Symbiotic microorganisms in Puto superbus (Leonardi, 1907) (Insecta, Hemiptera, Coccomorpha: Putoidae)

Insect Bacteriocytes: Adaptation, Development, and Evolution

Bacteriocytes are host cells specialized to harbor symbionts in certain insect taxa. The adaptation, development, and evolution of bacteriocytes underlie insect symbiosis maintenance. Bacteriocytes carry enriched host genes of insect and bacterial origin whose transcription can be regulated by microRNAs, which are involved in host-symbiont metabolic interactions. Recognition proteins of peptidoglycan, the bacterial cell wall component, and autophagy regulate symbiont abundance in bacteriocytes. Horizontally transferred genes expressed in bacteriocytes influence the metabolism of symbiont peptidoglycan, which may affect the bacteriocyte immune response against symbionts. Bacteriocytes release or transport symbionts into ovaries for symbiont vertical transmission. Bacteriocyte development and death, regulated by transcriptional factors, are variable in different insect species. The evolutionary origin of insect bacteriocytes remains unclear. Future research should elucidate bacteriocyte cell biology, the molecular interplay between bacteriocyte metabolic and immune functions, the genetic basis of bacteriocyte origin, and the coordination between bacteriocyte function and host biology in diverse symbioses.

Nutrient supplementation by genome-eroded Burkholderia symbionts of scale insects

Hemipterans are known as hosts to bacterial or fungal symbionts that supplement their unbalanced diet with essential nutrients. Among them, scale insects (Coccomorpha) are characterized by a particularly large diversity of symbiotic systems. Here, using microscopic and genomic approaches, we functionally characterized the symbionts of two scale insects belonging to the Eriococcidae family, Acanthococcus aceris and Gossyparia spuria. These species host Burkholderia bacteria that are localized in the cytoplasm of the fat body cells. Metagenome sequencing revealed very similar and highly reduced genomes (<900KBp) with a low GC content (~38%), making them the smallest and most AT-biased Burkholderia genomes yet sequenced. In their eroded genomes, both symbionts retain biosynthetic pathways for the essential amino acids leucine, isoleucine, valine, threonine, lysine, arginine, histidine, phenylalanine, and precursors for the semi-essential amino acid tyrosine, as well as the cobalamin-dependent methionine synthase MetH. A tryptophan biosynthesis pathway is conserved in the symbiont of G. spuria, but appeared pseudogenized in A. aceris, suggesting differential availability of tryptophan in the two host species' diets. In addition to the pathways for essential amino acid biosynthesis, both symbionts maintain biosynthetic pathways for multiple cofactors, including riboflavin, cobalamin, thiamine, and folate. The localization of Burkholderia symbionts and their genome traits indicate that the symbiosis between Burkholderia and eriococcids is younger than other hemipteran symbioses, but is functionally convergent. Our results add to the emerging picture of dynamic symbiont replacements in sap-sucking Hemiptera and highlight Burkholderia as widespread and versatile intra- and extracellular symbionts of animals, plants, and fungi.

Solitary bee larvae modify bacterial diversity of pollen provisions in the stem-nesting bee, Osmia cornifrons (Megachilidae)

Microbes, including diverse bacteria and fungi, play an important role in the health of both solitary and social bees. Among solitary bee species, in which larvae remain in a closed brood cell throughout development, experiments that modified or eliminated the brood cell microbiome through sterilization indicated that microbes contribute substantially to larval nutrition and are in some cases essential for larval development. To better understand how feeding larvae impact the microbial community of their pollen/nectar provisions, we examine the temporal shift in the bacterial community in the presence and absence of actively feeding larvae of the solitary, stem-nesting bee, Osmia cornifrons (Megachilidae). Our results indicate that the O. cornifrons brood cell bacterial community is initially diverse. However, larval solitary bees modify the microbial community of their pollen/nectar provisions over time by suppressing or eliminating rare taxa while favoring bacterial endosymbionts of insects and diverse plant pathogens, perhaps through improved conditions or competitive release. We suspect that the proliferation of opportunistic plant pathogens may improve nutrient availability of developing larvae through degradation of pollen. Thus, the health and development of solitary bees may be interconnected with pollen bacterial diversity and perhaps with the propagation of plant pathogens.

