Sawtooth beetles live in symbiosis with bacteria. Their bacterial partners provide important building blocks for the formation of the insect’s exoskeleton, which protects the beetles from their enemies as well as from desiccation. In a new study, a team of scientists from Johannes Gutenberg University in Mainz, the Max Planck Institute for Chemical Ecology in Jena and the National Institute of Advanced Industrial Science and Technology in Japan demonstrate that glyphosate inhibits Symbiotic bacteria of the cereal beetle. . Beetles exposed to weedkiller no longer receive the basic building blocks they need from bacteria. The study shows that glyphosate has the potential to indirectly harm insects by targeting their bacterial partners and thus contribute to their decline.
Insects need microbial partners to survive
Organisms do not exist in isolation but in a complex web of ecological interactions. These interactions should be kept in mind when assessing the impact of human activities. Insects in particular benefit in various ways from microbial symbionts, primarily through complementary nutrients and chemical defenses. However, the benefits derived from these symbiotic associations can also make the insect more vulnerable. Insect hosts often become so dependent on their microbial partners that they can hardly survive on their own. Without symbiosis, the development of insects can be delayed or prevented, their sensitivity to natural enemies can be increased, their reproductive potential and competitiveness with conspecifics can be impaired, or they can die.
Glyphosate prevents symbiotic bacteria from providing the building blocks of the insect exoskeleton
Glyphosate is currently one of the most widely used pesticides in agriculture, despite growing controversy over its potentially harmful side effects. It is believed to selectively suppress plant growth by inhibiting the biosynthesis of aromatic amino acids via the so-called shikimate pathway, a metabolic pathway occurring in plants and many microorganisms. Animals, on the other hand, do not encode the shikimate pathway and are therefore considered to be free from glyphosate. However, many animals engage in mutualistic interactions with symbiotic microbes that depend on the shikimate pathway to produce the amino acids needed by these animals. “An impact of glyphosate on animals via their essential bacterial partners who use or even specialize in the metabolic pathway of shikimate seems obvious, once the interaction of the two partners is understood,” explains Tobias Engl, one of the lead authors of the study. ‘study. Glyphosate has previously been shown to have a negative effect on microorganisms in the bee gut, making bees more susceptible to various stressors. In addition, mutualistic bacteria in several taxa of herbivorous insects provide an essential advantage: the aromatic amino acid tyrosine necessary for the biosynthesis of the insect exoskeleton (cuticle). This dependence on symbiosis, in turn, can make insect herbivores particularly vulnerable to glyphosate in agriculture: the herbicide inhibits the shikimate pathway in their symbiotic bacteria and, as a result, lacks important amino acids necessary for its development. cuticle formation.
Is dependence on their symbiotic partners the Achilles heel of insects?
In the current study, scientists show that glyphosate negatively influences mutualistic bacteria harbored by the sawtooth beetle Oryzaephilus surinamensis, an economically important pest of stored grain products. Exposure to the herbicide completely abolished the mutual benefit that symbiotic bacteria provide for cuticle formation, an insect’s primary protection against stresses, such as drought or attack by predators.
The researchers sequenced the genome of the symbiont associated with O. surinamensis. The tiny genome of the symbiont (300 kbp) contains the genetic instruction for a metabolism specializing in the synthesis of aromatic amino acids via the shikimate pathway. Interestingly, the genome strongly resembles that of the palm weevil’s bacterial partners, which are also involved in cuticle formation, although these symbionts belong to a different group of bacteria.
“Finding a functionally equivalent symbiotic relationship that arose independently in two different bacterial phyla and distant beetle families highlights the importance of cuticle-supporting symbioses in beetles,” says Julian Kiefer, the study’s first author. Scientists experimentally tested the functional benefits for the host by supplementing the beetles’ diet with aromatic amino acids, thereby compensating for the elimination of their symbionts. Conversely, exposure to glyphosate inhibited the establishment of symbiotic bacteria throughout the development of the beetle and completely abolished the mutual benefit for cuticle synthesis.
“When we observed the detrimental impact of glyphosate exposure on this symbiotic association, we wondered if glyphosate posed a general threat to insects that depend on their microbial partners,” explains Engl. Using phylogenetic analyzes, the authors demonstrate that many obligate mutualistic bacteria associated with various different insect hosts do indeed encode a glyphosate-sensitive enzyme in the shikimate pathway, suggesting that the sensitivity of its microbial symbionts to glyphosate represents a heel of ‘Achilles for an insect.
We are currently seeing a massive decline in insects. The abundance of insects is decreasing, as is the diversity of insects, not to mention the impact of the disappearance of bees, beetles and other insects at higher levels of the food chain. The new findings highlight that the use of the herbicide glyphosate in agriculture endangers the vital symbiotic relationships between insects and microorganisms and therefore poses a serious threat to our ecosystems.