An exploration of the fungal assemblage in each life history stage of the butterfly, Lycaeides melissa (Lycaenidae), as well as its host plant Astragalus canadensis (Fabaceae)

Low microbial diversity, yeast prevalence, and nematode-trapping fungal presence in fungal colonization and leaf microbiome of Serjania erecta

Medicinal plant microbiomes undergo selection due to secondary metabolite presence. Resident endophytic/epiphytic microorganisms directly influence plant's bioactive compound synthesis. Hypothesizing low microbial diversity in Serjania erecta leaves, we assessed leaf colonization by epiphytic and endophytic fungi. Given its traditional medicinal importance, we estimated diversity in the endophytic fungal microbiome. Analyses included scanning electron microscopy (SEM), isolation of cultivable species, and metagenomics. Epiphytic fungi interacted with S. erecta leaf tissues, horizontally transmitted via stomata/trichome bases, expressing traits for nematode trapping. Cultivable endophytic fungi, known for phytopathogenic habits, didn't induce dysbiosis symptoms. This study confirms low leaf microbiome diversity in S. erecta, with a tendency towards more fungal species, likely due to antibacterial secondary metabolite selection. The classification of Halicephalobus sp. sequence corroborated the presence of nematode eggs on the epidermal surface of S. erecta by SEM. In addition, we confirmed the presence of methanogenic archaea and a considerable number of methanotrophs of the genus Methylobacterium. The metagenomic study of endophytic fungi highlighted plant growth-promoting yeasts, mainly Malassezia, Leucosporidium, Meyerozyma, and Hannaella. Studying endophytic fungi and S. erecta microbiomes can elucidate their impact on beneficial bioactive compound production, on the other hand, it is possible that the bioactive compounds produced by this plant can recruit specific microorganisms, impacting the biological system.

Toward an Integrated Understanding of the Lepidoptera Microbiome

Research over the past 30 years has led to a widespread acceptance that insects establish widespread and diverse associations with microorganisms. More recently, microbiome research has been accelerating in lepidopteran systems, leading to a greater understanding of both endosymbiont and gut microorganisms and how they contribute to integral aspects of the host. Lepidoptera are associated with a robust assemblage of microorganisms, some of which may be stable and routinely detected in larval and adult hosts, while others are ephemeral and transient. Certain microorganisms that populate Lepidoptera can contribute significantly to the hosts' performance and fitness, while others are inconsequential. We emphasize the context-dependent nature of the interactions between players. While our review discusses the contemporary literature, there are major avenues yet to be explored to determine both the fundamental aspects of host-microbe interactions and potential applications for the lepidopteran microbiome; we describe these avenues after our synthesis.

Sources of Fungal Symbionts in the Microbiome of a Mobile Insect Host, Spodoptera frugiperda

The sources of fungal symbionts of insects are not well understood, yet the acquisition and assembly of fungal communities in mobile insect hosts have important implications for the ecology of migratory insects and their plant hosts. To determine potential sources of fungi associated with the fall armyworm (Spodoptera frugiperda), we characterized the fungal communities associated with four different ecological compartments (insects, infested leaves, uninfested leaves, and soil) and estimated the contributions of each of these potential sources to the insect's fungal microbiome. Results show that insect fungal community composition was distinct from and more varied than the composition of fungal communities in the environment of those insects (plants and soil). Among the sources evaluated, on average we found a surprisingly large apparent contribution from other congeneric S. frugiperda insect larvae (ca. 25%) compared to the contribution from soil or plant sources (< 5%). However, a large proportion of the insect microbiome could not be attributed to the sampled sources and was instead attributed to unknown sources (ca. 50%). Surprisingly, we found little evidence for exchange of fungal taxa, with the exception of a Fusarium oxysporum and a Cladosporium sp. OTU, between larvae and the infested leaves on which they fed. Together, our results suggest that mobile insects such as S. frugiperda obtain their fungal symbionts from a variety of sources, not limited to plants and soil, but including conspecific insects and other unsampled environmental sources, and that transmission among insects may play an important role in acquisition of fungal symbionts.

