Beetles rely on unique ‘back pockets’ to keep bacterial symbionts safe during metamorphosis

Beetles of the genus Lagria need a little help from their bacterial friends during their immature life stage. But keeping them in the same place throughout life is not feasible. This is because beetles are holometabolic insects that undergo a complete physical reorganization (metamorphosis) as pupae.

Here, scientists show for the first time that the beetles have developed an ingenious solution to this problem: female dolls keep their symbiotic bacteria in specialized pockets on their backs. When they show up as adultsthey shake the bacteria out of these pockets, backwards and then into their genital area.

“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 Dr. Laura V Flórez, a researcher in the Department of Plant and Environmental Sciences at the University of Copenhagen. “By adapting unique ‘pockets’ on their backs, Lagria beetles manage to preserve their protective symbionts and facilitate their movement into newly developed adult organs during pupation.”

Unique ‘back pockets’

Females of many Lagria species carry a mix of beneficial bacteria in their accessory glands, a pair of glands next to the fallopian tube. When females lay eggs, the bacteria are ‘squeezed’ from the glands and deposited on the surface of the eggs. Antibiotics produced by the bacteria protect the eggs, larvae and pupae of the beetles against fungi. In one of the species studied here, L. villosa, the largest component of the symbiotic mix is ​​a strain of Burkholderia bacteria called Lv-StB, which has lost the genes and cell structures for motility and is unlikely to survive long outside the beetles.

Flórez and colleagues show that in L. villosa and L. hirta female pupae, the symbionts usually live in the three bilobed sacs on the back of the thorax, where they can be fed by the beetles. Such ‘back pockets’ in larvae and pupae are not known in any other insect. In female pupae, symbionts also congregate in a fourth place, between bristles at the back of the head. The sacs are only rudimentary in male pupae and contain few or no symbionts. In adult females, the symbionts live exclusively in the accessory glands, which are absent in males.

‘Long and winding road’

“The symbionts move from the highly exposed egg surface to colonize the sacs on the back of the larvae and pupae. Eventually they end up in specialized glands associated with the reproductive system of adult females‘, summarized first author Rebekka S Janke, a doctoral student at the Johannes Gutenberg University of Mainz.

But how do the bacteria colonize the auxiliary glands after pupation? To answer this question, the authors spread approximately 1 m of polystyrene fluorescent beads, 1.0 m wide, over early pupae. They then show that after emergence the majority of these beads ended up on the tip of the abdomen. The authors conclude that, like presumably the symbionts, the beads are pushed toward the genitals by friction during the emergence process. The mechanism by which the symbionts subsequently colonize the accessory glands of females is not yet known.

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

Co-author Dr. Martin Kaltenpoth, a professor at the Max Planck Institute of Chemical Ecology in Jena, added: “To better understand how beneficial symbionts are transmitted and maintained within and between generations, we need to identify which host and symbiote factors influence the symbiote branch. For example, does the host select for specific symbiotes? And by what mechanisms can immobile symbionts colonize the symbiotic organs?”