Same host-plant, different sterols: variation in sterol metabolism in an insect herbivore community

Evolutionary formation of melatonin and vitamin D in early life forms: insects take centre stage

Melatonin, a product of tryptophan metabolism via serotonin, is a molecule with an indole backbone that is widely produced by bacteria, unicellular eukaryotic organisms, plants, fungi and all animal taxa. Aside from its role in the regulation of circadian rhythms, it has diverse biological actions including regulation of cytoprotective responses and other functions crucial for survival across different species. The latter properties are also shared by its metabolites including kynuric products generated by reactive oxygen species or phototransfomation induced by ultraviolet radiation. Vitamins D and related photoproducts originate from phototransformation of ∆5,7 sterols, of which 7-dehydrocholesterol and ergosterol are examples. Their ∆5,7 bonds in the B ring absorb solar ultraviolet radiation [290-315 nm, ultraviolet B (UVB) radiation] resulting in B ring opening to produce previtamin D, also referred to as a secosteroid. Once formed, previtamin D can either undergo thermal-induced isomerization to vitamin D or absorb UVB radiation to be transformed into photoproducts including lumisterol and tachysterol. Vitamin D, as well as the previtamin D photoproducts lumisterol and tachysterol, are hydroxylated by cyochrome P450 (CYP) enzymes to produce biologically active hydroxyderivatives. The best known of these is 1,25-dihydroxyvitamin D (1,25(OH)2D) for which the major function in vertebrates is regulation of calcium and phosphorus metabolism. Herein we review data on melatonin production and metabolism and discuss their functions in insects. We discuss production of previtamin D and vitamin D, and their photoproducts in fungi, plants and insects, as well as mechanisms for their enzymatic activation and suggest possible biological functions for them in these groups of organisms. For the detection of these secosteroids and their precursors and photoderivatives, as well as melatonin metabolites, we focus on honey produced by bees and on body extracts of Drosophila melanogaster. Common biological functions for melatonin derivatives and secosteroids such as cytoprotective and photoprotective actions in insects are discussed. We provide hypotheses for the photoproduction of other secosteroids and of kynuric metabolites of melatonin, based on the known photobiology of ∆5,7 sterols and of the indole ring, respectively. We also offer possible mechanisms of actions for these unique molecules and summarise differences and similarities of melatoninergic and secosteroidogenic pathways in diverse organisms including insects.

Mycobiomes of two distinct clades of ambrosia gall midges (Diptera: Cecidomyiidae) are species-specific in larvae but similar in nutritive mycelia

IMPORTANCE

Ambrosia gall midges are endophagous insect herbivores whose larvae live enclosed within a single gall for their entire development period. They may exhibit phytomycetophagy, a remarkable feeding mode that involves the consumption of plant biomass and mycelia of their cultivated gall symbionts. Thus, AGMs are ideal model organisms for studying the role of microorganisms in the evolution of host specificity in insects. However, compared to other fungus-farming insects, insect-fungus mutualism in AGMs has been neglected. Our study is the first to use DNA metabarcoding to characterize the complete mycobiome of the entire system of the gall-forming insects as we profiled gall surfaces, nutritive mycelia, and larvae. Interestingly, larval mycobiomes were significantly different from their nutritive mycelia, although Botryosphaeria dothidea dominated the nutritive mycelia, regardless of the evolutionary separation of the tribes studied. Therefore, we confirmed a long-time hypothesized paradigm for the important evolutionary association of this fungus with AGMs.

Sterol Regulation of Development and 20-Hydroxyecdysone Biosynthetic and Signaling Genes in Drosophila melanogaster

Ecdysteroids are crucial in regulating the growth and development of insects. In the fruit fly Drosophila melanogaster, both C27 and C28 ecdysteroids have been identified. While the biosynthetic pathway of the C27 ecdysteroid 20-hydroxyecdysone (20E) from cholesterol is relatively well understood, the biosynthetic pathway of C28 ecdysteroids from C28 or C29 dietary sterols remains unknown. In this study, we found that different dietary sterols (including the C27 sterols cholesterol and 7-dehydrocholesterol, the C28 sterols brassicasterol, campesterol, and ergosterol, and the C29 sterols β-sitosterol, α-spinasterol, and stigmasterol) differentially affected the expression of 20E biosynthetic genes to varying degrees, but similarly activated 20E primary response gene expression in D. melanogaster Kc cells. We also found that a single dietary sterol was sufficient to support D. melanogaster growth and development. Furthermore, the expression levels of some 20E biosynthetic genes were significantly altered, whereas the expression of 20E signaling primary response genes remained unaffected when flies were reared on lipid-depleted diets supplemented with single sterol types. Overall, our study provided preliminary clues to suggest that the same enzymatic system responsible for the classical C27 ecdysteroid 20E biosynthetic pathway also participated in the conversion of C28 and C29 dietary sterols into C28 ecdysteroids.

