A turtle beetle in the tropical rainforest of Panama has a pupa covered with a white, waxy substance. Scientists once believed that the substance was the secretion of tortoise beetles. Until 2020, scientists from a German Institute of Biology discovered that this white substance would actually grow in a petri dish. That means it’s not a discharge. So what exactly is it?
research doesn’t stop
Through scanning electron microscope observation and gene sequencing, scientists discovered that this layer of white substance was actually a plant pathogen – Fusarium oxysporum (a white filamentous fungus). Although the fungus can be detected throughout the tortoise beetle’s life cycle, the scientists focused on the pupation stage of the tortoise beetle because, shortly after the onset of pupation, they observed Fusarium oxysporum increased by 1000 times! Why does Fusarium oxysporum grow wildly at this time?
Turtle beetles don’t produce protective cocoons, so scientists speculate that Fusarium oxysporum may help protect the beetle from predators such as ants during the pupation stage. In order to confirm this conjecture, scientists conducted experiments in the tropical rainforest of Panama-they removed the fungi from the surface of 49 tortoise beetle pupae, and then placed them in sealed cages (to prevent predators from entering) and open cages. In addition, 49 tortoise beetle pupae that had not been cleared of the fungus were also “lived” in these two cages. As a result, the tortoise beetles in the sealed cages all survived, but the ones in the open cages were not so lucky: those that “associated with the fungus” performed well, and their survival rate during the four-day test period The survival rate of the tortoise beetle without fungus was only 43%. It can be seen that although the small Fusarium oxysporum looks inconspicuous, it does protect the turtle beetle.
Filamentous Fusarium oxysporum under the microscope
After confirming the above conjecture, scientists have a new question: Fusarium oxysporum itself is pathogenic to plants, so after establishing a symbiotic relationship with the tortoise beetle, does its pathogenicity still exist? If you’re not sure, experiment and see. So they exposed sweet potato plants, the natural host of the tortoise beetle, to Fusarium oxysporum, and found that the fungus consistently induced wilt in the plants for three weeks. They then placed ten fungus-free sweet potato plants with two tortoise beetles that had just “broke out” from their pupal shells, and the tortoise beetles spread the plant pathogen to about 80 percent of the plants within four weeks. Sweet potato plant leaves. It seems that Fusarium oxysporum did take the “free ride” of the tortoise beetle and successfully reached their common host plant. What surprised scientists, though, was that the genome of Fusarium oxysporum was much smaller than that of other Fusarium species. Other strains have a large number of genes that help them adopt different “strategies” to “colonize” different host plants. Scientists speculate that it may be because Fusarium oxysporum can easily “migrate” from one plant to another with the help of tortoise beetles, so it does not need these auxiliary genes, and its genome is so small.
Certain insects develop protective cocoons. Earth cocoon of the great horned beetle
White and golden cocoons
Sweet potato plants suffering from wilt disease
After two years of research, in August 2022, scientists finally identified the complex symbiosis between Fusarium oxysporum and the turtle beetle — they form a fungal symbiont that is beneficial to one host but harmful to the other . Fusarium oxysporum forms a protective layer around the pupae of the tortoise beetle that somehow wards off natural enemies of the tortoise beetle, thereby preventing predation of the larvae at the pupation stage; When they emerge, they spread Fusarium oxysporum into their common host plants, expanding their parasitic range. That is, the humble Fusarium oxysporum plays a dual role as a “guardian” of the turtle beetle on the one hand and a destroyer of the host plant on the other.
It’s inconceivable that the tortoise beetle could spread a pathogen that kills the plants it lives on, but scientists think the beetle may prefer plants infested with Fusarium oxysporum because the wilt disease weakens the plant’s ability to thrive. Defenses of herbivores. What’s more, with the “guardian” of Fusarium oxysporum, the pupation stage tortoise beetle can “stable life” on the host plant. Even if the host plant withers, the tortoise beetle can easily transfer to a new plant. on the plant.
Fusarium oxysporum forms a protective layer around the pupa of the turtle beetle
However, as to how Fusarium oxysporum protects the turtle beetle from predators, scientists have not yet given an answer. But by studying the genome of Fusarium oxysporum, they discovered genes that produce metabolites with insecticidal properties. Next, scientists need to further explore the specific defense mechanism of Fusarium oxysporum and whether this fungal symbiont affects other members of the ecological community.