Nutritional symbionts confer structural defence against predation and fungal infection in a grain pest beetle

Many insects benefit from bacterial symbionts that provide essential nutrients and thereby extend the hosts’ adaptive potential and their ability to cope with challenging environments. However, the implications of nutritional symbioses for the hosts’ defence against natural enemies remain largely unstudied. Here, we investigated if the cuticle-enhancing nutritional symbiosis of the saw-toothed grain beetle Oryzaephilus surinamensis confers protection against predation and fungal infection. We exposed age-defined symbiotic and symbiont-depleted (aposymbiotic) beetles to two antagonists that must actively penetrate the cuticle for a successful attack: wolf spiders (Lycosidae) and the fungal entomopathogen Beauveria bassiana. While young beetles suffered from high predation and fungal infection rates regardless of symbiont presence, symbiotic beetles were able to escape this period of vulnerability and reach high survival probabilities significantly faster than aposymbiotic beetles. To understand the mechanistic basis underlying these differences, we conducted a time-series analysis of cuticle development in symbiotic and aposymbiotic beetles by measuring cuticular melanisation and thickness. The results reveal that the symbionts accelerate their host's cuticle formation and thereby enable it to quickly reach a cuticle quality threshold that confers structural protection against predation and fungal infection. Considering the widespread occurrence of cuticle enhancement via symbiont-mediated tyrosine supplementation in beetles and other insects, our findings demonstrate how nutritional symbioses can have important ecological implications reaching beyond the immediate nutrient provisioning benefits.

Structure of an ant-myrmecophile-microbe community

Superorganismal ant colonies play host to a menagerie of symbiotic arthropods, termed myrmecophiles, which exhibit varying degrees of social integration into colony life. Such systems permit examination of how animal community interactions influence microbial assemblages. Here, we present an ecologically and phylogenetically comprehensive characterization of an ant-myrmecophile-microbe community in Southern California. Using 16S rRNA profiling, we find that microbiotas of the velvety tree ant (Liometopum occidentale) and its cohort of myrmecophiles are distinguished by species-specific characteristics but nevertheless bear signatures of their behavioral interactions. We found that the host ant microbiome was diverse at all taxonomic levels; that of a myrmecophilous cricket was moderately diverse, while microbiotas of three myrmecophilous rove beetles (Staphylinidae), which have convergently evolved symbiosis with Liometopum, were dominated by intracellular endosymbionts. Yet, despite these compositional differences, similarities between ant and myrmecophile microbiotas correlated with the nature and intimacy of their behavioral relationships. Physical interactions such as grooming and trophallaxis likely facilitate cross-species extracellular microbial sharing. Further, phylogenetic comparisons of microbiotas from myrmecophile rove beetles and outgroups revealed a lack of co-cladogenesis of beetles and intracellular endosymbionts, and limited evidence for convergence among the myrmecophiles' intracellular microbiotas. Comparative genomic analyses of the dominant Rickettsia endosymbiont of the most highly socially integrated myrmecophile imply possible functions unrelated to nutrient-provisioning in the host beetle's specialized lifestyle. Our findings indicate that myrmecophile microbiotas evolve largely independently of the constraints of deep evolutionary history, and that the transition to life inside colonies, including social interactions with hosts, plays a significant role in structuring bacterial assemblages of these symbiotic insects.

Host-associated microbial diversity in New Zealand cicadas uncovers elevational structure and replacement of obligate bacterial endosymbionts by Ophiocordyceps fungal pathogens

Host-microbe interactions influence eukaryotic evolution, particularly in the sap-sucking insects that often rely on obligate microbial symbionts to provision deficient nutrients in their diets. Cicadas (Hemiptera: Auchenorrhyncha: Cicadidae) specialize on xylem fluid and derive many essential amino acids and vitamins from intracellular bacteria or fungi (Hodgkinia, Sulcia, and Ophiocordyceps) that are propagated via transmission from mothers to offspring. Despite the beneficial role of these symbionts in nutrient provisioning, they are generally not considered to function within the gut where microbiota may play an important dietary role during insect diversification. Here, we investigate the relative impact of host phylogeny and ecology on gut microbial diversity in cicadas by sequencing 16S ribosomal RNA gene amplicons from 197 wild-collected cicadas and new mitochondrial genomes across 38 New Zealand cicada species, including natural hybrids between one species pair. We find a lack of phylogenetic structure and hybrid effects but a significant role of elevation in explaining variation in gut microbiota. Additionally, we provide evidence of Hodgkinia loss with gains of Ophiocordyceps fungal pathogens in all New Zealand cicadas examined that suggests convergent domestications of fungal pathogens. This highlights the macroevolutionary instability of obligate symbiosis and the relative importance of ecology rather than phylogeny for structuring gut microbial diversity in cicadas.

Skipping the Insect Vector: Plant Stolon Transmission of the Phytopathogen ‘Ca. Phlomobacter fragariae’ from the Arsenophonus Clade of Insect Endosymbionts

The genus Arsenophonus represents one of the most widespread clades of insect endosymbionts, including reproductive manipulators and bacteriocyte-associated primary endosymbionts. Two strains belonging to the Arsenophonus clade have been identified as insect-vectored plant pathogens of strawberry and sugar beet. The bacteria accumulate in the phloem of infected plants, ultimately causing leaf yellows and necrosis. These symbionts therefore represent excellent model systems to investigate the evolutionary transition from a purely insect-associated endosymbiont towards an insect-vectored phytopathogen. Using quantitative PCR and transmission electron microscopy, we demonstrate that ‘Candidatus Phlomobacter fragariae’, bacterial symbiont of the planthopper Cixius wagneri and the causative agent of Strawberry Marginal Chlorosis disease, can be transmitted from an infected strawberry plant to multiple daughter plants through stolons. Stolons are horizontally growing stems enabling the nutrient provisioning of daughter plants during their early growth phase. Our results show that Phlomobacter was abundant in the phloem sieve elements of stolons and was efficiently transmitted to daughter plants, which rapidly developed disease symptoms. From an evolutionary perspective, Phlomobacter is, therefore, not only able to survive within the plant after transmission by the insect vector, but can even be transmitted to new plant generations, independently from its ancestral insect host.