Alternative Transmission Patterns in Independently Acquired Nutritional Cosymbionts of Dictyopharidae Planthoppers

Sap-sucking hemipterans host specialized, heritable microorganisms that supplement their diet with essential nutrients. These microbes show unusual features that provide a unique perspective on the coevolution of host-symbiont systems but are still poorly understood. Here, we combine microscopy with high-throughput sequencing to revisit 80-year-old reports on the diversity of symbiont transmission modes in a broadly distributed planthopper family, Dictyopharidae. We show that in seven species examined, the ancestral nutritional symbionts Sulcia and Vidania producing essential amino acids are complemented by co-primary symbionts, either Arsenophonus or Sodalis, acquired several times independently by different host lineages and contributing to the biosynthesis of B vitamins. These symbionts reside within separate bacteriomes within the abdominal cavity, although in females Vidania also occupies bacteriocytes in the rectal organ. Notably, the symbionts are transovarially transmitted from mothers to offspring in two alternative ways. In most examined species, all nutritional symbionts simultaneously infect the posterior end of the full-grown oocytes and next gather in their perivitelline space. In contrast, in other species, Sodalis colonizes the cytoplasm of the anterior pole of young oocytes, forming a cluster separate from the "symbiont ball" formed by late-invading Sulcia and Vidania. Our results show how newly arriving microbes may utilize different strategies to establish long-term heritable symbiosis. IMPORTANCE Sup-sucking hemipterans host ancient heritable microorganisms that supplement their unbalanced diet with essential nutrients and have repeatedly been complemented or replaced by other microorganisms. These symbionts need to be reliably transmitted to subsequent generations through the reproductive system, and often they end up using the same route as the most ancient ones. We show for the first time that in a single family of planthoppers, the complementing symbionts that have established infections independently utilize different transmission strategies, one of them novel, with the transmission of different microbes separated spatially and temporally. These data show how newly arriving microbes may utilize different strategies to establish long-term heritable symbioses.

Fungal Associates of Soft Scale Insects (Coccomorpha: Coccidae)

Ophiocordyceps fungi are commonly known as virulent, specialized entomopathogens; however, recent studies indicate that fungi belonging to the Ophiocordycypitaceae family may also reside in symbiotic interaction with their host insect. In this paper, we demonstrate that Ophiocordyceps fungi may be obligatory symbionts of sap-sucking hemipterans. We investigated the symbiotic systems of eight Polish species of scale insects of Coccidae family: Parthenolecanium corni, Parthenolecanium fletcheri, Parthenolecanium pomeranicum, Psilococcus ruber, Sphaerolecanium prunasti, Eriopeltis festucae, Lecanopsis formicarum and Eulecanium tiliae. Our histological, ultrastructural and molecular analyses showed that all these species host fungal symbionts in the fat body cells. Analyses of ITS2 and Beta-tubulin gene sequences, as well as fluorescence in situ hybridization, confirmed that they should all be classified to the genus Ophiocordyceps. The essential role of the fungal symbionts observed in the biology of the soft scale insects examined was confirmed by their transovarial transmission between generations. In this paper, the consecutive stages of fungal symbiont transmission were analyzed under TEM for the first time.

Simulating the evolutionary trajectories of metabolic pathways for insect symbionts in the genus Sodalis

Insect-bacterial symbioses are ubiquitous, but there is still much to uncover about how these relationships establish, persist and evolve. The tsetse endosymbiont Sodalis glossinidius displays intriguing metabolic adaptations to its microenvironment, but the process by which this relationship evolved remains to be elucidated. The recent chance discovery of the free-living species of the genus Sodalis, Sodalis praecaptivus, provides a serendipitous starting point from which to investigate the evolution of this symbiosis. Here, we present a flux balance model for S. praecaptivus and empirically verify its predictions. Metabolic modelling is used in combination with a multi-objective evolutionary algorithm to explore the trajectories that S. glossinidius may have undertaken from this starting point after becoming internalized. The order in which key genes are lost is shown to influence the evolved populations, providing possible targets for future in vitro genetic manipulation. This method provides a detailed perspective on possible evolutionary trajectories for S. glossinidius in this fundamental process of evolutionary and ecological change.

The Diversity of Symbiotic Systems in Scale Insects

Most scale insects, like many other plant sap-sucking hemipterans, harbor obligate symbionts of bacterial or fungal origin, which synthesize and provide the host with substances missing in their restricted diet. Histological, ultrastructural, and molecular analyses have revealed that scale insects differ in the type of symbionts, the localization of symbionts in the host body, and the mode of transmission of symbionts from one generation to the next. Symbiotic microorganisms may be distributed in the cells of the fat body, midgut epithelium, inside the cells of other symbionts, or the specialized cells of a mesodermal origin, termed bacteriocytes. In most scale insects, their symbiotic associates are inherited transovarially, wherein the mode of transmission may have a different course-the symbionts may invade larval ovaries containing undifferentiated germ cells or ovaries of adult females containing vitellogenic or choriogenic oocytes.

