Abstract
BackgroundBlochmannia are obligately intracellular bacterial mutualists of ants of the tribe Camponotini. Blochmannia perform key nutritional functions for the host, including synthesis of several essential amino acids. We used Illumina technology to sequence the genome of Blochmannia associated with Camponotus vafer.ResultsAlthough Blochmannia vafer retains many nutritional functions, it is missing glutamine synthetase (glnA), a component of the nitrogen recycling pathway encoded by the previously sequenced B. floridanus and B. pennsylvanicus. With the exception of Ureaplasma, B. vafer is the only sequenced bacterium to date that encodes urease but lacks the ability to assimilate ammonia into glutamine or glutamate. Loss of glnA occurred in a deletion hotspot near the putative replication origin. Overall, compared to the likely gene set of their common ancestor, 31 genes are missing or eroded in B. vafer, compared to 28 in B. floridanus and four in B. pennsylvanicus. Three genes (queA, visC and yggS) show convergent loss or erosion, suggesting relaxed selection for their functions. Eight B. vafer genes contain frameshifts in homopolymeric tracts that may be corrected by transcriptional slippage. Two of these encode DNA replication proteins: dnaX, which we infer is also frameshifted in B. floridanus, and dnaG.ConclusionsComparing the B. vafer genome with B. pennsylvanicus and B. floridanus refines the core genes shared within the mutualist group, thereby clarifying functions required across ant host species. This third genome also allows us to track gene loss and erosion in a phylogenetic context to more fully understand processes of genome reduction.
Highlights
Blochmannia are obligately intracellular bacterial mutualists of ants of the tribe Camponotini
Independent frameshift events in ytfM are supported by the different location of the indel in B. pennsylvanicus, compared to B. floridanus and B. vafer (Figure 4). (iii) Convergent patterns of gene loss or erosion among Blochmannia In addition to examining the history of frameshifted genes, we identified several examples of convergent gene loss and erosion among the three Blochmannia genomes (Figure 5)
Comparisons of the B. vafer genome with the previously published B. floridanus and B. pennsylvanicus genomes clarified the core set of genes and functions shared by this mutualist group
Summary
Blochmannia are obligately intracellular bacterial mutualists of ants of the tribe Camponotini. Blochmannia perform key nutritional functions for the host, including synthesis of several essential amino acids. Candidatus Blochmannia species (hereafter, Blochmannia) are obligately intracellular, primary mutualists of ants belonging to the tribe Camponotini [1,2]. Blochmannia have been found in every species of Camponotus surveyed to date, as well as other genera within Camponotini, suggesting an ancient and stable association with the ant tribe [3]. Similar to other primary obligate mutualists of insects, Blochmannia genomes are extremely reduced in size (705-792 kb), show strong AT bias (27-29.5% GC content), and undergo accelerated rates of molecular evolution [6,7,10]. Independent loss of the same gene from multiple lineages would suggest relaxed selection on gene function, rather than stochastic effects of drift
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