Tobias Engl

Nutritional symbioses have repeatedly evolved in insects, yet how hosts regulate these partnerships to balance benefits across development and environments remains unclear. Ants provide an exceptional system to address this question, as several lineages maintain ancient symbionts that can be naturally lost without harming the host. The invasive tramp ant Cardiocondyla obscurior provides a compelling example of this phenomenon, as the vertically transmitted symbiont Candidatus Westeberhardia cardiocondylae can be lost at the species, colony, and even individual level. By implementing a range of molecular techniques, including dual-RNA sequencing, fluorescent in-situ hybridisations of RNA and qPCR, in conjunction with experimental manipulations of diet, we reveal the regulatory dynamics and function of this labile association. Symbionts provide shikimate-derived nutrients that enhance colony resilience under protein limitation, while hosts appear to regulate symbiont abundance during protein scarcity and cuticle formation to optimise resource allocation. In the symbiont’s absence, ants compensate by upregulating genes enabling tyrosine acquisition from external sources. This metabolic flexibility allows colonies to exploit symbiont-derived and environmental nutrients, sustaining both growth and survival across nutritional contexts. Our findings reveal dynamic host regulation as a mechanism for sustaining the persistence and adaptability of ancient symbioses.

Many insects rely on symbiotic fungi to occupy specialized ecological niches, yet the evolutionary dynamics of these partnerships remain poorly resolved for most lineages. The beetle family Ptinidae, split into the morphologically distinct Spider beetles and Deathwatch beetles, has long been known to harbor fungal endosymbionts based on early microscopy, but few associations have been confirmed with molecular methods. Here, we combined ultra-conserved element (UCE) phylogenomics with ITS amplicon sequencing to test whether fungal endosymbionts are conserved across Ptinidae and whether they have cospeciated with their hosts. Our UCE phylogeny supports Spider beetles and Deathwatch beetles as monophyletic clades but indicates that some aspects of subfamily-level taxonomy may merit closer examination. Screening for three known symbiotic fungal genera ( Symbiotaphrina , Meyerozyma , Nakazawaea ) revealed Symbiotaphrina in most Deathwatch beetles but no Spider beetles, while the other two genera were present but uncommon. Despite widespread Symbiotaphrina infection, we found no phylogenetic mirroring between host and symbiont trees, indicating an absence of codiversification. Instead, distantly related hosts frequently shared closely related symbionts, consistent with diffuse, mixed-mode transmission involving both vertical and horizontal symbiont exchange. This pattern parallels those documented in fungus-farming termites, ambrosia beetles, ants, and woodwasps, suggesting that diffuse, mixed-mode symbiosis may be a general hallmark of long-term insect-fungal associations. We further identify an unidentified Helotiales group as a candidate novel endosymbiont, recovered consistently within a clade comprising Anobium , Hemicoelus , and Ptilinus . Together, these findings reframe Deathwatch beetle-fungal associations as a dynamic, evolutionarily labile symbiosis rather than a fixed partnership.

With advances in imaging technologies and the growing availability of specialized lights and cameras, biofluorescence is increasingly recognized as a widespread trait with potential roles in the behavioural ecology of many tetrapods. Here, we report the first evidence of biofluorescence in the fire salamander (Salamandra salamandra), one of Europe’s most studied amphibians. Under UV illumination, S. salamandra exhibited conspicuous external fluorescence, mainly on the ventral and lateral surfaces and in glandular secretions. Histological analyses revealed that UV–violet excitation produces a broad blue–green fluorescence in the epidermis and granular glands, matching the emission of secretions and the speckled external pattern corresponding to glandular ducts. Fluorescence lifetime imaging revealed that glandular fluorophores differ from those in the epidermis and are also ubiquitous in blood components, suggesting that fluorescent compounds may be transported via the circulatory system, a feature previously known only in tree frogs. Additionally, a secondary yellow fluorescence was detected in the dermal chromatophore layer, excited exclusively by UV light. Notably, fluorescence from secretions persisted on substrates for over 24 h and fulfilled several criteria for ecological relevance, suggesting a communicative function for this trait and adding to the growing evidence of ecologically relevant fluorescence in amphibians.

