| 1. |
- Viklund, Johan, 1982-, et al.
(författare)
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Comparative and Phylogenomic Evidence that the Alphaproteobacterium HIMB59 is not a Member of the Oceanic SAR11 Clade
- 2013
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Ingår i: PLOS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 8:11, s. e78858-
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Tidskriftsartikel (refereegranskat)abstract
- SAR11 is a globally abundant group of Alphaproteobacteria in the oceans that is taxonomically not well defined. It has been suggested SAR11 should be classified into the novel order Pelagibacterales. Features such as conservation of gene content and synteny have been taken as evidence that also the divergent member HIMB59 should be included in the order. However, this proposition is controversial since phylogenetic analyses have questioned the monophyly of this grouping. Here, we performed phylogenetic analyses and reinvestigated the genomic similarity of SAR11 and HIMB59. Our phylogenetic analysis confirmed that HIMB59 is not a sister group to the other SAR11 strains. By placing the comparison in the context of the evolution of the Alphaproteobacteria, we found that none of the measures of genomic similarity supports a clustering of HIMB59 and SAR11 to the exclusion of other Alphaproteobacteria. First, pairwise sequence similarity measures for the SAR11 and HIMB59 genomes were within the range observed for unrelated pairs of Alphaproteobacteria. Second, pairwise comparisons of gene contents revealed a higher similarity of SAR11 to several other alphaproteobacterial genomes than to HIMB59. Third, the SAR11 genomes are not more similar in gene order to the HIMB59 genome than what they are to several other alphaproteobacterial genomes. Finally, in contrast to earlier reports, we observed no sequence similarity between the hypervariable region HVR2 in the SAR11 genomes and the region located at the corresponding position in the HIMB59 genome. Based on these observations, we conclude that the alphaproteobacterium HIMB59 is not monophyletic with the SAR11 strains and that genome streamlining has evolved multiple times independently in Alphaproteobacteria adapted to the upper surface waters of the oceans.
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| 2. |
- Viklund, Johan, 1982-
(författare)
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Phylogenomics of Oceanic Bacteria
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The focus of this thesis has been the phylogenomics and evolution of the Alphaproteobacteria. This is a very diverse group which encompasses bacteria from intraceullar parasites, such as the Rickettsiales, to freeliving bacteria such as the most abundant bacteria on earth, the SAR11. The genome sizes of the Alphaproteobacteria range between 1 Mb and 10 Mb. This group is also connected to the origin of the mitochondria.Several studies have placed the SAR11 clade together with the Rickettsiales and mitochon- dria. Here I have shown that this placement is an artifact of compositional heterogeneity. When choosing genes or sites less affected by heterogeneity we find that the SAR11-clade instead groups with free-living alphaproteobacteria. Gene-content analysis showed that SAR11 was missing several genes for recombination and DNA-repair. The relationships within the SAR11- clade has also been examined and questioned. Specifically, we found no support for placing the taxon referred to as HIMB59 within the SAR11. Ocean metagenomes have been investigated to determine whether the SAR11-clade is a potential relative of the mitochondria. No such relationship was found.Further I have shown how important it is to take the phylogenetic relationships into account when doing statistical analyzes of genomes.The evolution of LD12, the freshwater representative of SAR11, was investigated. Phyloge- nies and synonymous substitution frequencies showed the presence of three distinct subclades within LD12. The recombination to mutation rate was found to be extremely low. This is re- markable in light of the very high rate in the oceanic SAR11. This is may be due to adaptation to a more specialized niche.Finally we have compared structure-based and sequence-based methods for orthology pre- diction. A high fraction of the orfan proteins were predicted to code for intrinsically disordered proteins.Many phylogenetic methods are sensitive to heterogeneity and this needs to be taken into ac- count when doing phylogenies. There have been at least three independent genome reductions in the Alphaproteobacteria. The frequency of recombination differ greatly between freshwater and oceanic SAR11. Forces affecting the size of bacterial genomes and mechanisms of evolu- tionary change depend on the environmental context.
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