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

Metabolism, digestion, and horizontal transfer: potential roles and interaction of symbiotic bacteria in the ladybird beetle Novius pumilus and their prey Icerya aegyptiaca

In this study, we first time sequenced and analyzed the 16S rRNA gene data of predator ladybird beetles Novius pumilus and globally distributed invasive pest Icerya aegyptiaca at different stages, and combined data with bacterial genome sequences in N. pumilus to explored the taxonomic distribution, alpha and beta diversity, differentially abundant bacteria, co-occurrence network, and putative functions of their microbial community. Our finding revealed that Candidatus Walczuchella, which exhibited a higher abundance in I. aegyptiaca, possessed several genes in essential amino acid biosynthesis and seemed to perform roles in providing nutrients to the host, similar to other obligate symbionts in scale insects. Lactococcus, Serratia, and Pseudomonas, more abundant in N. pumilus, were predicted to have genes related to hydrocarbon, fatty acids, and chitin degradation, which may assist their hosts in digesting the wax shell covering the scale insects. Notably, our result showed that Lactococcus had relatively higher abundances in adults and eggs compared to other stages in N. pumilus, indicating potential vertical transmission. Additionally, we found that Arsenophonus, known to influence sex ratios in whitefly and wasp, may also function in I. aegyptiaca, probably by influencing nutrient metabolism as it similarly had many genes corresponding to vitamin B and essential amino acid biosynthesis. Also, we observed a potential horizontal transfer of Arsenophonus between the scale insect and its predator, with a relatively high abundance in the ladybirds compared to other bacteria from the scale insects.IMPORTANCEThe composition and dynamic changes of microbiome in different developmental stages of ladybird beetles Novius pumilus with its prey Icerya aegyptiaca were detected. We found that Candidatus Walczuchella, abundant in I. aegyptiaca, probably provide nutrients to their host based on their amino acid biosynthesis-related genes. Abundant symbionts in N. pumilus, including Lactococcus, Serratia, and Pseudophonus, may help the host digest the scale insects with their hydrocarbon, fatty acid, and chitin degrading-related genes. A key endosymbiont Arsenophonus may play potential roles in the nutrient metabolisms and sex determination in I. aegyptiaca, and is possibly transferred from the scale insect to the predator.

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.

Variable organization of symbiont-containing tissue across planthoppers hosting different heritable endosymbionts

Sap-feeding hemipteran insects live in associations with diverse heritable symbiotic microorganisms (bacteria and fungi) that provide essential nutrients deficient in their hosts' diets. These symbionts typically reside in highly specialized organs called bacteriomes (with bacterial symbionts) or mycetomes (with fungal symbionts). The organization of these organs varies between insect clades that are ancestrally associated with different microbes. As these symbioses evolve and additional microorganisms complement or replace the ancient associates, the organization of the symbiont-containing tissue becomes even more variable. Planthoppers (Hemiptera: Fulgoromorpha) are ancestrally associated with bacterial symbionts Sulcia and Vidania, but in many of the planthopper lineages, these symbionts are now accompanied or have been replaced by other heritable bacteria (e.g., Sodalis, Arsenophonus, Purcelliella) or fungi. We know the identity of many of these microbes, but the symbiont distribution within the host tissues and the bacteriome organization have not been systematically studied using modern microscopy techniques. Here, we combine light, fluorescence, and transmission electron microscopy with phylogenomic data to compare symbiont tissue distributions and the bacteriome organization across planthoppers representing 15 families. We identify and describe seven primary types of symbiont localization and seven types of the organization of the bacteriome. We show that Sulcia and Vidania, when present, usually occupy distinct bacteriomes distributed within the body cavity. The more recently acquired gammaproteobacterial and fungal symbionts generally occupy separate groups of cells organized into distinct bacteriomes or mycetomes, distinct from those with Sulcia and Vidania. They can also be localized in the cytoplasm of fat body cells. Alphaproteobacterial symbionts colonize a wider range of host body habitats: Asaia-like symbionts often colonize the host gut lumen, whereas Wolbachia and Rickettsia are usually scattered across insect tissues and cell types, including cells containing other symbionts, bacteriome sheath, fat body cells, gut epithelium, as well as hemolymph. However, there are exceptions, including Gammaproteobacteria that share bacteriome with Vidania, or Alphaproteobacteria that colonize Sulcia cells. We discuss how planthopper symbiont localization correlates with their acquisition and replacement patterns and the symbionts' likely functions. We also discuss the evolutionary consequences, constraints, and significance of these findings.

Insight into the bacterial communities of the subterranean aphid Anoecia corni

Many insect species are associated with bacterial partners that can significantly influence their evolutionary ecology. Compared to other insect groups, aphids harbor a bacterial microbiota that has the reputation of being poorly diversified, generally limited to the presence of the obligate nutritional symbiont Buchnera aphidicola and some facultative symbionts. In this study, we analyzed the bacterial diversity associated with the dogwood-grass aphid Anoecia corni, an aphid species that spends much of its life cycle in a subterranean environment. Little is known about the bacterial diversity associated with aphids displaying such a lifestyle, and one hypothesis is that close contact with the vast microbial community of the rhizosphere could promote the acquisition of a richer bacterial diversity compared to other aphid species. Using 16S rRNA amplicon Illumina sequencing on specimens collected on wheat roots in Morocco, we identified 10 bacterial operational taxonomic units (OTUs) corresponding to five bacterial genera. In addition to the obligate symbiont Buchnera, we identified the facultative symbionts Serratia symbiotica and Wolbachia in certain aphid colonies. The detection of Wolbachia is unexpected as it is considered rare in aphids. Moreover, its biological significance remains unknown in these insects. Besides, we also detected Arsenophonus and Dactylopiibacterium carminicum. These results suggest that, despite its subterranean lifestyle, A. corni shelter a bacterial diversity mainly limited to bacterial endosymbionts.