Diversity of Phytosterols in Leaves of Wild Brassicaceae Species as Compared to Brassica napus Cultivars: Potential Traits for Insect Resistance and Abiotic Stress Tolerance

Phytosterols are natural compounds found in all higher plants that have a wide variety of roles in plant growth regulation and stress tolerance. The phytosterol composition can also influence the development and reproductive rate of strict herbivorous insects and other important agronomic traits such as temperature and drought tolerance in plants. In this study, we analysed the phytosterol composition in 18 Brassica napus (Rapeseed/canola) cultivars and 20 accessions belonging to 10 related wild Brassicaceae species to explore diverse and novel phytosterol profiles. Plants were grown in a controlled phytotron environment and their phytosterols were analysed using a saponification extraction method followed by GC-MS from the leaf samples. The B. napus cultivars showed slight diversity in eight phytosterols (>0.02%) due to the genotypic effect, whereas the wild accessions showed significant variability in their phytosterol profiles. Of interest, a number of wild accessions were found with high levels of campesterol (HIN20, HIN23, HUN27, HIN30, SARS2, and UPM6563), stigmasterol (UPM6813, UPM6563, ALBA17, and ALBA2), and isofucosterol (SARS12, SAR6, and DMU2). These changes in individual phytosterols, or ratios of phytosterols, can have a significant implication in plant tolerance to abiotic stress and plant insect resistance properties, which can be used in breeding for crop improvement.

Bamboo charcoal mediated plant secondary metabolites biosynthesis in tomato against South American tomato pinworm (Tuta absoluta)

Introduction

The south American tomato pinworm (Tuta absoluta), an invasive insect pest species, has established itself in more than 33 Chinese prefectures, where it is mainly damaging tomato crops. Immediate efforts have been initiated to find strategies to control this pest. Studies have shown that biochar (BC) amendment to soil can enhance plant growth and resistance to herbivory.

Methods

First, we quantified the morphological performance of tomato plants grown in different coconut bran and bamboo charcoal (v/v) combinations and selected the most beneficial one. Then we checked the T. absoluta survival on the tomato plants grown in the selected bamboo charcoal combination. Finally, we studied the leaf metabolite accumulation and gene expression changes in tomato plants after growing in the selected bamboo charcoal combination.

Results

We found that the 30:1 ratio of bamboo charcoal and coconut bran is the most beneficial to tomato growth as its amendment to soil increased tomato plant height, stem thickness, and chlorophyll content, whereas, the T. absoluta survival decreased. The metabolome profiles of BC tomato leaves showed an increased accumulation of flavonoids, terpenoids, and phenolic acids compared to CK. Transcriptome sequencing resulted in the identification of 244 differentially expressed genes. Most of the upregulated genes were associated with stress-related hub proteins, flavonoid biosynthesis, MAPK and phytohormone signaling, and terpenoid biosynthesis. Additionally, the expression of many genes related to signaling and defense was changed in response to the bamboo charcoal amendment.

Discussion

We conclude that bamboo charcoal induces biosynthesis of flavonoids, terpenoids, and phenolic acids, which improve plant growth and tolerance against T. absoluta, thus reducing the survival of destructive pests.

Characterization of Microbial Communities from the Alimentary Canal of Typhaea stercorea (L.) (Coleoptera: Mycetophagidae)

Simple Summary

Hairy fungus beetle, Typhaea stercorea, is a secondary post-harvest pest of stored grains that thrives by feeding on mytoxigenic fungi. Bacterial communities residing in the alimentary canal of most insects contribute to their host’s development. While there are many examples, little is known about the role of bacterial communities in the alimentary canal of T. stercorea. The objectives of this study were to (1) characterize the microbial communities residing in T. stercorea and (2) compare the microbial compositions of field-collected and laboratory-reared populations. In this study, we were able to identify bacterial communities that possess mycolytic properties and track mark changes in the microbiota profiles associated with development. The genus Pseudomonas was enriched in T. stercorea larvae compared to adults. Furthermore, field-collected T. sterocrea adults had a lower species richness than both larva and adult laboratory-reared T. sterocrea. Moreover, the gut microbial compositions of field-collected and laboratory-reared populations were vastly different. Overall, our results suggest that the environment and physiology can shift the microbial composition in the alimentary canal of T. stercorea.

