Silly microbiologist, genomes are for mutualists

New paper in PLoS Genetics of possible interest: PLoS Genetics: Population Genomics of the Facultatively Mutualistic Bacteria Sinorhizobium meliloti and S. medica.

The abstract does an OK job with the technical details:

Abstract:
The symbiosis between rhizobial bacteria and legume plants has served as a model for investigating the genetics of nitrogen fixation and the evolution of facultative mutualism. We used deep sequence coverage (>100×) to characterize genomic diversity at the nucleotide level among 12 Sinorhizobium medicae and 32 S. meliloti strains. Although these species are closely related and share host plants, based on the ratio of shared polymorphisms to fixed differences we found that horizontal gene transfer (HGT) between these species was confined almost exclusively to plasmid genes. Three multi-genic regions that show the strongest evidence of HGT harbor genes directly involved in establishing or maintaining the mutualism with host plants. In both species, nucleotide diversity is 1.5–2.5 times greater on the plasmids than chromosomes. Interestingly, nucleotide diversity in S. meliloti but not S. medicae is highly structured along the chromosome – with mean diversity (θπ) on one half of the chromosome five times greater than mean diversity on the other half. Based on the ratio of plasmid to chromosome diversity, this appears to be due to severely reduced diversity on the chromosome half with less diversity, which is consistent with extensive hitchhiking along with a selective sweep. Frequency-spectrum based tests identified 82 genes with a signature of adaptive evolution in one species or another but none of the genes were identified in both species. Based upon available functional information, several genes identified as targets of selection are likely to alter the symbiosis with the host plant, making them attractive targets for further functional characterization.

I think the author summary is a bit more, well, friendly:

Facultative mutualisms are relationships between two species that can live independently, but derive benefits when living together with their mutualistic partners. The facultative mutualism between rhizobial bacteria and legume plants contributes approximately half of all biologically fixed nitrogen, an essential plant nutrient, and is an important source of nitrogen to both natural and agricultural ecosystems. We resequenced the genomes of 44 strains of two closely related species of the genus Sinorhizobium that form facultative mutualisms with the model legme Medicago truncatula. These data provide one of the most complete examinations of genomic diversity segregating within microbial species that are not causative agents of human illness. Our analyses reveal that horizontal gene transfer, a common source of new genes in microbial species, disproportionately affects genes with direct roles in the rhizobia-plant symbiosis. Analyses of nucleotide diversity segregating within each species suggests that strong selection, along with genetic hitchhiking has sharply reduced diversity along an entire chromosome half in S. meliloti. Despite the two species' ecological similarity, we did not find evidence for selection acting on the same genetic targets. In addition to providing insight into the evolutionary history of rhizobial, this study shows the feasibility and potential power of applying population genomic analyses to microbial species.

I have highlighted the section dissing pathogen studies ...

As with every good paper, it starts with a tree

Selfish DNA, symbionts and parasites - some quick links

I was at a committee meeting yesterday for a great PhD student here at UC Davis, Michael Hornsby and the topic of selfish DNA came up.  After his meeting we sat down and looked for some new papers and review papers on the topic.  I just thought it might be of value to share some of these here:

• The Myth … of the Myth of Junk DNA (blog post by John Farrell)
• A formal theory of the selfish gene
• Capturing the superorganism: a formal theory of group adaptation.
• Bacterial toxin-antitoxin systems: more than selfish entities?
• Genomic islands: tools of bacterial horizontal gene transfer and evolution.
• A Brief History of the Status of Transposable Elements: From Junk DNA to Major Players in Evolution
• What traits are carried on mobile genetic elements, and why?

We also discussed briefly the evolution of mutualists and parasites and here are a few papers that came up:

• Genomics and evolution of heritable bacterial symbionts.
• From parasite to mutualist: rapid evolution of Wolbachia in natural populations of Drosophila.

If anyone knows of any other good recent papers or blog posts about selfish DNA or mutualists vs. parasites please post them here.  Thanks

Mutualisms Rule - So Says Olivia Judson at the Wild Side

Nice blog today on mutualisms by Olivia Judson who writes the Wild Side blog/column for the New York Times (I seem to be writing a lot about writers for the NY Times these days ... not sure what is going on with that). She even features one of my favorite organisms in the blog:

The clam Calyptogena magnifica, which lives on deep-sea vents, depends on a bacterium to supply it with nutrients; the bacterium is transmitted through the clam’s eggs

Last year we published a paper on the complete genome sequence of this symbiont (which I wrote about here when I was clearly in a whiny kind of mood). And Judson picks up on a part of the story on the clam that is rarely discussed - the symbionts are transmitted vertically from parent to offspring. Vertical transmission seems to be linked to multiple properties of the symbionts (see my discussion of this regarding the glassy winged sharpshooter symbionts here).

Judson's post is really worth checking out for the symbiosis fans out there. She does a good job of highlighting diversity and evolution of mutualisms in a relatively short post.


See my video of a dissection of a baby Calyptogena: