These beetles place symbiotic bacteria in “back pockets” during metamorphosis

Zoom / Certain species of beetles have evolved unusual “back pockets” to safely house symbiotic bacteria during metamorphosis – the only known example of this among insects.

Certain species of beetles have evolved unusual “back pockets” to safely house symbiotic bacteria during metamorphosis – the only known example of this among insects. It’s part of a mutually beneficial arrangement, because bacteria protect vulnerable caterpillars and pupae from fungi. Scientists also determined that only adult females retain these symbiotic bacteria, driving the population out of those posterior pockets by rubbing against the genital area as they emerge from their cocoons, according to a new research paper published in Frontiers in Physiology.

Flores of the University of Copenhagen, co-author: “We have shown how an insect can maintain beneficial microbial partners despite the radical rearrangement of body structures that occurs during metamorphosis.” By modifying the unique ‘pockets’ on their backs, Agray The beetles were able to preserve their protective symbionts and facilitate their transfer during pupation to the newly developed adult organs.”

There are many examples of microbial symbionts in nature. For example, the Hawaiian Bobtail Squid has a built-in flashlight to help the creature navigate those mysterious nocturnal waters, hunt for prey, and in turn hide from predators. It is a special organ on the underside, a suitable small cavity containing colonies of bacteria, Vibrio Fishery. Once that bacterial colony reaches a critical threshold, they all begin to glow, acting as a source of light for the squid. Aphids, tubeworms, borer wasps, grain weevils, and bean bugs have also developed symbiotic relationships with microbes for various purposes.

Then there are the beetles. Beetles are distinguished from other insects by their front wings that are hardened into the wing-casings called elytra. The beetles undergo complete metamorphosis, that is, extensive physical reorganization over several developmental stages: egg to larva, to pupa, to the emergence of an adult from that pupal stage. Therefore any symbiotic bacteria must adapt accordingly during these growth stages.

The current study focuses on two types of beetles in particular: Lagria Herta (L. hirta) And the Lagria Velos (L. villosa), both of which host a community of microbial symbionts throughout their life cycle. L. villosasymbionts dominate a particular strain of Burkholderia Bacteria that have lost the ability to move and may not have been able to survive for long outside the host beetles. Flórez and her research colleagues at Johannes Gutenberg University in Mainz, Germany wanted to learn more about how beetles maintain and protect their symbionts during metamorphosis.

Moving 3D computed tomography of a pupa showing posterior pockets containing symbiotic bacteria.

assemble the team L. hirta Ladybugs in Germany in 2020, being raised in an outdoor tank to simulate natural conditions. The offspring was collected in 2021 L. villosa Specimens were collected in Brazil in 2019 and cultured in plastic containers in a microclimate. Then the researchers compared the concentrations of symbiotic bacteria and the morphology of the exoskeletons in males and females.

They found that the beetles had three protective pockets of two lobes in the back of the chest during the larval and pupae stages to house the symbionts. Female beetles also hosted symbioses between the whiskers at the back of the head. But the adult males lost their symbionts, while the bacterial assemblages shifted to the genital area in the adult females. flowers and others. He concluded that there must be a mechanism for transporting symbionts to the outer surface – particularly for bacterial species that lack the ability to move – and intellectual friction may be key.

To test this hypothesis, five early L. villosa Pupae were inoculated with fluorescent beads to simulate the transmission of symbiotic bacteria. Two adult females were carefully dissected, with photographs taken at each step to trace the location of the beads. An adult male was also dissected as a control.

The results confirmed their hypothesis. Co-author Rebecca S. Janke, a graduate student at the Johannes Gutenberg University in Mainz: “The symbionts move from the heavily exposed egg surface to colonize the pockets on the backs of larvae and pupae.” Specialized glands associated with the adult female reproductive system. “When an adult female lays her eggs, these symbiotic bacteria are extruded from the glands and deposited on the surface of the eggs, protecting them during metamorphosis, and starting the life cycle over and over again.

This explains why only adult females keep their adult lives. “In adulthood, it appears that the main purpose of the symbiotic organs is to enable a successful transition to the egg stage and into the next generation,” said Florez. Since only females lay eggs, adult males do not need to carry these costly symbionts and a dead end for bacteria.”

DOI: Frontiers in Physiology, 2022. 10.3389/fphys.2022.979200 (about DOIs).

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