Abstract

The gut microbiomes of symbiotic insects typically mediate essential functions lacking in their hosts. Here, we describe the composition of microbes residing in the alimentary canal of the hairy fungus beetle, Typhaea stercorea (L.), at various life stages. This beetle is a post-harvest pest of stored grains that feeds on fungi and serves as a vector of mycotoxigenic fungi. It has been reported that the bacterial communities found in most insects’ alimentary canals contribute to nutrition, immune defenses, and protection from pathogens. Hence, bacterial symbionts may play a key role in the digestive system of T. stercorea. Using 16S rRNA amplicon sequencing, we examined the microbiota of T. stercorea. We found no difference in bacterial species richness between larvae and adults, but there were compositional differences across life stages (PERMANOVA:pseudo-F_(8,2) = 8.22; p = 0.026). The three most abundant bacteria found in the alimentary canal of the larvae and adults included Pseudomonas (47.67% and 0.21%, respectively), an unspecified genus of the Enterobacteriaceae family (46.60 % and 90.97%, respectively), and Enterobacter (3.89% and 5.75%, respectively). Furthermore, Pseudomonas spp. are the predominant bacteria in the larval stage. Our data indicated that field-collected T. stercorea tended to have lower species richness than laboratory-reared beetles (Shannon: H = 5.72; p = 0.057). Furthermore, the microbial communities of laboratory-reared insects resembled one another, whereas field-collected adults exhibited variability (PERMANOVA:pseudo-F_(10,3) = 4.41; p = 0.006). We provide evidence that the environment and physiology can shift the microbial composition in the alimentary canal of T. stercorea.

The Addition of Sterols and Cryoprotectants to Optimize a Diet Developed for Eldana saccharina Walker (Lepidoptera: Pyralidae) Using the Carcass Milling Technique

Various combinations and concentrations of cholesterol (C) and stigmasterol (S) were added into a base diet developed for Eldana saccharina. Survival of inoculated neonate was high on all diets (>92% at day 20 and >95% at day 27). Fastest larval development occurred on the minimum specification (MS) (+1.0 gS) and MS (+0.2 gC: 0.2 gS) diets (72 and 70% pupation respectively at day 20). Significantly slower development (15% pupation) occurred on the control diet at day 20. Female pupal weight increased when larvae fed on the MS (+0.1 gC), (+0.1 gS) and (+0.2 gC:0.2 gS) diets (0.2143 ± 0.00 g, 0.2271 ± 0.01 g and 0.2252 ± 0.01 g, respectively) as compared with the control diet (0.1886 ± 0.00 g). Adult emergence was significantly higher (100%) from the MS (+0.1 gS) and MS (+0.2 gC:0.2 gS) diets, as compared with the remaining sterol (95%) and control diets (97%). To potentially increase E. saccharina’s cold tolerance, inclusion of cryoprotectants L-proline (P) and trehalose (T) into the MS diet was investigated. Males from the MS (0.2 gP:0.2 gT), MS (0.5 gP:0.5 gT) and MS (1.0 gT) diets recovered fastest from chill coma treatment (204 ± 44 s, 215 ± 7 s and 215 ± 9 s, respectively) than those from the remaining cryoprotectant diets (305 ± 22 s). The addition of cryoprotectants severely reduced female fertility (<44%) when mated with non-chill coma exposed males. In contrast, eggs from females not exposed to chilling treatment were 84% fertile when mated with males from the same source. The MS (0.2 gC:0.2 gS) diet is the preferred choice to replace the currently used diet, reducing the larval growth period by 60% without negative effects on key life cycle parameters of E. saccharina.

Plant Metabolites Involved in the Differential Development of a Heliantheae-Specialist Insect

Balanced nutritional intake is essential to ensure that insects undergo adequate larval development and metamorphosis. Integrative multidisciplinary approaches have contributed valuable insights regarding the ecological and evolutionary outcomes of plant–insect interactions. To address the plant metabolites involved in the larval development of a specialist insect, we investigated the development of Chlosyne lacinia caterpillars fed on Heliantheae species (Tithonia diversifolia, Tridax procumbens and Aldama robusta) leaves and determined the chemical profile of plants and insects using a metabolomic approach. By means of LC-MS and GC-MS combined analyses, 51 metabolites were putatively identified in Heliantheae species and C. lacinia caterpillars and frass; these metabolites included flavonoids, sesquiterpene lactones, monoterpenoids, sesquiterpenoids, diterpenes, triterpenes, oxygenated terpene derivatives, steroids and lipid derivatives. The leading discriminant metabolites were diterpenes, which were detected only in A. robusta leaves and insects that were fed on this plant-based diet. Additionally, caterpillars fed on A. robusta leaves took longer to complete their development to the adult phase and exhibited a greater diapause rate. Hence, we hypothesized that diterpenes may be involved in the differential larval development. Our findings shed light on the plant metabolites that play roles in insect development and metabolism, opening new research avenues for integrative studies of insect nutritional ecology.

