By Jiri Hulcr
Who was the first farmer on Earth, over 60 million years ago? Which insect group includes the most invasive and deadly vectors of tree diseases? And which fungi have foregone sex and independence only to become a garden crop that smells like ripe fruit? Welcome to the diverse and bizarre world of the ambrosia symbiosis, a symbiosis between wood-boring beetles and nutritious fungal crops.
Research on the ambrosia symbiosis has recently seen a major increase because of the destructive species that keep invading non-native habitats (such as Asian species arriving in North America, and vice versa). Only five percent of the ~3,500 beetle species have been studied for their symbionts, so there is still a long way to go to understand the whole system. However, many of the new studies have already yielded significant discoveries. We now know that there are beetles that farm only a single fungus, while other beetles have entire gardens of various fungi. Several beetle species carry fungi that act as pathogens and kill the host trees. There are also beetles that steal fungi from others by tunneling along the first beetle’s gardens, and there are beetles that attack and devour other beetles in order to take over their gardens. And then there are thousands of beetle species of which we still know nearly nothing.
Even less is known about the fungal side of the symbiosis. It seems that many different fungi from different origins evolved the same strategy — to provide food for their beetle vectors, and in exchange get transferred into the next tree by the new beetle generation. The one thing that all of these nutritional symbionts seems to have in common is that they are NOT very good wood degraders. All known ambrosia fungi depend on recently dead trees that still have lots of nutrients in them. These fungi colonize the dead tree tissues when they are still juicy and nutritious, before they get colonized by real wood degraders.
However, in a paper in PLoS ONE, my co-authors and I report the discovery of a fungal clade that is different. The fungal symbiont of beetles in the genus Ambrosiodmus does not just extract easily available nutrients — it is actually capable of true wood decomposition. This fungus is much more metabolically competent than any other known ambrosia fungus. Instead of snatching easily available nutrients, it actually uses decayed wood, and turns it into nutritious food for its animal symbiont. Such fungi are prime candidates for further studies of their enzymatic apparatus. We may discover interesting chemical processes that can help us use wood for food!
Interestingly, this fungus is also a new species that was previously unknown to science. Its only known relative is a “bracket fungus” of the genus Flavodon — definitely not an ambrosia fungus. Something dramatic must have happened when an ancient Ambrosiodmus beetle and the original Flavodon met, evolutionarily speaking, that turned a free-living bracket fungus into a crop of a beetle.
This paper also serves as a model for efficient and methodologically modern characterization of wood borer-fungus symbioses. Many ambrosia beetle species and their fungi are becoming global threats to forest health, so we need to have a reliable way of assessing their fungal symbionts. In this work, we combined several complementary methods to look at this symbiosis to assure that our analysis is quick as well as correct. We used standard culturing but with a quantitative approach that used a dilution series to grow the main symbiont rather than contaminants. We employed whole-community DNA amplification and high-throughput sequencing for the detection of any potentially unculturable symbionts (our ~50,000 sequences per beetle individual were all nearly identical, suggesting a single species and a few genotypes of the symbiont). Finally, our colleagues Matthew Kasson and Dylan Short at West Virginia University dissected the 0.1 mm beetle heads and sliced them into five micrometer slices to get an accurate view of the actual fungi inside the “fungus pockets” in which the fungus inoculum is transported. The culturing, sequencing, and microscopic observations all confirmed that the wood-decaying fungus Flavodon is the main symbiont of the Ambrosiodmus ambrosia beetles.
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Jiri Hulcr is an assistant professor of forest entomology at the University of Florida’s School of Forest Resources and Conservation. His team studies wood-boring beetles, tree pathogens, insect symbionts, and their impact on forests and people. Jiri is also increasingly involved in science-policy dialogue and in extension and outreach for the benefit of our forests.