| 1. |
- Klasson, Lisa, 1977-
(författare)
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Genome Evolution in Maternally Inherited Insect Endosymbionts
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Symbiosis is a widely common phenomenon in nature and has undoubtedly contributed to the evolution of all organisms on earth. Symbiotic associations can be of varying character, such as parasitic or mutualistic, but all imply a close relationship. To study the evolution of genomes of insect endosymbionts, we have sequenced the genomes of the mutualist Buchnera aphidicola from the aphid Schizaphis graminum (Sg) and the reproductive manipulator Wolbachia pipientis strain wRi from Drosophila simulans that show strikingly different evolutionary patterns. The comparison between the genome of B.aphidicola (Sg) and the genome of B.aphidicola from the aphid Acyrthosiphon pisum (Ap), that are believed to have diverged 50 million years ago, revealed a perfect gene order conservation and loss of only 14 genes in either of the lineages. In contrast, the rate of nucleotide turnover is very fast probably due to relaxed selection and loss of DNA repair genes. The genomic stasis observed in Buchnera was attributed to the loss of repeats and of the gene recA. In striking contrast to the genomes of B.aphidicola, a vast amount of repeats were found in the genome sequence of W.pipientis strain wMel. The comparison between the genomes of W.pipientis strain wRi and W.pipientis strain wMel shows that a lot of rearrangements have occurred since their divergence. The massive amount of repeats might stem from relaxed selection pressure but possibly also from selection to create variability via recombination. Comparisons between pairs of genomes from closely related bacteria showed that the stability of gene order and content is connected to an intracellular lifestyle and indicated that homologous recombination between repeats is an important mechanisms for causing intrachromosomal rearrangements. Our studies show that the lifestyle of a bacterium to a great extent shapes the evolution of their genetic material and future capabilities to adapt to new environments.
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| 2. |
- Klasson, Lisa, et al.
(författare)
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Genome evolution of Wolbachia strain wPip from the Culex pipiens group.
- 2008
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Ingår i: Molecular biology and evolution. - : Oxford University Press (OUP). - 0737-4038 .- 1537-1719. ; 25:9, s. 1877-87
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Tidskriftsartikel (refereegranskat)abstract
- The obligate intracellular bacterium Wolbachia pipientis strain wPip induces cytoplasmic incompatibility (CI), patterns of crossing sterility, in the Culex pipiens group of mosquitoes. The complete sequence is presented of the 1.48-Mbp genome of wPip which encodes 1386 coding sequences (CDSs), representing the first genome sequence of a B-supergroup Wolbachia. Comparisons were made with the smaller genomes of Wolbachia strains wMel of Drosophila melanogaster, an A-supergroup Wolbachia that is also a CI inducer, and wBm, a mutualist of Brugia malayi nematodes that belongs to the D-supergroup of Wolbachia. Despite extensive gene order rearrangement, a core set of Wolbachia genes shared between the 3 genomes can be identified and contrasts with a flexible gene pool where rapid evolution has taken place. There are much more extensive prophage and ankyrin repeat encoding (ANK) gene components of the wPip genome compared with wMel and wBm, and both are likely to be of considerable importance in wPip biology. Five WO-B-like prophage regions are present and contain some genes that are identical or highly similar in multiple prophage copies, whereas other genes are unique, and it is likely that extensive recombination, duplication, and insertion have occurred between copies. A much larger number of genes encode ankyrin repeat (ANK) proteins in wPip, with 60 present compared with 23 in wMel, many of which are within or close to the prophage regions. It is likely that this pattern is partly a result of expansions in the wPip lineage, due for example to gene duplication, but their presence is in some cases more ancient. The wPip genome underlines the considerable evolutionary flexibility of Wolbachia, providing clear evidence for the rapid evolution of ANK-encoding genes and of prophage regions. This host-Wolbachia system, with its complex patterns of sterility induced between populations, now provides an excellent model for unraveling the molecular systems underlying host reproductive manipulation.
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| 3. |
- Klasson, Lisa, et al.
(författare)
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Horizontal gene transfer between Wolbachia and the mosquito Aedes aegypti.
- 2009
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Ingår i: BMC Genomics. - : Springer Science and Business Media LLC. - 1471-2164. ; 10, s. 33-
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND: The evolutionary importance of horizontal gene transfer (HGT) from Wolbachia endosymbiotic bacteria to their eukaryotic hosts is a topic of considerable interest and debate. Recent transfers of genome fragments from Wolbachia into insect chromosomes have been reported, but it has been argued that these fragments may be on an evolutionary trajectory to degradation and loss.RESULTS: We have discovered a case of HGT, involving two adjacent genes, between the genomes of Wolbachia and the currently Wolbachia-uninfected mosquito Aedes aegypti, an important human disease vector. The lower level of sequence identity between Wolbachia and insect, the transcription of all the genes involved, and the fact that we have identified homologs of the two genes in another Aedes species (Ae. mascarensis), suggest that these genes are being expressed after an extended evolutionary period since horizontal transfer, and therefore that the transfer has functional significance. The association of these genes with Wolbachia prophage regions also provides a mechanism for the transfer.CONCLUSION: The data support the argument that HGT between Wolbachia endosymbiotic bacteria and their hosts has produced evolutionary innovation.