Many insects benefit from ancient nutrient-supplementing endosymbionts. While symbiont losses and replacements occur on evolutionary timescales, their dynamics remain enigmatic due to the lack of experimentally tractable systems. Here, we report on the experimental establishment of the culturable bacterium Sodalis praecaptivus in a grain pest beetle ( Oryzaephilus surinamensis ) and its effect on the native symbiont Shikimatogenerans silvanidophilus , which produces the tyrosine precursor prephenate. Injection of Sodalis into female beetles led to systemic intracellular infection and efficient transovarial vertical transmission but reduced host survival and reproduction. Interestingly, Sodalis also invaded the host’s bacteriomes, causing irregular morphology and rapid loss of Shikimatogenerans within three beetle generations. Transcriptomics revealed a strong upregulation of host immune effectors upon Sodalis infection, but little reaction from Shikimatogenerans , indicating that the ancient symbiont is incapable of responding adaptively to the introduced competitor. The rapid elimination of the native symbiont in O. surinamensis showcases the fragility of ancient beneficial symbioses and experimentally recapitulates a crucial step towards a functional symbiont replacement.

Beewolf wasps rely on an ancient defensive symbiosis with Streptomyces bacteria that protect their larvae from fungal infection. Female beewolves apply the bacteria to the brood-cell ceiling, and larvae later transfer the symbionts onto the cocoon surface, where they produce antifungal metabolites. Here, we investigated the mechanism of symbiont transfer from the beewolf brood cell to the larval cocoon and characterized the microbial community dynamics across different beewolf life stages and during larval hibernation. Fluorescence in situ hybridization revealed that the symbionts are transiently taken up into the proximal midgut lumen and then regurgitated onto the cocoon during the spinning process. High-throughput sequencing showed that the bacterial community of beewolf feeding larvae resembles that of the honeybee prey, whereas that of adults and diapausing larvae is dominated by Wolbachia. Moreover, the cocoon bacterial community is initially dominated by the defensive Streptomyces philanthi symbiont, but when larvae excrete the gut content inside the cocoon, other bacterial taxa including Lactobacillus, Gilliamella, and Bartonella shift the community composition toward dominance by Pseudomonadota. Our findings provide new insights into the transmission route of an ancient extracellular symbiont and its potential competition with other bacteria in this long-term defensive symbiosis.

The stored product pests Lasioderma serricorne and Stegobium paniceum (Coleoptera: Ptinidae: Anobiinae) have been described to harbour Symbiotaphrina kochii and Symbiotaphrina buchneri yeast-like symbionts (YLS) respectively, based on axenic cultivation from symbiotic organs. While the nutritional benefits provided by the YLS are well characterized, molecular studies focusing on variability or stability in the systems are lacking. Here we address that research gap by investigating the diversity and stability of Anobiid – Symbiotaphrina symbiosis in multiple populations. Amplicon sequencing of the fungal internal transcribed spacer (ITS) from collected and lab-reared populations revealed that populations from different origins were associated with similar YLS strains. These associations remained stable during rearing. However, only one L. serricorne population was associated with Sy. kochii , the others were associated with Sy. buchneri. Most St. paniceum samples were associated with a Symbiotaphrina species that could neither be identified as Sy. buchneri , nor Sy. kochii. Cultivation and subsequent phylogenetic analysis of the partial rRNA operon of YLS revealed a novel Symbiotaphrina clade. Our results indicate more flexible associations than previously assumed, however only with members of the genus Symbiotaphrina . The ecological relevance needs further analyses, but highly variable in vitro growth could indicate extensive differences in YLS capabilities.

