Who Needs a Cocoon When You Have a Protective Fungal Coat

Instead of forming a cocoon like a moth, or a chrysalis like a butterfly, a tortoise leaf beetle uses a fungus to protect it during metamorphosis, and in return, propagates the fungal pathogen on host plants

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Is nature always cruel and unforgiving? Is it always every individual for themself? Not quite. Although there is a high amount of competition in nature, there are also relationships that form between species (called symbiosis) that can be beneficial to both species involved. There are classic examples of symbiosis, like the anemone and clownfish, but new ones are still being discovered and really excite the science community. Recently published in the journal Current Biology, there was a new discovery of a symbiotic relationship with unique characteristics between a beetle and a fungus that is beneficial to both organisms.

When both species in a symbiotic relationship benefit, it is referred to as mutualistic [compared to commensalism, where one organism benefits and the other is unaffected – or parasitism, where one organism benefits while the other is harmed]. In the insect world, it is not uncommon for mutualistic relationships to occur between an insect and microorganisms. Many microbes can serve as defensive strategies for the insect, such as protection from pathogens and parasites, helping to boost the host immune system, or by producing chemicals that add to the defensive biochemistry of the host. These microorganisms can oftentimes be more heavily associated with the insect during specific stages of their life cycle.

Most insects have distinct stages in their life cycle as they undergo metamorphosis, which is a drastic transformation in morphology when insects transition from an immature form to an adult form. These stages can consist of the egg stage leading to a juvenile stage (e.g. caterpillar) leading to a pupal stage (e.g. cocoon) leading to an adult stage (e.g. moth). For beetles (and other insects), there have been many findings of symbiont-mediated protection for the egg and juvenile stage of their life cycle, however, protection during the pupal stage has remained mostly unfound. Now, in this recent study, scientists discovered this type of example for Chelymorpha alternanas pupae (a species of tortoise leaf beetle) and a fungus.

This finding started with a straightforward observation. Scientists noticed that C. alternanas is unusually covered in a white, fuzzy substance during its pupal stage, which stands out from its similar life cycle and morphology compared to other beetles. In previous reports, scientists referred to the substance as a waxy secretion, yet upon closer evaluation, researchers were thinking it was actually fungal growth.

So, to identify if the white substance was a fungus, the scientists first explored the composition of microorganisms associated with the beetle’s pupal stage by sequencing the DNA of its microbiome. This method identified previously known bacterial symbionts and a large amount of the soil-dwelling fungus called Fusarium oxysporum. Finding this fungal species by DNA lined up with the fact that F. oxysporum can consistently be isolated from the white, fuzzy substance on the pupae. The scientists went on to show that F. oxysporum was associated with the beetle in all stages of its life cycle but was 1000 times more abundant ~1-2 days after pupation started and remained stable for the remainder of metamorphosis.

This rapid growth and persistence of the fungus during the pupal stage was suggestive of a pupal-specific role for F. oxysporum. The scientists hypothesized that the role may be to protect the pupae from predators and parasites like ants and parasitoid wasps. This idea partly came from the fact that other types of tortoise leaf beetles and insects have defense mechanisms for their pupal stage that C. alternanas lack (such as fecal shields or the evolution of maternal care).

To test this idea, the scientists generated symbiont-free beetles by treating the pupated insects with a fungicidal solution that suppressed fungal growth and did not adversely affect the development of the beetle. The scientists then did field experiments in the beetle’s natural environment (Gamboa, Republic of Panama) with both fungicidal treated and untreated beetles. The beetles were placed in cages that were either exposed or sealed from the environment, creating 4 experimental groups. The scientists then compared the survival rates between experimental groups throughout 4 days and found the beetles that were both treated and exposed showed much weaker survivorship than the other 3 groups – highlighting the protective role of F. oxysporum for the beetle host. So, it turns out the fungus is the beetle’s bodyguard.

But does the fungus get anything out of this arrangement? It does. Some strains of F. oxysporum are pathogenic (or infection causing) to plants. After the scientists sequenced the genome of this particular strain and compared it to other known genomes, they found it is most closely related to pathogenic strains that cause wilt disease in cabbage, broccoli, radish, and cotton. Because some fungi infect plants after being transferred by an insect vector, the scientist guessed this may be occurring in this relationship as well. The scientists tested if the beetle could propagate the fungus on sweet potato plants (a type of plant in which the fungus thrives) and found that exposure of previously uninfected potato plants to post-pupal stage beetles indeed leads to infection. This means the fungus watches the beetle’s back while the beetle is vulnerable, and in return, the beetle will act like a taxi service and drop the fungus off in a place where it can thrive.

Interestingly, the scientists were able to see the dual life of the fungus as a symbiont and pathogen written in its genes. They sequenced the fungus’s genome and found many plant-pathogen related genes and found genes that produce compounds that may act like insecticides to defend the beetle. Finding these potential insecticide producing genes creates the need for future work to explore the structure and function of these molecules as they could be potentially made into insecticides for agricultural use. Additionally, if they are found to act as insecticides, then the scientists from the study say they will aim to understand how the beetle is resisting the chemical, while other insects cannot. Perhaps this can teach us about biological mechanisms related to pesticide resistance, which is of concern in agriculture.

Overall, the study identified a new symbiotic relationship with unique characteristics between a beetle and a fungus. It is truly fascinating how different insects have such diverse ways of protecting themselves during the vulnerable pupal stage. In the past, the word metamorphosis would usually trigger thoughts of a moth emerging from a cocoon or a butterfly emerging from a chrysalis. Now, however, metamorphosis will also trigger the thought of a beetle putting on a fuzzy-fungal coat to stay safe during its transformation.