Predator Performance and Fitness Is Dictated by Herbivore Prey Type Plus Indirect Effects of their Host Plant

Animals, including herbivores and predators, use diet-mixing to balance their macro- and micronutrient intake. Recent work demonstrated that lady beetles fed only pea aphids from fava beans had reduced fitness caused by a deficiency of dietary sterols. However, beetles redressed this deficit by eating fava bean leaves. In the current study we used Coccinella septempunctata as a model to test the hypotheses that pea aphids are a poor sterol resource independent of their host plant, and that fava beans produce low quality prey regardless of aphid species. Additionally, we tested the reproductive rescue capacity of alfalfa and barley foliage compared to fava, and profiled the sterols of phloem exudates, foliage, and aphids reared on these different hosts. Beetle fecundity and egg viability was significantly better when provided pea aphids reared on alfalfa (compared to fava beans) and green peach aphids reared on fava plants. Alfalfa and barley leaves were not consumed by beetles and did not support beetle reproduction. The sterol profile of aphids largely reflected their host plant phloem. However, green peach aphids from fava acquired 125-times more sterol than pea aphids from fava. Our findings show how the sterol content of different host-plants can affect the third trophic level. Our results suggest that 1) prey quality varies depending on prey species, even when they occur on the same plant, 2) plant species can mediate prey quality, 3) host plant-mediated effects on prey quality partially drive omnivory, and 4) diet-mixing benefits growth and reproduction by redressing micronutrient deficits.

Insect Sterol Nutrition: Physiological Mechanisms, Ecology, and Applications

Insects, like all eukaryotes, require sterols for structural and metabolic purposes. However, insects, like all arthropods, cannot make sterols. Cholesterol is the dominant tissue sterol for most insects; insect herbivores produce cholesterol by metabolizing phytosterols, but not always with high efficiency. Many insects grow on a mixed-sterol diet, but this ability varies depending on the types and ratio of dietary sterols. Dietary sterol uptake, transport, and metabolism are regulated by several proteins and processes that are relatively conserved across eukaryotes. Sterol requirements also impact insect ecology and behavior. There is potential to exploit insect sterol requirements to (a) control insect pests in agricultural systems and (b) better understand sterol biology, including in humans. We suggest that future studies focus on the genetic mechanism of sterol metabolism and reverse transportation, characterizing sterol distribution and function at the cellular level, the role of bacterial symbionts in sterol metabolism, and interrupting sterol trafficking for pest control.

Multiscale Simulations of Biological Membranes: The Challenge To Understand Biological Phenomena in a Living Substance

Biological membranes are tricky to investigate. They are complex in terms of molecular composition and structure, functional over a wide range of time scales, and characterized by nonequilibrium conditions. Because of all of these features, simulations are a great technique to study biomembrane behavior. A significant part of the functional processes in biological membranes takes place at the molecular level; thus computer simulations are the method of choice to explore how their properties emerge from specific molecular features and how the interplay among the numerous molecules gives rise to function over spatial and time scales larger than the molecular ones. In this review, we focus on this broad theme. We discuss the current state-of-the-art of biomembrane simulations that, until now, have largely focused on a rather narrow picture of the complexity of the membranes. Given this, we also discuss the challenges that we should unravel in the foreseeable future. Numerous features such as the actin-cytoskeleton network, the glycocalyx network, and nonequilibrium transport under ATP-driven conditions have so far received very little attention; however, the potential of simulations to solve them would be exceptionally high. A major milestone for this research would be that one day we could say that computer simulations genuinely research biological membranes, not just lipid bilayers.

Adaptive Divergence in a Defense Symbiosis Driven from the Top Down

Most studies of adaptive radiation in animals focus on resource competition as the primary driver of trait divergence. The roles of other ecological interactions in shaping divergent phenotypes during such radiations have received less attention. We evaluate natural enemies as primary agents of diversifying selection on the phenotypes of an actively diverging lineage of gall midges on tall goldenrod. In this system, the gall of the midge consists of a biotrophic fungal symbiont that develops on host-plant leaves and forms distinctly variable protective carapaces over midge larvae. Through field studies, we show that fungal gall morphology, which is induced by midges (i.e., it is an extended phenotype), is under directional and diversifying selection by parasitoid enemies. Overall, natural enemies disruptively select for either small or large galls, mainly along the axis of gall thickness. These results imply that predators are driving the evolution of phenotypic diversity in symbiotic defense traits in this system and that divergence in defensive morphology may provide ecological opportunities that help to fuel the adaptive radiation of this genus of midges on goldenrods. This enemy-driven phenotypic divergence in a diversifying lineage illustrates the potential importance of consumer-resource and symbiotic species interactions in adaptive radiation.

Sterol targeting drugs reveal life cycle stage-specific differences in trypanosome lipid rafts

Cilia play important roles in cell signaling, facilitated by the unique lipid environment of a ciliary membrane containing high concentrations of sterol-rich lipid rafts. The African trypanosome Trypanosoma brucei is a single-celled eukaryote with a single cilium/flagellum. We tested whether flagellar sterol enrichment results from selective flagellar partitioning of specific sterol species or from general enrichment of all sterols. While all sterols are enriched in the flagellum, cholesterol is especially enriched. T. brucei cycles between its mammalian host (bloodstream cell), in which it scavenges cholesterol, and its tsetse fly host (procyclic cell), in which it both scavenges cholesterol and synthesizes ergosterol. We wondered whether the insect and mammalian life cycle stages possess chemically different lipid rafts due to different sterol utilization. Treatment of bloodstream parasites with cholesterol-specific methyl-β-cyclodextrin disrupts both membrane liquid order and localization of a raft-associated ciliary membrane calcium sensor. Treatment with ergosterol-specific amphotericin B does not. The opposite results were observed with ergosterol-rich procyclic cells. Further, these agents have opposite effects on flagellar sterol enrichment and cell metabolism in the two life cycle stages. These findings illuminate differences in the lipid rafts of an organism employing life cycle-specific sterols and have implications for treatment.

The importance of pollen chemistry in evolutionary host shifts of bees

Although bee-plant associations are generally maintained through speciation processes, host shifts have occurred during evolution. Understanding shifts between both phylogenetically and morphologically unrelated plants (i.e., host-saltation) is especially important since they could have been key processes in the origin and radiation of bees. Probably far from being a random process, such host-saltation might be driven by hidden constraints associated with plant traits. We selected two clades of oligolectic bees (i.e., Colletes succinctus group and Melitta leporina group) foraging on co-flowering but unrelated host-plants to test this hypothesis. We analyzed floral scent, floral color and chemical composition of pollen from host and non-host plants of these two clades. We did not find evidence for host-plant evolution in the Melitta leporina group driven by one of the assayed floral traits. On the contrary, hosts of the C. succinctus group display similar primary nutritive content of pollen (i.e., amino acids and sterols) but not similar floral scent or color, suggesting that shared pollen chemistry probably mediates saltation in this clade. Our study revealed that constraints shaping floral associations are diverse and clearly depend on species life-history traits, but evidence suggests that pollen chemistry may act as a major floral filter and guide evolutionary host-shifts.

Diet micronutrient balance matters: How the ratio of dietary sterols/steroids affects development, growth and reproduction in two lepidopteran insects

Insects lack the ability to synthesize sterols de novo so they acquire this essential nutrient from their food. Cholesterol is the dominant sterol found in most insects, but in plant vegetative tissue it makes up only a small fraction of the total sterol profile. Instead, plants mostly contain phytosterols; plant-feeding insects generate the majority of their cholesterol by metabolizing phytosterols. However, not all phytosterols are readily converted to cholesterol, and some are even deleterious when ingested above a threshold level. In a recent study we showed that caterpillars reared on tobacco accumulating novel sterols/steroids exhibited reduced performance, even when suitable sterols were present. In the current study we examined how the dominant sterols (cholesterol and stigmasterol) and steroids (cholestanol and cholestanone) typical of the modified tobacco plants affected two insect herbivores (Heliothis virescens and Helicoverpa zea). The sterols/steroids were incorporated into synthetic diets singly, as well as in various combinations, ratios and amounts. For each insect species, a range of performance values was recorded for two generations, with the eggs from the 1st-generation adults as the source of neonates for the 2nd-generation. Performance on the novel steroids (cholestanol and cholestanone) was extremely poor compared to suitable sterols (cholesterol and stigmasterol). Additionally, performance tended to decrease as the ratio of the novel dietary steroids increased. We discuss how the balance of different dietary sterols/steroids affected our two caterpillar species, relate this back to recent studies on sterol/steroid metabolism in these two species, and consider the potential application of sterol/steroid modification in crops.

Developmental changes in the sterol composition and the glycerol content of cuticular and internal lipids of three species of flies

The glycerol concentration and the composition of cuticular and internal sterols in three medically and forensically important fly species, viz., Musca domestica, Sarcophaga carnaria, and Calliphora vicina, were analyzed. The cuticular and internal lipid extracts were separated by HPLC-LLSD, after which the sterol fraction was characterized by GC/MS in total ion current (TIC) mode. The cuticular lipids of M. domestica larvae contained seven sterols, while in pupae and females, six sterols were identified. Five sterols were found in the cuticular lipids of M. domestica males. The internal lipids of M. domestica larvae and pupae contained six and seven sterols, respectively, while those of male and female flies contained only five sterols. Sitosterol, cholesterol, and campesterol were the dominant sterols in M. domestica, while campestanol, stigmasterol, sitostanol, and fucosterol were identified in low concentrations or in traces. In contrast, cuticular and internal lipids of S. carnaria and C. vicina contained only cholesterol. Glycerol was identified in all stages of M. domestica, S. carnaria, and C. vicina. For all the three examined fly species, the present study clearly showed species-specific developmental changes in the composition of cuticular and internal sterols as well as in the glycerol concentration.

A dietary test of putative deleterious sterols for the aphid Myzus persicae

The aphid Myzus persicae displays high mortality on tobacco plants bearing a transgene which results in the accumulation of the ketosteroids cholestan-3-one and cholest-4-en-3-one in the phloem sap. To test whether the ketosteroids are the basis of the plant resistance to the aphids, M. persicae were reared on chemically-defined diets with different steroid contents at 0.1-10 µg ml(-1). Relative to sterol-free diet and dietary supplements of the two ketosteroids and two phytosterols, dietary cholesterol significantly extended aphid lifespan and increased fecundity at one or more dietary concentrations tested. Median lifespan was 50% lower on the diet supplemented with cholest-4-en-3-one than on the cholesterol-supplemented diet. Aphid feeding rate did not vary significantly across the treatments, indicative of no anti-feedant effect of any sterol/steroid. Aphids reared on diets containing equal amounts of cholesterol and cholest-4-en-3-one showed fecundity equivalent to aphids on diets containing only cholesterol. Aphids were reared on diets that reproduced the relative steroid abundance in the phloem sap of the control and modified tobacco plants, and their performance on the two diet formulations was broadly equivalent. We conclude that, at the concentrations tested, plant ketosteroids support weaker aphid performance than cholesterol, but do not cause acute toxicity to the aphids. In plants, the ketosteroids may act synergistically with plant factors absent from artificial diets but are unlikely to be solely responsible for resistance of modified tobacco plants.

How firebrats (Thysanura: Lepismatidae) detect and nutritionally benefit from their microbial symbionts Enterobacter cloacae and Mycotypha microspora

The phylogenetically ancient firebrats, Thermobia domestica (Packard) (Thysanura: Lepismatidae), lack any form of long-distance communication, yet are able to locate mates in sustained hot and humid microhabitats, typically within human habitations where they feed on dried goods, including cellulosic substrates. We have recently shown that firebrats aggregate in response to two symbiotic microorganisms in their feces, the bacterium Enterobacter cloacae and the fungus Mycotypha microspora. Our objectives were to determine how firebrats detect M. microspora and E. cloacae, and whether these microbial symbionts nutritionally benefit firebrats. Applied to a glass surface in bioassays, E. cloacae as well as the isolated exopolysaccharide of E. cloacae induced arrestment of firebrats, whereas M. microspora induced arrestment only in the presence of cellulosic substrate. When M. microspora and E. cloacae were grown aerobically on cellulose agar, only M. microspora yielded zones of clearing indicative of enzymatic cellulose degradation. Firebrats also arrested in response to d-glucose, which is a constituent of the exopolysaccharide and which is produced by the cellulase of M. microspora during cellulose degradation. First- to third-instar nymphs of firebrats that were fed E. cloacae, M. microspora, or a nutrient-rich diet developed equally well. By consuming E. cloacae and M. microspora, and by spreading them through feces, firebrats appear able to occupy nutrient-poor habitats that otherwise would not support development of their offspring.

Molecular phylogeny, biogeography, and host plant shifts in the bee genus Melitta (Hymenoptera: Anthophila)

New molecular studies suggested that the family Melittidae is either a paraphyletic group from which all the other bees are derived, or the sister clade to all other existing bees. Studying the historical biogeography and evolution of each major lineage within this group is a key step to understand the origin and early radiation of bees. Melitta is the largest genus of melittid bees, for which a robust molecular phylogeny and a biogeographic analysis are still lacking. Here, we derive a phylogenetic hypothesis from the sequences of seven independent DNA fragments of mitochondrial and nuclear origin. This phylogenetic hypothesis is then used to infer the evolution of the species range and of the host-plant shifts in Melitta. Our results confirmed the monophyly of Melitta, but did not recover all previously defined clades within the genus. We propose new taxa by splitting the genus in three subgenera (including two new subgenera described in the Appendix: Afromelitta subgen. nov., Plesiomelitta subgen. nov.) and describe two new species: Melitta avontuurensis sp. n. and M. richtersveldensis sp. n. Regarding the evolution of host-plant use, our analysis suggests that all species currently specialized on one plant family originated from an ancestor that was specialized on Fabaceae plants. The inferred biogeographic history for the genus supported an African origin. In concordance with previous studies identifying Africa as the geographic origin for many clades of bees, our data bring new evidence for an African origin of melittid bees.

