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Aphids acquired symbiotic genes via lateral gene transfer.

Nikoh N, Nakabachi A - BMC Biol. (2009)

Bottom Line: Sequence similarity searches demonstrated that these fully sequenced transcripts are significantly similar to the bacterial genes ldcA (product, LD-carboxypeptidase) and rlpA (product, rare lipoprotein A), respectively.Real-time quantitative RT-PCR demonstrated that ldcA and rlpA are expressed 11.6 and 154-fold higher in the bacteriocyte than in the whole body, respectively.LdcA is an enzyme required for recycling murein (peptidoglycan), which is a component of the bacterial cell wall.

View Article: PubMed Central - HTML - PubMed

Affiliation: Division of Natural Sciences, The Open University of Japan, Chiba, Japan. nikoh@u-air.ac.jp

ABSTRACT

Background: Aphids possess bacteriocytes, which are cells specifically differentiated to harbour the obligate mutualist Buchnera aphidicola (gamma-Proteobacteria). Buchnera has lost many of the genes that appear to be essential for bacterial life. From the bacteriocyte of the pea aphid Acyrthosiphon pisum, we previously identified two clusters of expressed sequence tags that display similarity only to bacterial genes. Southern blot analysis demonstrated that they are encoded in the aphid genome. In this study, in order to assess the possibility of lateral gene transfer, we determined the full-length sequences of these transcripts, and performed detailed structural and phylogenetic analyses. We further examined their expression levels in the bacteriocyte using real-time quantitative RT-PCR.

Results: Sequence similarity searches demonstrated that these fully sequenced transcripts are significantly similar to the bacterial genes ldcA (product, LD-carboxypeptidase) and rlpA (product, rare lipoprotein A), respectively. Buchnera lacks these genes, whereas many other bacteria, including Escherichia coli, a close relative of Buchnera, possess both ldcA and rlpA. Molecular phylogenetic analysis clearly demonstrated that the aphid ldcA was derived from a rickettsial bacterium closely related to the extant Wolbachia spp. (alpha-Proteobacteria, Rickettsiales), which are intracellular symbionts of various lineages of arthropods. The evolutionary origin of rlpA was not fully resolved, but it was clearly demonstrated that its double-psi beta-barrel domain is of bacterial origin. Real-time quantitative RT-PCR demonstrated that ldcA and rlpA are expressed 11.6 and 154-fold higher in the bacteriocyte than in the whole body, respectively. LdcA is an enzyme required for recycling murein (peptidoglycan), which is a component of the bacterial cell wall. As Buchnera possesses a cell wall composed of murein but lacks ldcA, a high level of expression of the aphid ldcA in the bacteriocyte may be essential to maintain Buchnera. Although the function of RlpA is not well known, conspicuous up-regulation of the aphid rlpA in the bacteriocyte implies that this gene is also essential for Buchnera.

Conclusion: In this study, we obtained several lines of evidence indicating that aphids acquired genes from bacteria via lateral gene transfer and that these genes are used to maintain the obligately mutualistic bacterium, Buchnera.

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Phylogenetic position of aphid RlpA. A total of 76 aligned amino acid sites were subjected to the analysis. A neighbour-joining tree is shown, while the ML tree and BP tree exhibited substantially the same topologies. On each node, bootstrap support values over 50% are shown (NJ above, ML below). Thickened nodes indicate the Bayesian posterior probabilities are > 0.95. Taxonomic positions (eubacterial taxonomy unless otherwise stated) are shown in brackets. α, β, γ, and ε indicate proteobacterial classes.
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Figure 4: Phylogenetic position of aphid RlpA. A total of 76 aligned amino acid sites were subjected to the analysis. A neighbour-joining tree is shown, while the ML tree and BP tree exhibited substantially the same topologies. On each node, bootstrap support values over 50% are shown (NJ above, ML below). Thickened nodes indicate the Bayesian posterior probabilities are > 0.95. Taxonomic positions (eubacterial taxonomy unless otherwise stated) are shown in brackets. α, β, γ, and ε indicate proteobacterial classes.

Mentions: The amino acid sequences of putative RlpAs of A. pisum, A. gossypii, and T. citricida were subjected to molecular phylogenetic analysis with RlpAs of various bacterial lineages (Figure 4). The highly conserved DPBB domains were aligned and used for this analysis. The phylogenetic positions of aphid RlpAs were not clearly resolved with a high level of statistical support. However, to date, no rlpA genes have been observed in any eukaryotes, except aphids. Moreover, the phylogenetic tree showed that the aphid rlpAs are monophyletic and that the phylogenetic relationships were congruent with the species tree of aphids [45]. This suggests that the common ancestor of these three aphid species acquired the rlpA gene from a bacterium via LGT. The relatively low resolution of the phylogenetic positions of the aphid rlpA may be partly due to the high evolutionary rate of the aphid lineages (see below).


