A symbiotic relationship with bacteria helps Lagria beetles protect themselves from harmful fungi in early life stages. But how do the females save the useful symbionts through the metamorphosis in order to finally pass them on to their own offspring? A study shows that they have developed special back pockets for this. The bacteria survive there while the larva develops into a beetle. Due to the friction during hatching, they are transported mechanically to the genital tract of the female - and from there they are finally transferred to the eggs.
Symbioses between animals and bacteria are widespread. The useful microorganisms can take on numerous tasks, from supporting digestion to protecting against pathogens. We humans come into contact with many helpful bacteria at birth, some of which will colonize us for the rest of our lives. Many insects are also dependent on such symbioses. However, they face a special challenge: How can they preserve their bacteria while they themselves develop from a larva to an adult animal? Because in the metamorphosis during the pupal stage, almost their entire body is rebuilt.
Safer storage
A team led by Rebekka Janke from the Johannes Gutenberg University of Mainz has now discovered how beetles of the Lagria genus solve this problem. "We show here how an insect can retain useful microbial partners despite the drastic remodeling of body structures during metamorphosis," says Janke's colleague Laura Flórez. "Using unique 'bags' on their backs, female Lagria beetles manage to retain their symbionts and eventually transport them to newly developed adult organs."
To arrive at this finding, the researchers first examined male and female larvae and pupae of Lagria beetles using micro-CT images. They noticed that the females each have three tiny pockets on their backs, which the males only have rudimentarily. Further investigation revealed that these pockets contained numerous symbiotic bacteria. Using adult beetles, the researchers also demonstrated that the beneficial bacteria were only found in the female's genital tract, in a pair of glands right next to the fallopian tube. When the females lay eggs, the bacteria are squeezed out with them, are deposited on the surface of the eggs and can thus be passed on to the next generation.
By friction to the genital tract
The bacteria are important for the beetle offspring because they produce a natural antibiotic that prevents the eggs, larvae and pupae from being attacked by fungi. Adult beetles apparently no longer need this protection. "In the adult stage, the main purpose of the symbiotic organisms seems to be to enable successful transmission to the egg stage and the next generation," says Flórez. "Since only the females lay eggs, the adult males don't have to carry these potentially costly symbionts. They are a dead end for the bacteria.”
But how do the bacteria get from the back pouches of the females into the genital tract? To find out, the researchers sprinkled the early pupae with tiny, fluorescent plastic beads. With a diameter of one micrometer, the globules were similar in size to the bacteria - and like many of the symbionts, they could not move on their own. Janke's team now observed the distribution of the beads. The result: The balls were pushed towards the abdomen of the females simply by the friction during hatching. "This suggests that translocation of symbionts on the outer surface is possible without the need for the bacteria themselves to be motile or specially adapted," the authors say.
Other research questions
It is still unclear how the bacteria subsequently colonize the genital tract of the females. The team wants to deal with this in future studies. "In order to better understand how useful symbionts are transmitted and maintained within and across generations, we need to find out which host and symbiont factors control the establishment of symbionts," says Janke's colleague Martin Kaltenpoth. It is conceivable, for example, that the host selects certain symbionts and allows them to colonize the target organs despite their own immobility.
Source: Rebekka Janke (Johannes Gutenberg University Mainz) et al., Frontiers in Physiology, doi: 10.3389/fphys.2022.979200