Sterol/steroid metabolism and absorption in a generalist and specialist caterpillar: effects of dietary sterol/steroid structure, mixture and ratio

Insects cannot synthesize sterols de novo, so they typically require a dietary source. Cholesterol is the dominant sterol in most insects, but because plants contain only small amounts of cholesterol, plant-feeding insects generate most of their cholesterol by metabolizing plant sterols. Plants almost always contain mixtures of different sterols, but some are not readily metabolized to cholesterol. Here we explore, in two separate experiments, how dietary phytosterols and phytosteroids, in different mixtures, ratios, and amounts, affect insect herbivore sterol/steroid metabolism and absorption; we use two caterpillars species - one a generalist (Heliothis virescens), the other a specialist (Manduca sexta). In our first experiment caterpillars were reared on two tobacco lines - one expressing a typical phystosterol profile, the other expressing high amounts/ratios of stanols and 3-ketosteroids. Caterpillars reared on the control tobacco contained mostly cholesterol, but those reared on the modified tobacco had reduced amounts of cholesterol, and lower total sterol/steroid body profiles. In our second experiment, caterpillars were reared on artificial diets containing known amounts of cholesterol, stigmasterol, cholestanol and/or cholestanone, either singly or in various combinations and ratios. Cholesterol and stigmasterol-reared moths were mostly cholesterol, while cholestanol-reared moths were mostly cholestanol. Moth tissue cholesterol concentration tended to decrease as the ratio of dietary cholestanol and/or cholestanone increased. In both moths cholestanone was metabolized into cholestanol and epicholestanol. Interestingly, M. sexta generated much more cholestanol than epicholestanol, while H. virescens did the opposite. Finally, total tissue steroid levels were significantly reduced in moths reared on diets containing very high levels of cholestanol. We discuss how dietary sterol/steroid structural differences are important with respect to sterol/steroid metabolism and uptake, including species-specific differences.

Symbiosis catalyses niche expansion and diversification

Interactions between species are important catalysts of the evolutionary processes that generate the remarkable diversity of life. Symbioses, conspicuous and inherently interesting forms of species interaction, are pervasive throughout the tree of life. However, nearly all studies of the impact of species interactions on diversification have concentrated on competition and predation leaving unclear the importance of symbiotic interaction. Here, I show that, as predicted by evolutionary theories of symbiosis and diversification, multiple origins of a key innovation, symbiosis between gall-inducing insects and fungi, catalysed both expansion in resource use (niche expansion) and diversification. Symbiotic lineages have undergone a more than sevenfold expansion in the range of host-plant taxa they use relative to lineages without such fungal symbionts, as defined by the genetic distance between host plants. Furthermore, symbiotic gall-inducing insects are more than 17 times as diverse as their non-symbiotic relatives. These results demonstrate that the evolution of symbiotic interaction leads to niche expansion, which in turn catalyses diversification.

Plant phloem sterol content: forms, putative functions, and implications for phloem-feeding insects

All eukaryotes contain sterols, which serve as structural components in cell membranes, and as precursors for important hormones. Plant vegetative tissues are known to contain mixtures of sterols, but very little is known about the sterol composition of phloem. Plants are food for many animals, but plant-feeding arthropods (including phloem-feeding insets) are unique among animals in that they have lost the ability to synthesize sterols, and must therefore acquire these essential nutrients from their food, or via endosymbionts. Our paper starts by providing a very brief overview of variation in plant sterol content, and how different sterols can affect insect herbivores, including those specializing on phloem. We then describe an experiment, where we bulk collected phloem sap exudate from bean and tobacco, and analyzed its sterol content. This approach revealed two significant observations concerning phloem sterols. First, the phloem exudate from each plant was found to contain sterols in three different fractions - free sterols, sterols conjugated to lipids (acylated), and sterols conjugated to carbohydrates (glycosylated). Second, for both plants, cholesterol was identified as the dominant sterol in each phloem exudate fraction; the remaining sterols in each fraction were a mixture of common phytosterols. We discuss our phloem exudate sterol profiles in a plant physiology/biochemistry context, and how it relates to the nutritional physiology/ecology of phloem-feeding insects. We close by proposing important next steps that will advance our knowledge concerning plant phloem sterol biology, and how phloem-sterol content might affect phloem-feeding insects.

