Tortoise Beetles Form Symbiotic Alliances With Fungus

By D.C. Demetre •  Updated: 08/22/22 •  3 min read

The tortoise or leaf beetle Chelymorpha alternans has a unique partnership with fungus, a new study from German and Panamanian scientists reveals. The microbe furnishes a layer around the beetle’s pupae, protecting it during metamorphosis.

A pupa is an insect’s life stage representing the transformation between immature and mature stages. The four stages are egg, larva, pupa, and imago.

Since pupae cannot move, they are primarily defenceless. Pupae either hide in a cocoon, camouflage themselves in the environment, or go underground for protection.

Entomologists knew that some insects depend on microbial protection while immobile in developmental stages, such as eggs. But the role of microbes in ensuring defence during an insect’s metamorphosis remained unclear.

Leading A Double Life

leaf beetle

A leaf beetle. Credit: Max Planck Institute for Biology Tübingen

Biologists from the Max Planck Institute for Biology in Tübingen, the University of Tübingen, and the Smithsonian Tropical Research Institute in Panama, found a symbiosis between the sac fungi Fusarium oxysporum and Chelymorpha alternans. The fungus protects the pupae of the leaf beetle from predators.

The cost of this transaction for the beetles? As adults, they spread the fungus to its host plants, contributing to its transmission.

“The fungus retained a metabolic profile that reflects its dual lifestyle. Our findings show a mutualism ensuring pupal protection for an herbivorous beetle on the one hand, in exchange for symbiont dissemination and propagation on the other hand,”

said senior author Hassan Salem, Research Group Leader at the Max Planck Institute for Biology.

The Microbe Metamorphosis Defence

Birds and rodents are threats to pupae, but it is the smaller predators and parasites like ground beetles, ants and wasps that stalk them in the wild.

“Structural and chemical adaptations are known to protect pupae against predators and other threats. But microbes appear to also play an important role when we consider how a beetle defends itself during metamorphosis,”

said first author Aileen Berasategui, University of Tübingen.

Widespread Plant Infection

A dense microbial growth seems to form at the beginning of pupation, which intrigued the research team. Genetic sequencing, as well as culture-based analysis, revealed the growth to be the fungi Fusarium oxysporum.

Field studies in Panama were then conducted to understand and confirm their hypothesis of a mutual partnership. To compare the two, they recorded survival rates of pupae with and without the protective fungus.

After a follow-up investigation using sweet potato plants, the researchers concluded that the leaf beetle carries and distributes the fungus to uninfected plants. The beetles have the fungus on their legs during the adult stage, resulting in widespread infection of the plants.

Further Questions

Chelymorpha alternans belongs to the Cassidinae subfamily of the Chrysomelidae leaf beetle family. Various members of this group seem to carry the microbial coat that covers pupae which is the tell-tale feature of the symbiosis with Fusarium oxysporum.

A 2018 study, also done in Panama, found that only around 43% of the larvae survive. The pupae were observed to take around 5 days to reach the adult stage, and the average survival rate of pupae was just over 83%.

It is still unknown when the symbiosis evolved and how it is maintained, but these are questions that members of the research team are eager to answer.

 

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