Diversity and Functional Roles of the Gut Microbiota in Lepidopteran Insects

Lepidopteran insects are one of the most widespread and speciose lineages on Earth, with many common pests and beneficial insect species. The evolutionary success of their diversification depends on the essential functions of gut microorganisms. This diverse gut microbiota of lepidopteran insects provides benefits in nutrition and reproductive regulation and plays an important role in the defence against pathogens, enhancing host immune homeostasis. In addition, gut symbionts have shown promising applications in the development of novel tools for biological control, biodegradation of waste, and blocking the transmission of insect-borne diseases. Even though most microbial symbionts are unculturable, the rapidly expanding catalogue of microbial genomes and the application of modern genetic techniques offer a viable alternative for studying these microbes. Here, we discuss the gut structure and microbial diversity of lepidopteran insects, as well as advances in the understanding of symbiotic relationships and interactions between hosts and symbionts. Furthermore, we provide an overview of the function of the gut microbiota, including in host nutrition and metabolism, immune defence, and potential mechanisms of detoxification. Due to the relevance of lepidopteran pests in agricultural production, it can be expected that the research on the interactions between lepidopteran insects and their gut microbiota will be used for biological pest control and protection of beneficial insects in the future.

Heliconius Butterflies Host Characteristic and Phylogenetically Structured Adult-Stage Microbiomes

Lepidoptera (butterflies and moths) are diverse and ecologically important, yet we know little about how they interact with microbes as adults. Due to metamorphosis, the form and function of their adult-stage microbiomes might be very different from those of microbiomes in the larval stage (caterpillars). We studied adult-stage microbiomes of Heliconius and closely related passion-vine butterflies (Heliconiini), which are an important model system in evolutionary biology. To characterize the structure and dynamics of heliconiine microbiomes, we used field collections of wild butterflies, 16S rRNA gene sequencing, quantitative PCR, and shotgun metagenomics. We found that Heliconius butterflies harbor simple and abundant bacterial communities that are moderately consistent among conspecific individuals and over time. Heliconiine microbiomes also exhibited a strong signal of the host phylogeny, with a major distinction between Heliconius and other butterflies. These patterns were largely driven by differing relative abundances of bacterial phylotypes shared among host species and genera, as opposed to the presence or absence of host-specific phylotypes. We suggest that the phylogenetic structure in heliconiine microbiomes arises from conserved host traits that differentially filter microbes from the environment. While the relative importance of different traits remains unclear, our data indicate that pollen feeding (unique to Heliconius) is not a primary driver. Using shotgun metagenomics, we also discovered trypanosomatids and microsporidia to be prevalent in butterfly guts, raising the possibility of antagonistic interactions between eukaryotic parasites and colocalized gut bacteria. Our discovery of characteristic and phylogenetically structured microbiomes provides a foundation for tests of adult-stage microbiome function, a poorly understood aspect of lepidopteran biology.IMPORTANCE Many insects host microbiomes with important ecological functions. However, the prevalence of this phenomenon is unclear because in many insect taxa, microbiomes have been studied in only part of the life cycle, if at all. A prominent example is butterflies and moths, in which the composition and functional role of adult-stage microbiomes are largely unknown. We comprehensively characterized microbiomes in adult passion-vine butterflies. Butterfly-associated bacterial communities are generally abundant in guts, consistent within populations, and composed of taxa widely shared among hosts. More closely related butterflies harbor more similar microbiomes, with the most dramatic shift in microbiome composition occurring in tandem with a suite of ecological and life history traits unique to the genus Heliconius Butterflies are also frequently infected with previously undescribed eukaryotic parasites, which may interact with bacteria in important ways. These findings advance our understanding of butterfly biology and insect-microbe interactions generally.

Host plant-dependent effects of microbes and phytochemistry on the insect immune response

Herbivorous insects can defend themselves against pathogens via an immune response, which is influenced by the nutritional quality and phytochemistry of the host plant. However, it is unclear how these aspects of diet interact to influence the insect immune response and what role is played by ingested foliar microbes. We examined dietary protein, phytochemistry, and the caterpillar microbiome to understand variation in immune response of the Melissa blue butterfly, Lycaeides melissa. We also asked if these factors have host plant-specific effects by measuring L. melissa immune response when reared on a recently colonized exotic host plant (Medicago sativa) as compared to the immune response on an ancestral, native host (Astragalus canadensis). L. melissa did not experience immunological benefits directly related to consumption of the novel plant M. sativa. However, we did find negative, direct effects of phytochemical diversity and negative, direct effects of diet-derived microbial diversity on constitutive immune response for caterpillars fed M. sativa, as measured by phenoloxidase activity. Foliar protein did not directly influence the immune response, but did do so indirectly by increasing weight gain. Our results highlight the important effects of host diet on caterpillar physiology and raise the possibility that foliar microbiota, despite being rapidly passed through the gut, can affect the caterpillar immune response.

