It Takes a Thief. And 50 Million Years of Practice

Roughly 50 million years before the first Neolithic human  grain in the ground on purpose, three insect groups—ants, beetles, and termites—evolved the ability to practice agriculture with fungi. When humans started planting nearly 12,000 years ago, it changed the trajectory of life on earth, and today our species dominates its environment with a visible sense of superiority. Insects have been at agriculture for tens of millions of years longer than we have, and we are just beginning to understand their tools and traditions. We don’t know have a sense of their own purpose or not, but we do know that their collective act has had consequences that are still playing out today. Humans may seem to rule the planet, but hidden from their eye most of the time there are insects that dominate the undergrowth.

Fungiculture originated in the beetle family tree at least seven independent times (by comparison, it only originated once each in ant and termite lineages), which Dr. Ulrich Mueller of the University of Texas at Austin says is “perhaps not surprising, given the sheer diversity of beetle species and given the importance of feeding specializations in beetle diversification.” [1]

About 40% of all insect species are in the family Coleoptera, more commonly known to us as beetles. Fungiculture is carried out by the 3,400 species of beetles known as ambrosia beetles, which line the wooden walls of their burrows, or galleries, with fungi that absorb nutrients from the wood. The various fungi strains have been called ambrosia since the late 19th Century.

The black stains that the fungal growths leave on wood are not at all reminiscent of the gods’ nectar in Greek mythology, but despite the unsightly blemishes the fungi causes in wood, some of the ambrosia fungi raised in petri dishes grow as beautiful, fluffy pink mold; this may be the reason for the name. But more likely, it seems, is the fact that the beetles delight in these fungi enough to build their kingdoms in, around, and with this fungus. It is their life source.

The 3,400 species of ambrosia beetles are not all directly related. They are classified based on their symbiotic dependence on fungi to survive in wood—all ambrosia beetle clades are weevils or bark beetles that have independently evolved fungiculture.

To do this, ambrosia beetles use mycangia, which are pouches specialized for carrying fungal spores to the burrow. Most species of ambrosia beetle have these mycangia in or around their mandibles, wing cases, or the middle of their thorax. “Only certain fungal species appear to survive in the mycangia, selectively eliminating unwanted fungi,” said Dr. Mueller.

Dr. Nicole Gerardo of Emory University has been Dr. Mueller’s colleague on several research expeditions. “The fungi serve as the beetle’s primary food source,” she explained, “and are essential for the completion of the beetle life cycle.”

Ambrosia beetles are in turn critical to the tropical forest life cycle because they are the principal degraders of wood. The beetles provide a great service to the development of trees and their surrounding sources of nutrients. Of course, not all species are happy about this: from a human perspective they can cause great damage to timber industries as invasive species.

Known to foresters as “pinworms,” ambrosia beetles rarely attack living trees. “The biggest commercial loss caused by ambrosia beetles is in the degradation of lumber due to the presence of the dark-stained pinholes,” said T.L. Shore, of the Canadian Forestry Service. “The loss is particularly significant because the beetles bore into the sapwood where the most valuable clear lumber is located.” [2]

In a way, this makes them a good research subject, because scientists and forestry professionals benefit from the knowledge gained. And with that knowledge, tweaked a bit into everyday language, the rest of us can enjoy the following show, its colorful cast of characters, and its intrigue:

The more interesting reason to research the ambrosia beetle was discovered in 2009 by entomologists Jiri Hulcr and Anthony Cognato, who until recently worked together at Michigan State University (Dr. Hulcr is now at North Carolina State University, another state where the forests provide both scientific and economic reasons to understand this phenomenon).

The two scientists and their team found several species of ambrosia beetle in Malaysia and Papua New Guinea that do not gather and transport ambrosia fungi the way other species do. [3]

Dr. Hulcr and Dr. Cognato showed that these beetles—at least 16 species of them—“seek established brood galleries of ‘host’ beetle species, and create their own galleries adjacent to the host tunnels.” The thieving beetles then divert the gardens of their “host” into their own tunnels so that they can take advantage of the fruiting structures that contain nutrients from the wood not otherwise available to the beetles.

Hulcr and Cognato describe the devious beetle’s behavior as mycocleptism—a combination of the Greek mýkēs for mushroom or fungus, and kleptēs for thief. Scientists now refer to the thieving species as mycocleptae; a fancy Greek name for the largest known burglary ring on the planet. But naming is not the same as understanding. One underlying question, depending on who is funding the research, is what all these petty heists aggregate to: scientists want to understand the various ecosystemic impacts whereas foresters want to know how to best arrest and prosecute.

Hulcr and Cognato’s research team established that practically all the mycocleptic beetles burrow within 1 centimeter of their host’s galleries. Roughly half of the mycocleptic beetles that created independent galleries (that is, further than a centimeter away) were found dead. Without hosts to steal from, the mycocleptae probably had insufficient food and starved. Or so the scientists seem to think. Forget what you know about forensic genius from television crime scene investigation shows—these scientists do some of the same kind of work, but on a much grander scale of detail, and using evidence trails that sometimes are millions of years old.

