Insects have their own set of friendly microbes that help keep them healthy, just like us.
But instead of growing larger like us sentient vertebrates, insects undergo extreme body deformation to metamorphose into their older life stages.
As you might expect, these life-changing contortions complicate the living arrangements of microbes.
This convoluted process can completely distort and drastically change the surrounding organs and other tissues, so the microbes along for the ride would possibly not have a survivable journey, much less a fun one.
Evolutionary ecologist Rebekka Janke of the Johannes Gutenberg University Mainz and her colleagues took a closer look at this transformative process in the dark beetle Lagria villosa to find out what happens to its little helpers during this upheaval.
While dark beetles have many symbiotic species within their microbiome, they are particularly dependent on Burkholderia bacteria for successful reproduction.
The beetle’s eggs and larvae are vulnerable to infection, but Burkholderia, which female beetles secrete from glands near their ovaries into their eggs, keeps their offspring safe by producing polyketide chemicals that have antimicrobials
As a result, a particular strain of Burkholderia, B. gladioli, or Lv-StB, has become accustomed to living a comfortable life inside the beetle that has lost all ability to move on its own: its genes and structures cells for motility. they have almost completely disappeared, so it also depends on the beetles for its own survival.
Janke and the team tracked what happened to the microbes using fluorescent markers and microCT scans, and a sample of the bacteria’s DNA.
After mother beetles cover their eggs with their bacteria-containing gland, the microbes spend about six days exposed on the surface of the eggs, fighting off parasitic bacteria and hungry fungi.
Once the beetles hatch, the bacteria gather in three deep folds on the lower back in the larva’s outer cuticle, like back pockets. These folds not only provide protection for the symbiotes; they also contain glandular cells that probably help nourish the bacteria with secretions.
frameborder=”0″ allow=”accelerometer; automatic playback; clipboard-writing; encrypted media; gyroscope; “Picture-in-picture” in full screen>
But even these pockets wrinkle during extreme metamorphosis. However, they allow some of the bacteria to escape to the surface, this time in pupae, ready to make their way to the adult beetle’s reproductive organs.
The researchers did not detect any B. gladioli in the guts of the pupae, so travel to the glands of their hosts is clearly not via internal pathways.
So the researchers put tiny, symbiont-sized fluorescent polystyrene beads into the developing pupae. Most of the beads ended up around the tip of the beetles’ abdomens once they emerged as adults, after their cases opened exactly where the back pockets were.
“By modifying the unique ‘pockets’ on their backs, Lagria beetles manage to maintain their protective symbionts and facilitate their relocation during pupation to newly developed adult organs,” says evolutionary ecologist Laura Flórez of the University of Copenhagen.
The final stage of the bacteria’s journey to the adult glands remains a mystery, and most of this process occurs only in female beetles. Males begin to lose bacteria from the pupal stage: their posterior pockets are much smaller and shallower, and male adults have no symbiotes.
Cuticle pocket shapes in female (left) and male (right) larvae and pupae. (Janke et al., Frontiers in Physiology, 2022)
“In the adult stage, the main purpose of the symbiotic organs seems to be to enable successful transmission to the egg stage and the next generation,” explains Flórez. “Because only females lay eggs, adult males do not need to carry these potentially costly symbionts and are a dead end for the bacteria.”
In social insects such as ants, if individuals lose some of their microbial companions, others in the group can be reseeded; the new study results provide an example of how solitary insects avoid this potential loss during their most vulnerable life stages.
“These findings indicate that the ecological importance of symbionts likely drove the evolution of specialized host structures to house and maintain bacteria during metamorphosis,” Janke and colleagues conclude in their paper.
This research was published in Frontiers in Physiology.