Beetles have "backpacks" to protect their eggs

Not all microbes bring disease and discomfort for animals; in fact, many are very useful, or even essential for survival. Beetles of the genus Lagria are dependent on their symbiotic bacterial bodyguards for reproductive success, and therefore have to keep them close. This is not a simple matter as the beetles metamorphose completely during their life cycle, meaning that their tissues get reorganized, new organs develop and their exoskeletons get shed. All of this makes it difficult for the useful microbes to tag along to beetle adulthood.

Female beetles lay their eggs in humid soil, under leaf litter, where the eggs would certainly be colonized by fungi that are present among the decomposing leaves. This would lead to the decay and death of the eggs. Instead, a female carries several species of symbiotic bacteria in glands near the oviduct and, when she lays the eggs, bacteria get squeezed out and coat the surface of the eggs. These bacteria produce several types of antibiotic substances that protect the eggs, and then the larvae and pupae, from fungal colonization. 

Until now, however, it has not been understood how the symbiotic bacteria remain with a beetle offspring as it changes through the egg, larva and pupa stages, such that the bacteria are present in the adult female’s reproductive organs when it is time for her to lay her own eggs. A new study by researchers from Germany and Denmark has now identified three unique storage pockets that are present, like backpacks, on the dorsal surface of each larva and that ensure the essential bacteria are carried along during the life cycle. 

“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 at the Department of Plant and Environmental Sciences of the University of Copenhagen. “By modifying unique ‘pockets’ on their backs, Lagria beetles manage to keep their protective symbionts and facilitate their relocation, during pupation, to newly developed adult organs.”

Females of many Lagria species carry a mix of beneficial bacteria in their paired glands next to the oviduct. 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 cellular structures for motility, and probably can’t survive for long outside the beetles.

Flórez and colleagues show, in their publication in the journal Frontiers in Physiology, that L. villosa and L. hirta female pupae house their symbionts mostly inside the three two-lobed pockets on the back of the thorax, where they may be nourished by the beetles. Such ‘back pockets’ in larvae and pupae aren’t known in any other insect. In female pupae, symbionts also congregate in a fourth spot, between bristles at the back of the head. The pockets are only vestigial in male pupae, and contain few or no symbionts. Clearly, the males have no role in laying eggs and therefore do not need to store useful bacteria for this purpose. 

“The symbionts go from the highly exposed egg surface to colonize the pockets on the back of the larvae and pupae. Finally, 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, exactly, do the microbes get from the storage pockets on the pupa’s back, down to the genital organs at the tip of the abdomen of an adult female? 

To answer this question, the experts scattered miniscule fluorescent beads, each only 1.0 μm wide, across the surface of early pupae. They found that, after the pupa had emerged, the majority of these beads ended up on the tip of the abdomen. The researchers conclude that the beads, like presumably the symbionts, are shuffled towards the genitals by friction during the emergence process. The mechanism by which the symbionts then 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 onto the egg stage and to the next generation. Since only females lay eggs, male adults do not need to carry these potentially costly symbionts and are a dead-end for the bacteria,” explained Flórez.

“To better understand how beneficial symbionts are transmitted and maintained within and across generations, we’ll need to identify which host and symbiont factors regulate symbiont establishment,” said study co-author Dr. Martin Kaltenpoth. “For example, does the host select for specific symbionts? And through which mechanisms can immotile symbionts colonize the symbiotic organs?”

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By Alison Bosman, Earth.com Staff Writer