Certain species of beetles have evolved unusual “back pockets” to safely house symbiotic bacteria during metamorphosis — the only known occurrence in insects. This is part of a mutually beneficial arrangement, as the bacteria protect the fragile larvae and pupae from the fungus. According to a new paper published in the journal Frontiers in Physiology, the scientists also determined that only adult females retain these commensal bacteria, and when they emerge from the pupae, by rubbing into the genital area, the population is removed from these rears. Wash out in the bag.
“We show how insects maintain beneficial microbial partners despite dramatic rearrangements of body structure during metamorphosis,” said co-author Laura V. Flórez of the University of Copenhagen. “By modifying Unique ‘pocket’ on the back, Laglia Beetles manage to maintain their protective symbionts and facilitate their migration to newly developed adult organs during pupation. “
There are many examples of microbial symbionts in nature. The Hawaiian bobtail squid, for example, has a built-in flashlight that helps the creature navigate dark nighttime waters in search of prey and, in turn, avoid predators. It is a special organ at the bottom, a small cavity that conveniently holds bacterial colonies, Vibrio fischeri. Once this bacterial colony reaches a critical threshold, they begin to glow, becoming a light source for the squid. Aphids, tube worms, digging wasps, cereal weevils, and bean worms have also evolved symbiotic relationships with microbes for a variety of purposes.
Then there are beetles. What distinguishes the beetle from other insects is that the forewings harden into wing shells called elytra. The beetle undergoes complete metamorphosis, an overall body reorganization over the course of several developmental stages: egg to larva, to pupa, and then to an adult emerging from the pupal stage. Therefore, any symbiotic bacteria will need to adapt accordingly during these developmental stages.
The current study focused specifically on two beetle species: Lagria hirta (L.hirta) and Crape myrtle (L. villosa), both possess a community of microbial symbionts throughout their life cycle. L. villosaof symbionts dominated by a specific strain Burkholderia Inactivated bacteria may not survive long outside of the host beetle. Flórez and her research colleagues at Johannes Gutenberg University in Mainz, Germany, wanted to learn more about how beetles maintain and protect their symbionts throughout metamorphosis.
The team collected L.hirta Beetles in Germany in 2020, reared in outdoor terrariums to simulate natural conditions. Descendants to be collected in 2021. L. villosa Specimens were collected in Brazil in 2019 and housed in plastic containers in a climate chamber. The researchers then compared the concentrations of male and female symbiotic bacteria and the morphological structure of the exoskeleton.
They found that in the larval and pupal stages, the beetles have three two-lobed protective pockets at the back of the thorax to house the symbionts. Female beetles also host symbionts between the bristles on the back of their heads. But adult males lost their symbionts, and bacterial populations moved to the genital areas of adult females.Flores et al. concluded that there must be a mechanism to relocate symbionts on the outer surface—especially for those bacteria that lack motility—and suggested that friction may be the key.
To test this hypothesis, five early L. villosa Pupae were inoculated with fluorescent beads to simulate the spread of symbiotic bacteria. Two of the adult females that emerged were carefully dissected, with images taken at each step to track the location of the beads. A male adult was also dissected as a control.
The results confirmed their hypothesis. “The symbionts move from the highly exposed egg surface into the pockets on the back of the larvae and pupae,” says co-author Rebekka S. Janke, a graduate student at Johannes Gutenberg University Mainz. Finally, they eventually enter to reproduce with adult females. System-related special glands. “When adult females lay eggs, these symbiotic bacteria are squeezed out of the glands and deposited on the surface of the eggs, protecting them throughout metamorphosis, and the life cycle begins again.
This explains why only adult females retain their symbionts into adulthood. “At the adult stage, the main purpose of the symbiotic organ appears to be to be able to successfully propagate to the egg stage and the next generation,” Flores said. “Since only females lay eggs, male adults do not need to carry these potentially expensive symbionts, a dead end for bacteria.”
DOI: Frontiers in Physiology, 2022. 10.3389/fphys.2022.979200 (on DOI).