At the beginning of March, the ‘The cellular mechanics of symbiosis’ took place, the first EMBO | EMBL Symposium of its kind. 111 on-site participants and 51 virtual participants joined scientific organisers Toby Kiers, John McCutcheon, and Thomas Richards for talks on how symbionts are harnessed for benefit, what commonalities there are in symbiosis, and how to know when a symbiosis is actually a symbiosis. Two poster sessions were held, and from these emerged four winners of the best poster prize. We are happy to introduce you to two of them, Inès and Liliya!

Specificity of an obligate leaf beetle‑bacterial symbiosis

Presenter: Inès Pons

Symbiont specificity is predicted to stabilize obligate insect microbe symbioses. Despite the prevalence of nutritional and defensive symbioses in insects, mechanistic insights into mutualism specificity are often hampered by the limited ability to disrupt and experimentally exchange symbionts across different host species. Here, we leverage the tractability afforded by an extracellular symbiont transmission route in tortoise leaf beetles (Coleoptera: Cassidinae) to quantify the specificity governing their obligate symbiosis with Candidatus Stammera capleta. Cassidines derive pectin degrading enzymes from Stammera, allowing them to exploit a leafy diet rich in recalcitrant polysaccharides. Stammera is localized in specialized organs connected to the foregut and extracellularly transmitted from mother to progeny via egg caplets. Caplet removal disrupts symbiont transmission, enabling downstream manipulation and exchange of Stammera across different host species. We conducted experimental transfections in aposymbiotic eggs of the tortoise beetle, Chelymorpha alternans, using Stammera derived from three additional beetle species. Symbiont transfection was successful across all treatments, highlighting that Stammera can be exchanged across host species. However, we observe differences in the efficacy of symbiont colonization depending on the genetic distance of nonnative Stammera strains relative to the native symbiont. Reflecting this, the fitness of aposymbiotic larvae is restored following symbiont transfection of Stammera, but only in treatments bearing strains that are closely related to the native symbiont. The positive correlation between symbiont genetic distance and host survivorship highlights the strong effect of symbiont genotype on host development. Overall, our findings confirm that the high degree of host symbiont specificity plays a significant role in the obligate symbiont maintenance over long evolutionary timescales.

Due to the confidentiality of the unpublished data, we cannot share the poster.

Seasonal wild dance of dual-obligate symbionts in the pear psyllid Cacopsylla pyricola (Hemiptera: Psylloidea): Titers of symbionts tell different stories

Presenter: Liliya Serbina

Cacopsylla pyricola (Hemiptera: Psylloidea) is a multivoltine pear psyllid that spends its entire life cycle on pear trees, producing several summer generations and one morphologically different overwintering generation. This species is also a vector of ‘Candidatus Phytoplasma pyri’, the causative agent of Pear Decline disease. As most sap-feeding insects, C. pyricola maintains obligate relationships with two endosymbiotic bacteria, Ca. Carsonella ruddii and Ca. Psyllophila symbiotica, that provide the host with essential nutrients. Using quantitative PCR, we explored the seasonal dynamics of these symbionts in a natural population of C. pyricola, collected across an entire year, encompassing all generations of this species. Among all tested individuals, immatures harboured the highest titer of both symbionts, while the lowest symbiont density was observed in males. The density of Carsonella remained high and relatively stable throughout the vegetative period, whereas its density significantly dropped during the non-vegetative period, overlapping with C. pyricola’s reproductive diapause. In contrast, Psyllophila titer was significantly higher than Carsonella titer and exhibited fluctuations throughout the sampling year related to host age. Despite a tightly integrated metabolic complementarity between Carsonella and Psyllophila, our findings suggest that their density dynamics are regulated by different forces that could be linked to differences in their metabolic roles at different life stages of the host. Taken together, this study provides insights into host-symbiont interactions and could thus contribute to the development of novel strategies to control plant disease vectors.

Due to the confidentiality of the unpublished data, we cannot share the poster.

The EMBO | EMBL Symposium ‘The cellular mechanics of symbiosis’ took place 8 – 11 March 2023 at EMBL Heidelberg and virtually.