For an example of their technique, which grows out of needing solid explanation for cause and effect, consider the following: To make sure that the fungus strains found in the mycocleptic galleries were taken from host beetles’ burrows, Hulcr and Cognato gathered 29 galleries and performed DNA testing on the tissue samples. They first found 10 large fallen branches or dead trees that contained host beetles, and then each group of galleries “was excised from the tree and dissected by hand. Beetles from each gallery were stored in a separate vial and identified in the laboratory,” explained Dr. Cognato. For the fungal samples, they scraped a mixture of wood and fungi from the gallery walls and ground it in a chemical solution before running it through a DNA extractor.

According to their results, the fungi collected in the galleries of mycocleptae and their hosts were more similar than the fungi found in different galleries of members of the same host species. That is, if mycocleptic beetle X was living adjacent to it’s non-mycocleptic host species Y, the fungi in X and Y’s burrows would be more genetically similar than that of Y and another Y living a few centimeters down the decaying log.

Based on their research of the largest mycocleptic genus Diuncus in the ambrosia subtribe Xyleborini, Hulcr and Cognato found that members of Diuncus have none of the three types of mycangia, the pockets to transport spores. According to Dr. Hulcr, the fungal gardens of two separate mycocleptic beetles of the same species contain different  fungi strains if the two mycocleptae are parasitizing different host species. “Mycocleptae probably do not carry their own symbionts [fungi spores],” said Dr. Hulcr.

To be able to affirm this assertion for all mycocleptae, Hulcr and Cognato’s team will have to conduct further research on the anatomy of mycocleptic species outside of Diuncus, embedding dozens of beetle heads in paraffin to take cross-sections with a microtome, which is a much smaller and more precise version of the machines used to slice ham at a deli.

Already Hulcr and Cognato have determined through the ambrosia subtribe Xyleborini genealogic tree that many Xyleborini species, including the ancestors of Diuncus, have mycangia, so the absence of mycangia in Diuncus may be a result of mycocleptic behavior, or vice versa.

Since there are some mycocleptae in Xyleborini genera other than Diuncus that do have mycangia, it is worth more closely studying the evolutionary rise and fall of the two traits.

Thieves need victims. It is fairly unlikely that mycocleptic beetles—especially those without mycangia—could survive without host beetles. Dr. Hulcr describes the relationship the two types of beetle have as varying “from neutral to parasitic. Many mycocleptae inflict loss on their hosts, ranging from destroying the host’s gallery to decreasing the fungal matter available to the host’s larvae.” But in fact it is not that simple. This relationship has mutual benefits between the fungi and the mycocleptae: there are gains beyond the beetle world too.

In a healthy gallery of C. aeneipennis, larval chambers line both sides of the horizontal maternal tunnel. The bottom larval chambers of C. aeneipennis in this case have been destroyed and replaced by a gallery and brood of the mycocleptic species. Blackened portions of wood are covered in fungi. Photo by Sarah M. Smith, MSU.

“Ambrosia beetles are promising for the study of symbiotic evolution,” Dr. Cognato explained. “They are the only fungus-growing insects to have evolved the strategy multiple times, and therefore provide an unparalleled framework for comparative studies.”

A mycocleptic ambrosia beetle exhibits both symbiotic and parasitic behavior, with fungi and fellow ambrosia beetles, respectively. The mycocleptae and the organisms they depend on could help explain areas of co-evolution that have not yet been explored, opening the way for a better understanding of the evolutionary relationship between fungi, plants, and different insect species.

“Most mycocleptae display notable host specificity,” said Dr. Hulcr, “although quantitative data are not available.” If this were proven to be the case with more extensive research, we would then face the question of how the thieves locate their targets. “Discernment of a single gallery of an unrelated beetle by mycocleptae suggests unknown sensory mechanisms,” Dr. Hulcr mused.

In order to better understand the methods and advantages of the burglaring beetles, it is clear that we must undertake more extensive research. As Dr. Cognato later affirmed, “our data suggest that our contemporary understanding of the ambrosial community is incomplete.” Perhaps the exploration of ambrosial species in tropical areas other than Papua New Guinea and Malaysia, such as the rainforests of Latin America, will yield mycocleptae with different origins that we can compare with. 


Sources:

[1] Ulrich G. Mueller and Nicole Gerardo. “Fungus-farming insects: Multiple origins and diverse evolutionary histories.” PNAS November 2002. Vol. 99 No. 24 pages 15247-1524.

[2] T.L. Shore. “Insects and Diseases of Canada’s Forests.” Natural Resources Canada March 2011. Web. April 2011. Retrieved from http://imfc.cfl.scf.rncan.gc.ca/insecte-insect-eng.asp?geID=2839

[3] Jiri Hulcr and Anthony I. Cognato. “Repeated Evolution of Crop Theft in Fungus-Farming Ambrosia Beetles.Evolution November 2010. Vol. 64, Issue 11, pages 3205–3212.

7 thoughts on “It Takes a Thief. And 50 Million Years of Practice

  1. Pingback: Zombie Ants |

  2. Pingback: Xandari Insects | Raxa Collective

  3. Pingback: Kleptothermy | Raxa Collective

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