The first symbiotic relationships were described in the 19th century. Nearly 150 years later, immense research efforts have highlighted that symbiotic relationships are ubiquitous and have particularly important ecological and evolutionary consequences. But considering individuals as holobionts, or ecosystems that combine host and symbiotic entities, is an on-going revolution. Within this framework, the 10th Congress of the International Symbiosis Society (ISS 10) was held in conjunction with the 3rd edition of the International Conference on Holobionts (HOLOBIONT 3) at the Palais des Congrès of Lyon (France) from July 25 to 29, 2022. This joint congress provided an unique opportunity to present the most exciting and recent discoveries in the fields of symbiosis and holobiont research. More than 300 attendees were present, and 82 speakers from 23 different countries were able to share their latest research, along with 16 invited speakers.

Rita Rebollo, Steering Committee Member

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Elisa Dell’Aglio Post doctoral researcher at BF2i laboratory, University of Lyon, France.

Can you tell us a bit about yourself and your research interests?

I have developed various research interests. My first love was for dissecting the importance of vitamin metabolism for plant responses to stress (both biotic and abiotic). After a few years spent studying NADP and vitamin B6 biosynthesis in Arabidopsis, I moved to a completely different field: the investigation of the molecular mechanisms that govern the interaction between the cereal weevil Sitophilus oryzae and the its bacterial mutualistic endosymbiont Sodalis pierantonius. The weevil is one of the major threats for stored cereals, as it eats and lays eggs inside cereal grains, and the endosymbiont has been shown to be quite useful to improve insect protection against biotic and abiotic stress. This is especially due to the fact that bacteria produce a surplus of amino acids that are used by the host to reinforce their cuticle, the external “armor” that protects adult insects once out of the cereal grains. In exchange, the endosymbionts obtain energy and shelter from their host weevil, which is why this interaction is called a “mutualistic symbiosis”.

What first interested you in this field of research?

I had been fascinated by an article published by the group a few years ago (Vigneron et al., Current Biology 2014) about how the endosymbionts are eliminated by the weevil once the benefits of keeping them were satisfied (i.e. after the biosynthesis of the cuticle is completed). I was impressed by the amount of creative work that was necessary in order to discover the mechanistic aspects of non-model organisms. For example, the endosymbiont cannot be cultivated in vitro, both partners cannot be genetically modified and the development of the host is difficult to monitor since it takes place inside cereal grains from the egg stage until early adulthood. Despite that, with some tricks and creativity we were able to expand our knowledge on the subject.

You won the Best Student Presentation award at ISS 10/HOLOBIONT 3, can you explain the research you presented?

I presented a continuation of the 2014 work. In particular, we have discovered that prior to endosymbiont clearance by the host, the endosymbionts proliferate uncontrollably. Our data indicate that this proliferation escapes host control and is rather a parasitic-like phase, which does not serve the insect in normal rearing conditions and can even be detrimental for host survival if coupled with nutrient scarcity. It is important to stress that many bacterial endosymbionts derive from former bacterial pathogens that have been domesticated during evolution, and so is the case for S. pierantonius. However, our results, together with other independent discoveries, suggest that the domestication of the cereal weevil’s endosymbiont is still ongoing and therefore that we have captured a unique moment in the continuum between parasitism and mutualism. (https://www.biorxiv.org/content/10.1101/2022.07.06.498660v1).

How will you continue to build on this research??

We are currently investigating the signalling pathways that might regulate host-endosymbiont communication at the cellular level. In particular, we are investigating the changes in gene expression, DNA methylation, and bacterial/eukaryotic small RNAs at various stages of the symbiosis and we are using advanced microscopy techniques to study bacterial/host molecular exchanges.

 

Nomthi Kanyile PhD Candidate at the Max Planck Institute for Chemical Ecology, Germany.

Can you tell us a bit about yourself and your research interests?

I am originally from Inanda, a township in South Africa. My research journey started with my BSc-Honours, where I investigated whether biochar could alleviate the toxicity of heavy metals to soil invertebrates. For my MSc, I spent time on the field with a wild population of African striped mice in the Succulent Karoo, a semi-desert region of South Africa, where I studied and described the behavioural ecology of bachelor male groups in this population. I then moved to Germany for my PhD where I currently study insect symbioses. Although my research background has spanned different fields, the pivot point has always been ecology. I am interested in the dynamic interaction of organisms intra-and-interspecifically, and how these interactions may be mediated by environmental factors.

What first interested you in this field of research?

After spending time in the field exploring some of the immediate and ultimate explanations for the existence of bachelor groups in striped mice, I was fascinated by how individuals of this population change their social and reproductive tactic depending on the prevailing environmental conditions. I then became broadly interested in understanding the physiological and molecular mechanisms that underpin adaptability in organisms occupying difficult environments. As such, I was drawn in by the concept of insects engaging in symbiosis with microbes as a way of extending their metabolic capabilities and thus their adaptive potential, allowing them to exploit recalcitrant resources that they would otherwise not be able to exploit. This can be studied from multidisciplinary perspectives, and I found this quite exciting!

You won the Best ECR Presentation award at ISS 10/HOLOBIONT 3, can you explain the research you presented?

The research focused on the role played by tyrosine-supplementing symbionts on the ecology of grain pest beetles. Tyrosine is a semi-essential amino acid required for the biosynthesis of the insect cuticle, which serves as the insect’s first barrier against the external environment. Interestingly, insects are unable to de novo synthesize tyrosine. They can either obtain it from their diet or through mutualistic associations with tyrosine-provisioning microbes. The grain pest beetles Oryzaephilus surinamensis and Sitophilus oryzae harbour intracellular bacterial symbionts that had previously been shown to contribute to cuticle biosynthesis.

Therefore, we hypothesized that the symbionts mediate the hosts interactions with antagonists that must actively penetrate the cuticle. By exposing symbiotic and symbiont-free (aposymbiotic) O. surinamensis to two such antagonists (wolf spiders and the entomopathogenic fungus Beauveria bassiana); we demonstrated that while both symbiotic and aposymbiotic young beetles suffer from high predation and fungal infection rates, symbiotic beetles escape this vulnerability earlier and reach high survival probabilities faster than aposymbiotic beetles. Moreover, we conducted a time-series analysis of cuticle development in symbiotic and aposymbiotic beetles to better understand the mechanistic basis of the observed differences in survival. This analysis revealed that cuticle development is drastically accelerated in the presence of the symbionts, correlating with the observed early escape of symbiotic beetles from high predation and fungal infection rates. Finally, we exposed symbiotic and aposymbiotic S. oryzae to long-term desiccation stress and measured survival, population growth and cuticular hydrocarbon profiles of both treatments. We showed that symbiosis improves host resistance to desiccation in this case. These findings demonstrate how nutritional symbioses can have important protective properties that reach beyond the immediate nutrient provisioning benefits.

(Kanyile, S.N., Engl, T. and Kaltenpoth, M., 2022. Nutritional symbionts enhance structural defence against predation and fungal infection in a grain pest beetle. Journal of Experimental Biology, 225(1), p.jeb243593).

What are your next steps?

The next steps involve further investigating the physiological response of beetles when exposed to dry stress, and how this response is mediated by the symbionts. I also intend to understand if the protective cuticle mediates intraspecific competition through cannibalism and whether protective effects are shared across species with similar symbioses. Finally, I will investigate the effects of environmental chemicals on the insect cuticle across different species.