The planthopper Pentastiridius leporinus has emerged as a severe crop pest, rapidly expanding both its host plant range and the affected areas in central Europe. Originating as a monophagous herbivore of reed grass, P. leporinus recently adopted polyphagous feeding and is now a pest of sugar beet, potato, carrot, and onion, suggesting rapid ecological niche expansion. P. leporinus vectors two bacterial pathogens: the γ-proteobacterium Candidatus Arsenophonus phytopathogenicus (CAP) and the stolbur phytoplasma Candidatus Phytoplasma solani (CPS), which are responsible for a range of disease syndromes, including syndrome basses richesses in sugar beet. We used long-read metagenomic sequencing to characterize the genomes of microbes associated with P. leporinus , resulting in the complete sequences of CAP and CPS, as well as obligate symbionts of the genera Purcelliella , Karelsulcia , and Vidania , and facultative symbionts Rickettsia and Wolbachia . The obligate symbionts are inferred to provide or contribute to the biosynthesis of 10 essential amino acids and to B vitamin. The genomes of CPS and CAP encode numerous pathogenicity factors, enabling the colonization of different hosts. Bacterial fluorescence in situ hybridization revealed the tissue distribution, cellular localization, relative abundance, and transmission patterns of these bacteria. The intracellular presence of all obligate symbionts in bacteriomes, the intracellular presence of Wolbachia , and the intranuclear localization of Rickettsia suggest vertical transmission. CPS was restricted to salivary glands, suggesting strict horizontal, plant-mediated transmission, whereas CAP colonized all tissue types, allowing for horizontal and vertical transmission. Our data suggest that P. leporinus hosts an exceptionally broad range of symbionts, encompassing mutualistic, commensal, and pathogenic interactions. IMPORTANCE: The planthopper Pentastiridius leporinus has recently expanded its host plant range and emerged as a severe pest of sugar beet and potato crops in central Europe, which is exacerbated by its capacity to vector bacterial pathogens to its host plants. Because microbial symbionts may play an important role for both the host plant shifts and the transmission of pathogens, we used metagenomic sequencing and fluorescence in situ hybridization to characterize the microbial community associated with P. leporinus . We detected three bacteriome-localized obligate symbionts that together provision all essential amino acids and several B-vitamins to the host, as well as two intracellular bacteria with a broader tissue distribution. In addition, we infer localization, transmission, and putative pathogenicity factors for the two major phytopathogens that are vectored by P. leporinus . Our results reveal a complex community of symbiotic bacteria that likely shapes the interaction of this emerging pest with its host plants.

Nutritional symbioses have repeatedly evolved in insects, yet how hosts regulate these partnerships to balance benefits across development and environments remains unclear. Ants provide an exceptional system to address this question, as several lineages maintain ancient symbionts that can be naturally lost without harming the host. Using Cardiocondyla obscurior, which carries and occasionally loses the vertically transmitted symbiont Cand. Westeberhardia cardiocondylae, we reveal the regulatory dynamics and function of this labile association. Symbionts provide shikimate-derived nutrients that enhance colony resilience under protein limitation, while hosts actively regulate symbiont abundance during protein scarcity and cuticle formation to optimise resource allocation. In the symbiont’s absence, ants compensate by upregulating genes enabling tyrosine acquisition from external sources. This metabolic flexibility allows colonies to exploit symbiont-derived and environmental nutrients, sustaining both growth and survival across nutritional contexts. Our findings reveal dynamic host regulation as a mechanism sustaining the persistence and adaptability of ancient symbioses.

The beetle family Ptinidae contains a number of economically important pests, such as the cigarette beetle Lasioderma serricorne , the drugstore beetle Stegobium paniceum , and the diverse spider beetles. Many of these species are stored product pests, which target a diverse range of food sources, from dried tobacco to books made with organic materials. Despite the threat that the 2,200 species of Ptinidae beetles pose, fewer than 50 have been surveyed for microbial symbionts, and only a handful have been screened using contemporary genomic methods. In this study, we screen 116 individual specimens that cover most subfamilies of Ptinidae, with outgroup beetles from closely related families Dermestidae, Endecatomidae, and Bostrichidae. We used 16S ribosomal RNA gene amplicon data to characterize the bacterial microbiomes of these specimens. The majority of these species had never been screened for microbes. We found that, unlike in their sister family, Bostrichidae, that has two mutualistic bacteria seen in most species, there are no consistent bacterial members of ptinid microbiomes. For specimens which had Wolbachia infections, we did additional screening using multilocus sequence typing and showed that our populations have different strains of Wolbachia than noted in previous publications. IMPORTANCE: Ptinid beetles are both household pests of pantry goods and economic pests of dried goods warehouses and cultural archives, such as libraries and museums. Currently, the most common pest control measures for ptinid beetles are phosphine and/or heat treatments. Many ptinid beetles have been observed to have increasing resistance to phosphine, and heat treatments are not appropriate for many of the goods commonly infested by ptinids. Pest control techniques focused on symbiotic bacteria have been shown to significantly decrease populations and often have the beneficial side effect of being more specific than other pest control techniques. This survey provides foundational information about the bacteria associated with diverse ptinid species, which may be used for future control efforts.