Molecular characterization, ultrastructure, and transovarial transmission of Tremblaya phenacola in six mealybugs of the Phenacoccinae subfamily (Insecta, Hemiptera, Coccomorpha)

Mealybugs (Hemiptera, Coccomorpha: Pseudococcidae) are plant sap-sucking insects which require close association with nutritional microorganisms for their proper development and reproduction. Here, we present the results of histological, ultrastructural, and molecular analyses of symbiotic systems of six mealybugs belonging to the Phenacoccinae subfamily: Phenacoccus aceris, Rhodania porifera, Coccura comari, Mirococcus clarus, Peliococcus calluneti, and Ceroputo pilosellae. Molecular analyses based on bacterial 16S rRNA genes have revealed that all the investigated species of Phenacoccinae are host to only one type of symbiotic bacteria-a large pleomorphic betaproteobacteria-Tremblaya phenacola. In all the species examined, bacteria are localized in the specialized cells of the host-insect termed bacteriocytes and are transovarially transmitted between generations. The mode of transovarial transmission is similar in all of the species investigated. Infection takes place in the neck region of the ovariole, between the tropharium and vitellarium. The co-phylogeny between mealybugs and bacteria Tremblaya has been also analyzed.

Bacterial associates of Orthezia urticae, Matsucoccus pini, and Steingelia gorodetskia - scale insects of archaeoccoid families Ortheziidae, Matsucoccidae, and Steingeliidae (Hemiptera, Coccomorpha)

The biological nature, ultrastructure, distribution, and mode of transmission between generations of the microorganisms associated with three species (Orthezia urticae, Matsucoccus pini, Steingelia gorodetskia) of primitive families (archaeococcoids = Orthezioidea) of scale insects were investigated by means of microscopic and molecular methods. In all the specimens of Orthezia urticae and Matsucoccus pini examined, bacteria Wolbachia were identified. In some examined specimens of O. urticae, apart from Wolbachia, bacteria Sodalis were detected. In Steingelia gorodetskia, the bacteria of the genus Sphingomonas were found. In contrast to most plant sap-sucking hemipterans, the bacterial associates of O. urticae, M. pini, and S. gorodetskia are not harbored in specialized bacteriocytes, but are dispersed in the cells of different organs. Ultrastructural observations have shown that bacteria Wolbachia in O. urticae and M. pini, Sodalis in O. urticae, and Sphingomonas in S. gorodetskia are transovarially transmitted from mother to progeny.

Symbiotic cornucopia of the monophagous planthopper Ommatidiotus dissimilis (Fallén, 1806) (Hemiptera: Fulgoromorpha: Caliscelidae)

In contrast to Cicadomorpha, in which numerous symbiotic bacteria have been identified and characterized, the symbionts of fulgoromorphans are poorly known. Here, we present the results of histological, ultrastructural, and molecular analyses of the symbiotic system of the planthopper Ommatidiotus dissimilis. Amplification, cloning, and sequencing of bacterial 16S RNA genes have revealed that O. dissimilis is host to five types of bacteria. Apart from bacteria Sulcia and Vidania, which are regarded as ancestral symbionts of Fulgoromorpha, three additional types of bacteria belonging to the genera Sodalis, Wolbachia, and Rickettsia have been detected. Histological and ultrastructural investigations have shown that bacteria Sulcia, Vidania, and Sodalis house separate bacteriocytes, whereas bacteria Wolbachia and Rickettsia are dispersed within various insect tissue. Additionally, bacteria belonging to the genus Vidania occupy the bacteriome localized in the lumen of the hindgut. Both molecular and microscopic analyses have revealed that all the symbionts are transovarially transmitted between generations.

Diversity of symbiotic microbiota in Deltocephalinae leafhoppers (Insecta, Hemiptera, Cicadellidae)

Symbiotic microorganisms associated with thirteen species of the subfamily Deltocephalinae were examined using microscopic and molecular techniques. Athysanus argentarius, Euscelis incisus, Doratura stylata, Arthaldeus pascuellus, Errastunus ocellaris, Jassargus flori, Jassargus pseudocellaris, Psammotettix alienus, Psammotettix confinis, Turrutus socialis and Verdanus abdominalis harbor two types of ancient bacteriome-associated microorganisms: bacteria Sulcia (phylum Bacteroidetes) and bacteria Nasuia (phylum Proteobacteria, class Betaproteobacteria). In Balclutha calamagrostis and Balclutha punctata, the bacterium Nasuia has not been detected. In the bacteriomes of both species of Balclutha examined, only bacteria Sulcia occur, whereas Sodalis-like symbionts (phylum Proteobacteria, class Gammaproteobacteria) are localized in the fat body cells, in close vicinity of the bacteriomes. To our knowledge, this is the first report of the co-existence in Deltocephalinae leafhoppers of the ancient symbiont Sulcia and the more recently acquired Sodalis-like bacterium. The obtained results provide further evidence indicating that Deltocephalinae leafhoppers are characterized by a large diversity of symbiotic systems, which results from symbiont acquisition and replacement. The obtained results are additionally discussed in phylogenetic context.