Aphids acquired symbiotic genes via lateral gene transfer.

Nikoh N, Nakabachi A - BMC Biol. (2009)

Phylogenetic position of aphid RlpA. A total of 76 aligned amino acid sites were subjected to the analysis. A neighbour-joining tree is shown, while the ML tree and BP tree exhibited substantially the same topologies. On each node, bootstrap support values over 50% are shown (NJ above, ML below). Thickened nodes indicate the Bayesian posterior probabilities are > 0.95. Taxonomic positions (eubacterial taxonomy unless otherwise stated) are shown in brackets. α, β, γ, and ε indicate proteobacterial classes.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2662799&req=5

Figure 4: Phylogenetic position of aphid RlpA. A total of 76 aligned amino acid sites were subjected to the analysis. A neighbour-joining tree is shown, while the ML tree and BP tree exhibited substantially the same topologies. On each node, bootstrap support values over 50% are shown (NJ above, ML below). Thickened nodes indicate the Bayesian posterior probabilities are > 0.95. Taxonomic positions (eubacterial taxonomy unless otherwise stated) are shown in brackets. α, β, γ, and ε indicate proteobacterial classes.
Mentions: The amino acid sequences of putative RlpAs of A. pisum, A. gossypii, and T. citricida were subjected to molecular phylogenetic analysis with RlpAs of various bacterial lineages (Figure 4). The highly conserved DPBB domains were aligned and used for this analysis. The phylogenetic positions of aphid RlpAs were not clearly resolved with a high level of statistical support. However, to date, no rlpA genes have been observed in any eukaryotes, except aphids. Moreover, the phylogenetic tree showed that the aphid rlpAs are monophyletic and that the phylogenetic relationships were congruent with the species tree of aphids [45]. This suggests that the common ancestor of these three aphid species acquired the rlpA gene from a bacterium via LGT. The relatively low resolution of the phylogenetic positions of the aphid rlpA may be partly due to the high evolutionary rate of the aphid lineages (see below).

Bottom Line: Sequence similarity searches demonstrated that these fully sequenced transcripts are significantly similar to the bacterial genes ldcA (product, LD-carboxypeptidase) and rlpA (product, rare lipoprotein A), respectively.Real-time quantitative RT-PCR demonstrated that ldcA and rlpA are expressed 11.6 and 154-fold higher in the bacteriocyte than in the whole body, respectively.LdcA is an enzyme required for recycling murein (peptidoglycan), which is a component of the bacterial cell wall.

View Article: PubMed Central - HTML - PubMed

Affiliation: Division of Natural Sciences, The Open University of Japan, Chiba, Japan. nikoh@u-air.ac.jp

ABSTRACT

Background: Aphids possess bacteriocytes, which are cells specifically differentiated to harbour the obligate mutualist Buchnera aphidicola (gamma-Proteobacteria). Buchnera has lost many of the genes that appear to be essential for bacterial life. From the bacteriocyte of the pea aphid Acyrthosiphon pisum, we previously identified two clusters of expressed sequence tags that display similarity only to bacterial genes. Southern blot analysis demonstrated that they are encoded in the aphid genome. In this study, in order to assess the possibility of lateral gene transfer, we determined the full-length sequences of these transcripts, and performed detailed structural and phylogenetic analyses. We further examined their expression levels in the bacteriocyte using real-time quantitative RT-PCR.

Results: Sequence similarity searches demonstrated that these fully sequenced transcripts are significantly similar to the bacterial genes ldcA (product, LD-carboxypeptidase) and rlpA (product, rare lipoprotein A), respectively. Buchnera lacks these genes, whereas many other bacteria, including Escherichia coli, a close relative of Buchnera, possess both ldcA and rlpA. Molecular phylogenetic analysis clearly demonstrated that the aphid ldcA was derived from a rickettsial bacterium closely related to the extant Wolbachia spp. (alpha-Proteobacteria, Rickettsiales), which are intracellular symbionts of various lineages of arthropods. The evolutionary origin of rlpA was not fully resolved, but it was clearly demonstrated that its double-psi beta-barrel domain is of bacterial origin. Real-time quantitative RT-PCR demonstrated that ldcA and rlpA are expressed 11.6 and 154-fold higher in the bacteriocyte than in the whole body, respectively. LdcA is an enzyme required for recycling murein (peptidoglycan), which is a component of the bacterial cell wall. As Buchnera possesses a cell wall composed of murein but lacks ldcA, a high level of expression of the aphid ldcA in the bacteriocyte may be essential to maintain Buchnera. Although the function of RlpA is not well known, conspicuous up-regulation of the aphid rlpA in the bacteriocyte implies that this gene is also essential for Buchnera.

Conclusion: In this study, we obtained several lines of evidence indicating that aphids acquired genes from bacteria via lateral gene transfer and that these genes are used to maintain the obligately mutualistic bacterium, Buchnera.

Show MeSH
Related in: MedlinePlus