Jumping genes underlie symbiont-mediated defense
Beetle eggs lying on Brazilian soil and a bright green sea-squirt in the middle of the South Pacific Ocean: as far as they might seem, both are protected by very similar bioactive substances thanks to jumping genes in their bacterial partners.
You can find the original paper published in Nature Communications here: go.nature.com/2Ky8Yns
Animals as different as ants, sponges or birds have taught us that symbiotic microbes can be great defenders against natural enemies. Indeed, by producing bioactive compounds microbial associates can make a life or death difference for their hosts.
After almost a century since the last publication on the symbionts of Lagria beetles, in 2017 we published a paper in Nature Communications describing the defensive role of these bacterial partners. We found that Burkholderia gladioli symbionts protect the eggs of invasive Lagria villosa against pathogenic fungi. Also, we noticed that there were multiple closely related B. gladioli strains co-existing in these beetles. This simultaneous presence of diverse symbiont strains in a single host is probably common among symbiotic systems, yet we know little about how this persists over time and whether it is of any advantage for the host, so it was something we certainly wanted to follow up on.
We were able to get one of the bacterial symbiont strains of L. villosa (strain A) to grow in pure culture, which came handy to start investigating the chemistry of this defense. A couple of novel compounds were elucidated and two other known substances with antimicrobial activity were found to be produced by this strain. This was a quite exciting first finding, and we thought it would be worthwhile to focus our attention on the association and the chemical mediators of defense under natural conditions. Judging from current literature on defensive symbioses, we seem to know relatively little about how they work in the field, and whether the microbial isolates and the compounds they produce in laboratory conditions are indeed of ecological relevance. In the Lagria-Burkholderia symbiosis, we were especially interested in the diversity of strains and their role in the soil environment, which brings forward a more complex scenario than what we had dealt with under tightly controlled conditions.
Focusing on the naturally dominant symbiont strain of L. villosa (strain B) came with a challenge, as this one is so far reluctant to grow on plate. Kirstin Scherlach and Christian Hertweck, the natural product chemists that have been on board with the project, had tracked a potentially interesting bioactive compound on the eggs from field-collected mothers. However, without chances to cultivate the main candidate strain, our best shot would be to put together a rather outrageous number of these eggs to figure out what compound it actually was. 28,000 eggs did the job, and the chemists ended up with a structure that seemed surprisingly familiar. Lagriamide, named after the beetles, looked very much like bistramides, a group of cytotoxic compounds found before in a species of tunicate – an immobile marine invertebrate – from the South Pacific Ocean. The cyanobacterial symbionts of this tunicate species presumably produce bistramides, although to date there is no direct evidence confirming that they actually do, and the genes associated to the biosynthesis of the compounds are also unknown.
Doing genomics on unculturable bacteria is certainly possible with current sequencing technologies, but it would not be a smooth road. A few months before, Jason Kwan from the University of Wisconsin had visited our lab in Mainz and I briefly talked with him about our work on Lagria and their Burkholderia symbionts. Knowing about his research on marine natural products from symbionts, I thought the bistramide-lagriamide puzzle would seem interesting to him. Indeed, we teamed up with Jason and a PhD student from his lab, Ian Miller. Using metagenomics and single-cell genomics, they were able to identify a gene cluster that fit pretty well to the predicted biosynthesis of lagriamide and fell within the genome of the strain B. What was especially exciting was that this gene cluster is located in a genomic island, meaning that they are “jumping genes” which were acquired from another organism. This kind of gene shuffling, called horizontal gene transfer, is known as an important factor often influencing the genome structure, metabolic abilities or even virulence of bacteria. However, this is one of the few described cases in which it explains the defensive potential of a symbiont.
Keeping the natural conditions under perspective, we also tested whether the drastic fungal inhibition we had seen before in single eggs on filter paper was still there when we put the egg clusters in soil from soybean plantations, where we usually collect the beetles. We confirmed that the symbionts protect the eggs and that lagriamide is involved, but we also saw that the presence of this compound is likely not the only means of protection. Other strains or compounds might contribute, which makes sense for a broad-spectrum defense and is in line with our previous findings.
We’ve learned that horizontal gene transfer can potentiate innovation in the context of defensive symbiosis, and it is possible that co-existing symbiont strains also have an important impact on effective and versatile protection. Lagria and their symbionts seem to still hold exciting answers -and questions- on defensive symbiosis, giving us a tour deep into the ocean and back onto a sunny green field.