Patrick Forterre. For the full reviews please go to the Reviewers' comments section.
Biology Direct 02/2011; 6:6. · 4.02 Impact Factor
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ABSTRACT: Genome size and gene content in bacteria are associated with their lifestyles. Obligate intracellular bacteria (i.e., mutualists and parasites) have small genomes that derived from larger free-living bacterial ancestors; however, the different steps of bacterial specialization from free-living to intracellular lifestyle have not been studied comprehensively. The growing number of available sequenced genomes makes it possible to perform a statistical comparative analysis of 317 genomes from bacteria with different lifestyles.
Compared to free-living bacteria, host-dependent bacteria exhibit fewer rRNA genes, more split rRNA operons and fewer transcriptional regulators, linked to slower growth rates. We found a function-dependent and non-random loss of the same 100 orthologous genes in all obligate intracellular bacteria. Thus, we showed that obligate intracellular bacteria from different phyla are converging according to their lifestyle. Their specialization is an irreversible phenomenon characterized by translation modification and massive gene loss, including the loss of transcriptional regulators. Although both mutualists and parasites converge by genome reduction, these obligate intracellular bacteria have lost distinct sets of genes in the context of their specific host associations: mutualists have significantly more genes that enable nutrient provisioning whereas parasites have genes that encode Types II, IV, and VI secretion pathways.
Our findings suggest that gene loss, rather than acquisition of virulence factors, has been a driving force in the adaptation of parasites to eukaryotic cells. This comparative genomic analysis helps to explore the strategies by which obligate intracellular genomes specialize to particular host-associations and contributes to advance our knowledge about the mechanisms of bacterial evolution.
Biology Direct 05/2009; 4:13. · 4.02 Impact Factor