The evolution of a placenta is not linked to increased brain size in poeciliid fishes

ABSTRACTMaternal investment is considered to have a direct influence on the size of energetically costly organs, including the brain. In placental organisms, offspring are supplied with nutrients during pre-natal development, potentially modulating brain size. However, the coevolution of the placenta and brain size remains largely unknown in non-mammalian taxa. Here, using eight poeciliid fish species, we test if species with placental structures invest more resources into offspring brain development than species without placental structures. We predict that matrotrophy may entail higher nutrient provisioning rates to the developing embryo than lecithotrophy, resulting in larger brain sizes in offspring of matrotrophic species, and that a relatively larger part of the total brain growth would occur at younger ages (leading to a shallower ontogenetic brain size allometry). We took non-invasive brain size measurements during the first four weeks of life, and compared these to somatic growth measurements. Contrary to our expectations, we did not find any differences in brain size between the two maternal strategies. Furthermore, we did not find any differences in how relative brain size changed over ontogenetic development, between placental and non-placental species. In contrast to the marsupial/placental transition, the species investigated here only exhibit pre-natal provisioning, which may reduce the potential for maternal investment into brain size. Consequently, our results suggest that coevolution between placental structures and juvenile brain size is not a general pattern.

PSIX-25 Documenting succession of the rumen microbial community in dairy calves

Dairy cattle rely exclusively on the microbiota within their gastrointestinal tract for nutrient provisioning as they lack the endogenous enzymes needed to convert their plant-based diet into an accessible form. The acquisition of a fully functioning gut microbiome early in life is critical to survival of these animals. The establishment of a calf’s gut microbiota has previously been characterized using proxies such as fecal sampling and destructive sampling methods, but it is unclear how accurate these methods are over time in the same animals. To address this, 10 dairy calves were cannulated at 3 weeks of age. Rumen liquid and rumen solid samples were collected biweekly in congruence with buccal swabs and fecal samples from 7–17 weeks of age and characterized using Illumina 16S rRNA V4 amplicon sequencing. Fecal and buccal samples contained similar amounts of shared operational taxonomic units (OTUs) to the rumen pre-weaning but separated post-weaning such that buccal samples contained nearly double the number of shared OTUs. Beta diversity showed that fecal communities more closely resemble the rumen than buccal but shift as the animals begin ruminating such that buccal communities more closely resemble the rumen. This suggests that fecal samples would serve as a more accurate proxy prior to weaning whereas buccal samples would more accurately represent the rumen after weaning. These data will be invaluable for researchers interested in understanding the acquisition, succession, and establishment of the calf rumen microbiota using non-invasive approaches.

Compartmentalization drives the evolution of symbiotic cooperation

Across the tree of life, hosts have evolved mechanisms to control and mediate interactions with symbiotic partners. We suggest that the evolution of physical structures that allow hosts to spatially separate symbionts, termed compartmentalization, is a common mechanism used by hosts. Such compartmentalization allows hosts to: (i) isolate symbionts and control their reproduction; (ii) reward cooperative symbionts and punish or stop interactions with non-cooperative symbionts; and (iii) reduce direct conflict among different symbionts strains in a single host. Compartmentalization has allowed hosts to increase the benefits that they obtain from symbiotic partners across a diversity of interactions, including legumes and rhizobia, plants and fungi, squid and Vibrio , insects and nutrient provisioning bacteria, plants and insects, and the human microbiome. In cases where compartmentalization has not evolved, we ask why not. We argue that when partners interact in a competitive hierarchy, or when hosts engage in partnerships which are less costly, compartmentalization is less likely to evolve. We conclude that compartmentalization is key to understanding the evolution of symbiotic cooperation. This article is part of the theme issue ‘The role of the microbiome in host evolution’.

Parallel evolution in the integration of a co-obligate aphid symbiosis

SUMMARYInsects evolve dependencies - often extreme - on microbes for nutrition. These include cases where insects harbour multiple endosymbionts that function collectively as a metabolic unit. How do these metabolic co-dependencies originate, and is there a predictable sequence of events leading to the integration of new symbionts? While dependency on multiple nutrient-provisioning symbionts has evolved numerous times across sap-feeding insects, there is only one known case of metabolic co-dependency in aphids, between Buchnera aphidicola and Serratia symbiotica in the Lachninae subfamily. Here we identify three additional independent transitions to the same co-obligate symbiosis in different aphids. A comparison of recent and ancient associations allows us to investigate intermediate stages of metabolic and physical integration between the typically facultative symbiont, Serratia, and the ancient obligate symbiont Buchnera. We find that these uniquely replicated evolutionary events support the idea that co-obligate associations initiate in a predictable manner, through parallel evolutionary processes. Specifically, we show (i) how the repeated losses of the riboflavin pathway in Buchnera leads to dependency on Serratia, (ii) evidence of a stepwise process of symbiont integration, whereby dependency evolves first, then essential amino acid pathways are lost (at ~30-60MYA), which coincides with increased physical integration of the companion symbiont; and (iii) dependency can evolve prior to specialised structures (e.g. bacteriocytes), and in one case with no direct nutritional basis. More generally, our results suggest the energetic costs of synthesising nutrients may provide a unified explanation for the sequence of gene loses that occur during the evolution of co-obligate symbiosis.