Meta-Omics Tools in the World of Insect-Microorganism Interactions

Microorganisms are able to influence several aspects of insects’ life, and this statement is gaining increasing strength, as research demonstrates it daily. At the same time, new sequencing technologies are now available at a lower cost per base, and bioinformatic procedures are becoming more user-friendly. This is triggering a huge effort in studying the microbial diversity associated to insects, and especially to economically important insect pests. The importance of the microbiome has been widely acknowledged for a wide range of animals, and also for insects this topic is gaining considerable importance. In addition to bacterial-associates, the insect-associated fungal communities are also gaining attention, especially those including plant pathogens. The use of meta-omics tools is not restricted to the description of the microbial world, but it can be also used in bio-surveillance, food safety assessment, or even to bring novelties to the industry. This mini-review aims to give a wide overview of how meta-omics tools are fostering advances in research on insect-microorganism interactions.

Alternate life history phases of a common seaweed have distinct microbial surface communities

Macroalgal life histories are complex, often involving the alternation of distinct free-living life history phases that differ in morphology, longevity and ploidy. The surfaces of marine macroalgae support diverse microbial biofilms, yet the degree of microbial variation between alternate phases is unknown. We quantified bacterial (16S rRNA gene) and microeukaryote (18S rRNA gene) communities on the surface of the common intertidal seaweed, Mastocarpus spp., which alternates between gametophyte (foliose, haploid) and sporophyte (encrusting, diploid) life history phases. A large portion (97%) of bacterial taxa on the surface Mastocarpus was also present in samples from the environment, indicating that macroalgal surface communities are largely assembled from the surrounding seawater. Still, changes in the relative abundance of bacterial taxa result in significantly different communities on alternate Mastocarpus life history phases, rocky substrate and seawater at all intertidal elevations. For microeukaryote assemblages, only high intertidal samples had significant differences between life history phases although sporophytes were not different from the rocky substrate at this elevation; gametophytes and sporophytes did not differ in microeukaryote communities in the mid and low zones. By sequencing three host genes, we identified three cryptic species of Mastocarpus in our data set, which co-occur in the mid-to-low intertidal zone. In these samples, M. alaskensis sporophytes harboured distinct bacterial communities compared to M. agardhii and M. intermedius sporophytes, which were not distinguishable. Conversely, microeukaryote communities did not differ among species.

Sources of Variation in the Gut Microbial Community of Lycaeides melissa Caterpillars

Microbes can mediate insect-plant interactions and have been implicated in major evolutionary transitions to herbivory. Whether microbes also play a role in more modest host shifts or expansions in herbivorous insects is less clear. Here we evaluate the potential for gut microbial communities to constrain or facilitate host plant use in the Melissa blue butterfly (Lycaeides melissa). We conducted a larval rearing experiment where caterpillars from two populations were fed plant tissue from two hosts. We used 16S rRNA sequencing to quantify the relative effects of sample type (frass versus whole caterpillar), diet (plant species), butterfly population and development (caterpillar age) on the composition and diversity of the caterpillar gut microbial communities, and secondly, to test for a relationship between microbial community and larval performance. Gut microbial communities varied over time (that is, with caterpillar age) and differed between frass and whole caterpillar samples. Diet (host plant) and butterfly population had much more limited effects on microbial communities. We found no evidence that gut microbe community composition was associated with caterpillar weight, and thus, our results provide no support for the hypothesis that variation in microbial community affects performance in L. melissa.

Microbial Communities of Lycaenid Butterflies Do Not Correlate with Larval Diet

Herbivores possess many counteradaptations to plant defenses, and a growing body of research describes the role of symbiotic gut bacteria in mediating herbivorous diets among insects. However, persistent bacterial symbioses have not been found in Lepidoptera, despite the fact that perhaps 99% of the species in this order are herbivorous. We surveyed bacterial communities in the guts of larvae from 31 species of lycaenid butterflies whose caterpillars had diets ranging from obligate carnivory to strict herbivory. Contrary to our expectations, we found that the bacterial communities of carnivorous and herbivorous caterpillars do not differ in richness, diversity, or composition. Many of the observed bacterial genera are commonly found in soil and plant surfaces, and we detected known homopteran endosymbionts in the guts of homopterophagous species, suggesting that larvae acquire gut bacteria from their food and environment. These results indicate that lycaenid butterflies do not rely on specific bacterial symbioses to mediate their diverse diets, and provide further evidence of taxonomically depauperate bacterial communities among Lepidoptera.