Role of the phloem in the biochemistry and ecophysiology of benzylisoquinoline alkaloid metabolism

Benzylisoquinoline alkaloids (BIAs) are a diverse group of biologically active specialized metabolites produced mainly in four plant families. BIA metabolism is likely of monophyletic origin and involves multiple enzymes yielding structurally diverse compounds. Several BIAs possess defensive properties against pathogenic microorganisms and herbivores. Opium poppy (Papaver somniferum: Papaveraceae) has emerged as a model system to investigate the cellular localization of BIA biosynthesis. Although alkaloids accumulate in the laticifer cytoplasm (latex) of opium poppy, corresponding biosynthetic enzymes and gene transcripts are localized to proximal sieve elements and companion cells, respectively. In contrast, BIA metabolism in the non-laticiferous meadow rue (Thalictrum flavum; Ranunculaceae) occurs independent of the phloem. Evidence points toward the adoption of diverse strategies for the biosynthesis and accumulation of alkaloids as defensive compounds. Recruitment of cell types involved in BIA metabolism, both within and external to the phloem, was likely driven by selection pressures unique to different taxa. The biochemistry, cell biology, ecophysiology, and evolution of BIA metabolism are considered in this context.

The physiology of sterol nutrition in the pea aphid Acyrthosiphon pisum

The phloem sap of fava bean (Vicia faba) plants utilized by the pea aphid Acyrthosiphon pisum contains three sterols, cholesterol, stigmasterol and sitosterol, in a 2:2:1 ratio. To investigate the nutritional value of these sterols, pea aphids were reared on chemically-defined diets containing each sterol at 0.1, 1 and 10μgml(-1) with a sterol-free diet as control. Larval growth rate and aphid lifespan did not vary significantly across the diets, indicating that sterol reserves can buffer some performance indices against a shortfall in dietary sterol over at least one generation. However, lifetime reproductive output was depressed in aphids on diets containing stigmasterol or no sterol, relative to diets supplemented with cholesterol or sitosterol. The cholesterol density of embryos in teneral adults was significantly higher than in the total body; and the number and biomass of embryos in aphids on diets with stigmasterol and no sterols were reduced relative to diets with cholesterol or sitosterol, indicating that the reproductive output of the pea aphid can be limited by the amount and composition of dietary sterol. In a complementary RNA-seq analysis of pea aphids reared on plants and diets with different sterol contents, 7.6% of the 17,417 detected gene transcripts were differentially expressed. Transcript abundance of genes with annotated function in sterol utilization did not vary significantly among treatments, suggesting that the metabolic response to dietary sterol may be mediated primarily at the level of enzyme function or metabolite concentration.

Rampant host- and defensive phenotype-associated diversification in a goldenrod gall midge

Natural selection can play an important role in the genetic divergence of populations and their subsequent speciation. Such adaptive diversification, or ecological speciation, might underlie the enormous diversity of plant-feeding insects that frequently experience strong selection pressures associated with host plant use as well as from natural enemies. This view is supported by increasing documentation of host-associated (genetic) differentiation in populations of plant-feeding insects using alternate hosts. Here, we examine evolutionary diversification in a single nominal taxon, the gall midge Asteromyia carbonifera (O.S.), with respect to host plant use and gall phenotype. Because galls can be viewed as extended defensive phenotypes of the midges, gall morphology is likely to be a reflection of selective pressures by enemies. Using phylogenetic and comparative analyses of mtDNA and nuclear sequence data, we find evidence that A. carbonifera populations are rapidly diversifying along host plant and gall morphological lines. At a broad scale, geography explains surprisingly little genetic variation, and there is little evidence of strict co-cladogenesis with their Solidago hosts. Gall morphology is relatively labile, distinct gall morphs have evolved repeatedly and colonized multiple hosts, and multiple genetically and morphologically distinct morphs frequently coexist on a single host plant species. These results suggest that Asteromyia carbonifera is in the midst of an adaptive radiation driven by multitrophic selective pressures. Similar complex community pressures are likely to play a role in the diversification of other herbivorous insect groups.

Symbiotic fungal flora in leaf galls induced by Illiciomyia yukawai (Diptera: Cecidomyiidae) and in its mycangia

We investigated the association between a gall midge, Illiciomyia yukawai, and its symbiotic fungi on Japanese star anise, Illicium anisatum. The number of fungal species isolated from the galls increased with development of the galls, whereas those from the leaves showed a different trend. Botryosphaeria dothidea was dominant in the galls from June to October, and after that Phomopsis sp. 1, Colletotrichum sp., and Pestalotiopsis sp. became dominant. Although B. dothidea was not isolated from the leaves, it was detected from mycangia (abdominal sternite VII) of egg-laying adults at a high isolation frequency (>90%). However, B. dothidea was not isolated from mycangia of adults emerging from galls that were enclosed by plastic bags. This indicates that I. yukawai is closely associated with B. dothidea and that its newly emerged adults do not take the fungus into mycangia directly from the galls where they had developed. Also, the fungus from the fungal layers of ambrosia galls has less ability to propagate on artificial media despite the presence of its mycelial mass in mature galls.