Coexistence of novel gammaproteobacterial and Arsenophonus symbionts in the scale insect Greenisca brachypodii (Hemiptera, Coccomorpha: Eriococcidae)

Scale insects are commonly associated with obligate, intracellular microorganisms which play important roles in complementing their hosts with essential nutrients. Here we characterized the symbiotic system of Greenisca brachypodii, a member of the family Eriococcidae. Histological and ultrastructural analyses have indicated that G. brachypodii is stably associated with coccoid and rod-shaped bacteria. Phylogenetic analyses have revealed that the coccoid bacteria represent a sister group to the secondary symbiont of the mealybug Melanococcus albizziae, whereas the rod-shaped symbionts are close relatives of Arsenophonus symbionts in insects - to our knowledge, this is the first report of the presence of Arsenophonus bacterium in scale insects. As a comparison of 16S and 23S rRNA genes sequences of the G. brachypodii coccoid symbiont with other gammaprotebacterial sequences showed only low similarity (∼90%), we propose the name 'Candidatus Kotejella greeniscae' for its tentative classification. Both symbionts are transovarially transmitted from one generation to the next. The infection takes place in the neck region of the ovariole. The bacteria migrate between follicular cells, as well as through the cytoplasm of those cells to the perivitelline space, where they form a characteristic 'symbiont ball'. Our findings provide evidence for a polyphyletic origin of symbionts of Eriococcidae.

Behavior of bacteriome symbionts during transovarial transmission and development of the Asian citrus psyllid

The Asian citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Liviidae) is a serious pest worldwide, transmitting Candidatus Liberibacter spp. (Alphaproteobacteria), the causative agents of a devastating citrus disease known as huanglongbing or greening disease. In a symbiotic organ called the bacteriome, D. citri possesses an organelle-like defensive symbiont, Candidatus Profftella armatura (Betaproteobacteria), and a nutritional symbiont, Ca. Carsonella ruddii (Gammaproteobacteria). Drastically reduced symbiont genomes and metabolic complementarity among the symbionts and D. citri indicate their mutually indispensable association. Moreover, horizontal gene transfer between the Profftella and Liberibacter lineages suggests ecological and evolutionary interactions between the bacteriome symbiont and the HLB pathogen. Using fluorescence in situ hybridization, we examined the behavior of Profftella and Carsonella during transovarial transmission and the development of D. citri. In the bacteriomes of sexually-mature female adults, symbionts transformed from an extremely elongated tubular form into spherical or short-rod forms, which migrated toward the ovary. The symbionts then formed mosaic masses, which entered at the posterior pole of the vitellogenic oocytes. After anatrepsis, Carsonella and Profftella migrated to the central and peripheral parts of the mass, respectively. Following the appearance of host nuclei, the mass cellularized, segregating Carsonella and Profftella in the central syncytium and peripheral uninucleate bacteriocytes, respectively. Subsequently, the uninucleate bacteriocytes harboring Profftella assembled at the posterior pole, while the syncytium, containing Carsonella, sat on the anterior side facing the germ band initiating katatrepsis. During dorsal closure, the syncytium was divided into uninuclear bacteriocytes, which surrounded the mass of bacteriocytes containing Profftella. Once fully surrounded, the bacteriocyte mass containing Profftella was fused into a syncytium. Prior to hatching, a pair of wing-like protrusions arose from both lateral sides of the bacteriome, which continued to grow throughout the nymphal stages. These findings provide a foundation for better understanding the intricate relationship between D. citri and its microbiota.

Transovarial Transmission of Symbionts in Insects

Many insects, on account of their unbalanced diet, live in obligate symbiotic associations with microorganisms (bacteria or yeast-like symbionts), which provide them with substances missing in the food they consume. In the body of host insect, symbiotic microorganisms may occur intracellularly (e.g., in specialized cells of mesodermal origin termed bacteriocytes, in fat body cells, in midgut epithelium) or extracellularly (e.g., in hemolymph, in midgut lumen). As a rule, symbionts are vertically transmitted to the next generation. In most insects, symbiotic microorganisms are transferred from mother to offspring transovarially within female germ cells. The results of numerous ultrastructural and molecular studies on symbiotic systems in different groups of insects have shown that they have a large diversity of symbiotic microorganisms and different strategies of their transmission from one generation to the next. This chapter reviews the modes of transovarial transmission of symbionts between generations in insects.