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| 4. |
- Klasson, Lisa, et al.
(författare)
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Strong asymmetric mutation bias in endosymbiont genomes coincide with loss of genes for replication restart pathways
- 2006
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Ingår i: Molecular biology and evolution. - : Oxford University Press (OUP). - 0737-4038 .- 1537-1719. ; 23:5, s. 1031-1039
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Tidskriftsartikel (refereegranskat)abstract
- A large majority of bacterial genomes show strand asymmetry, such that G and T preferentially accumulate on the leading strand. The mechanisms are unknown, but cytosine deaminations are thought to play an important role. Here, we have examined DNA strand asymmetry in three strains of the aphid endosymbiont Buchnera aphidicola. These are phylogenetically related, have similar genomic GC contents, and conserved gene order structures, yet B. aphidicola (Bp) shows a fourfold higher replication-induced strand bias than B. aphidicola (Sg) and (Ap). We rule out an increase in the overall substitution frequency as the major cause of the stronger strand bias in B. aphidicola (Bp). Instead, the results suggest that the higher GC skew in this species is caused by a different spectrum of mutations, including a relatively higher frequency of C to T mutations on the leading strand and/or of G to A mutations on the lagging strand. A comparative analysis of 20 γ-proteobacterial genomes revealed that endosymbiont genomes lacking recA and other genes involved in replication restart processes, such as priA, which codes for primosomal helicase PriA, displayed the strongest strand bias. We hypothesize that cytosine deaminations accumulate during single-strand exposure at arrested replication forks and that inefficient restart mechanisms may lead to high DNA strand asymmetry in bacterial genomes.
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| 5. |
- Klasson, Lisa, et al.
(författare)
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The mosaic genome structure of the Wolbachia wRi strain infecting Drosophila simulans
- 2009
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 106:14, s. 5725-5730
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Tidskriftsartikel (refereegranskat)abstract
- The obligate intracellular bacterium Wolbachia pipientis infects around 20% of all insect species. It is maternally inherited and induces reproductive alterations of insect populations by male killing, feminization, parthenogenesis, or cytoplasmic incompatibility. Here, we present the 1,445,873-bp genome of W. pipientis strain wRi that induces very strong cytoplasmic incompatibility in its natural host Drosophila simulans. A comparison with the previously sequenced genome of W. pipientis strain wMeI from Drosophila melanogaster identified 35 breakpoints associated with mobile elements and repeated sequences that are stable in Drosophila lines transinfected with wRi. Additionally, 450 genes with orthologs in wRi and wMeI were sequenced from the W. pipientis strain wUni, responsible for the induction of parthenogenesis in the parasitoid wasp Muscidifurax uniraptor. The comparison of these A-group Wolbachia strains uncovered the most highly recombining intracellular bacterial genomes known to date. This was manifested in a 500-fold variation in sequence divergences at synonymous sites, with different genes and gene segments supporting different strain relationships. The substitution-frequency profile resembled that of Neisseria meningitidis, which is characterized by rampant intraspecies recombination, rather than that of Rickettsia, where genes mostly diverge by nucleotide substitutions. The data further revealed diversification of ankyrin repeat genes by short tandem duplications and provided examples of horizontal gene transfer across A- and B- group strains that infect D. simulans. These results suggest that the transmission dynamics of Wolbachia and the opportunity for coinfections have created a freely recombining intracellular bacterial community with mosaic genomes.
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| 6. |
- Walker, Thomas, et al.
(författare)
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Ankyrin repeat domain-encoding genes in the wPip strain of Wolbachia from the Culex pipiens group.
- 2007
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Ingår i: BMC Biology. - : Springer Science and Business Media LLC. - 1741-7007. ; 5, s. 39-
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND: Wolbachia are obligate endosymbiotic bacteria maternally transmitted through the egg cytoplasm that are responsible for several reproductive disorders in their insect hosts, such as cytoplasmic incompatibility (CI) in infected mosquitoes. Species in the Culex pipiens complex display an unusually high number of Wolbachia-induced crossing types, and based on present data, only the wPip strain is present.RESULTS: The sequencing of the wPip strain of Wolbachia revealed the presence of 60 ankyrin repeat domain (ANK) encoding genes and expression studies of these genes were carried out in adult mosquitoes. One of these ANK genes, pk2, is shown to be part of an operon of three prophage-associated genes with sex-specific expression, and is present in two identical copies in the genome. Another homolog of pk2 is also present that is differentially expressed in different Cx. pipiens group strains. A further two ANK genes showed sex-specific regulation in wPip-infected Cx. pipiens group adults.CONCLUSION: The high number, variability and differential expression of ANK genes in wPip suggest an important role in Wolbachia biology, and the gene family provides both markers and promising candidates for the study of reproductive manipulation.
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