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Beetles use special pockets to store bacteria during metamorphosis

Researchers found that the beetles use these back pockets to store and then move the symbiotic bacteria throughout their life cycles.

(CN) — A type of female beetle uses an ingenious method of storing and then moving important bacteria to keep it safe during metamorphosis.

Scientists detail these new findings in the journal Frontiers in Physiology and explain how this type of female beetle uses a special pocket on its back to store and move the bacteria.

The studied beetles come from the genus known as Lagria, which carry helpful bacteria in a pair of glands that are used to coat the outside of the surface of laid eggs. This is done because the bacteria produce antibiotics that protect the eggs, larvae and pupae of the beetles against fungi prior to their final adult life stage.

The studied beetles are known as holometabolous insects, and their metamorphosis process consists of four life stages. First there’s the egg stage, then larva, followed by pupa until the insect finally reaches adulthood.

However, the bacteria need to be kept safe during the stages of metamorphosis because of the beetle goes through an overall body reorganization during the pupae stage.

To keep the bacteria alive, it is stored in specialized pockets on the beetles’ back and then, as they emerge as adults, friction moves the bacteria onto their genitals.

“Here we show how an insect can maintain beneficial microbial partners despite the drastic rearrangements of body structures that occur during metamorphosis,” said corresponding author Laura Flórez, a researcher at the University of Copenhagen’s department of plant and environmental sciences. “By modifying unique ‘pockets’ on their backs, Lagria beetles manage to keep their protective symbionts and facilitate their relocation during pupation to newly developed adult organs.”

The researchers point out that these back pockets are rather unique, as they are not seen in the larvae and pupae of other insects. The pockets are present but not utilized on the body of male beetles of the same kind.

Once all the bacterial shuffling has concluded, the symbionts live inside special glands of the female beetle where it will be stored until used on a set of eggs, starting the cycle over.

This entire process was summarized by first author of the research Rebekka Janke, a doctoral student at the Johannes Gutenberg University of Mainz.

“The symbionts go from the highly exposed egg surface to colonize the pockets on the back of the larvae and pupae. Finally, they end up in specialized glands associated with the reproductive system of adult females,” said Janke.

Knowing that the adult female beetles held onto the bacteria throughout metamorphosis, the researchers needed to figure out how it survived such a dynamic process.

To figure this out, the researchers spread tiny fluorescent beads across beetles in the pupae stage, and observed that a large portion of the beads, like the bacteria, emerged on the tip of the abdomen of the beetle.

Researchers were able to determine that the bacteria were moved towards the genitalia of the beetle by friction during the emergence process where the insect reaches the adult stage of its life cycle.

“In the adult stage, the main purpose of the symbiotic organs seems to be to enable successful transmission onto the egg stage and to the next generation, and not for defense against microbes. Since only females lay eggs, male adults do not need to carry these potentially costly symbionts and are a dead-end for the bacteria,” said Flórez.

Last author Martin Kaltenpoth, a professor at the Max Planck Institute for Chemical Ecology in Jena, Germany, pointed forward to the further questions that need to be answered to better understand the beetles and their symbionts.

“To better understand how beneficial symbionts are transmitted and maintained within and across generations, we’ll need to identify which host and symbiont factors regulate symbiont establishment,” Kaltenpoth said. “For example, does the host select for specific symbionts? And through which mechanisms can immotile symbionts colonize the symbiotic organs?”

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