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.

Lists of names of prokaryotic Candidatus taxa

We here present annotated lists of names of Candidatus taxa of prokaryotes with ranks between subspecies and class, proposed between the mid-1990s, when the provisional status of Candidatus taxa was first established, and the end of 2018. Where necessary, corrected names are proposed that comply with the current provisions of the International Code of Nomenclature of Prokaryotes and its Orthography appendix. These lists, as well as updated lists of newly published names of Candidatus taxa with additions and corrections to the current lists to be published periodically in the International Journal of Systematic and Evolutionary Microbiology, may serve as the basis for the valid publication of the Candidatus names if and when the current proposals to expand the type material for naming of prokaryotes to also include gene sequences of yet-uncultivated taxa is accepted by the International Committee on Systematics of Prokaryotes.

Complex symbiotic systems of two treehopper species: Centrotus cornutus (Linnaeus, 1758) and Gargara genistae (Fabricius, 1775) (Hemiptera: Cicadomorpha: Membracoidea: Membracidae)

The aim of the conducted study was to describe the symbiotic systems (the types of symbionts, distribution in the body of the host insect, the transovarial transmission between generations) of two treehoppers: Centrotus cornutus and Gargara genistae by means of microscopic and molecular techniques. We found that each of them is host to four species of bacteriome-inhabiting symbionts. In C. cornutus, ancestral bacterial symbionts Sulcia and Nasuia are accompanied by an additional symbiont-the bacterium Arsenophonus. In the bacteriomes of G. genistae, apart from Sulcia and Nasuia, bacterium Serratia is present. To our knowledge, this is the first report regarding the occurrence of Serratia as a symbiont in Hemiptera: Auchenorrhyncha. Bacteria Sulcia and Nasuia are harbored in their own bacteriocytes, whereas Arsenophonus and Serratia both inhabit their own bacteriocytes and also co-reside with bacteria Nasuia. We observed that both bacteria Arsenophonus and Serratia undergo autophagic degradation. We found that in both of the species examined, in the cytoplasm and nuclei of all of the cells of the bacteriome, bacteria Rickettsia are present. Our histological and ultrastructural observations revealed that all the bacteriome-associated symbionts of C. cornutus and G. genistae are transovarially transmitted from mother to offspring.

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.

A symbiont's guide to the germline

Microbial symbioses exhibit astounding adaptations, yet all symbionts face the problem of how to reliably associate with host offspring every generation. A common strategy is vertical transmission, in which symbionts are directly transmitted from the female to her offspring. The diversity of symbionts and vertical transmission mechanisms is as expansive as the diversity of eukaryotic host taxa that house them. However, there are several common themes among these mechanisms based on the degree to which symbionts associate with the host germline during transmission. In this review, we detail three distinct vertical transmission strategies, starting with associations that are transmitted from host somatic cells to offspring somatic cells, either due to lacking a germline or avoiding it. A second strategy involves somatically-localized symbionts that migrate into the germline during host development. The third strategy we discuss is one in which the symbiont maintains continuous association with the germline throughout development. Unexpectedly, the vast majority of documented vertically inherited symbionts rely on the second strategy: soma-to-germline migration. Given that not all eukaryotes contain a sequestered germline and instead produce offspring from somatic stem cell lineages, this soma-to-germline migration is discussed in the context of multicellular evolution. Lastly, as recent genomics data have revealed an abundance of horizontal gene transfer events from symbiotic and non-symbiotic bacteria to host genomes, we discuss their impact on eukaryotic host evolution.

Changes in Endosymbiont Complexity Drive Host-Level Compensatory Adaptations in Cicadas

For insects that depend on one or more bacterial endosymbionts for survival, it is critical that these bacteria are faithfully transmitted between insect generations. Cicadas harbor two essential bacterial endosymbionts, "Candidatus Sulcia muelleri" and "Candidatus Hodgkinia cicadicola." In some cicada species, Hodgkinia has fragmented into multiple distinct but interdependent cellular and genomic lineages that can differ in abundance by more than two orders of magnitude. This complexity presents a potential problem for the host cicada, because low-abundance but essential Hodgkinia lineages risk being lost during the symbiont transmission bottleneck from mother to egg. Here we show that all cicada eggs seem to receive the full complement of Hodgkinia lineages, and that in cicadas with more complex Hodgkinia this outcome is achieved by increasing the number of Hodgkinia cells transmitted by up to 6-fold. We further show that cicada species with varying Hodgkinia complexity do not visibly alter their transmission mechanism at the resolution of cell biological structures. Together these data suggest that a major cicada adaptation to changes in endosymbiont complexity is an increase in the number of Hodgkinia cells transmitted to each egg. We hypothesize that the requirement to increase the symbiont titer is one of the costs associated with Hodgkinia fragmentation.IMPORTANCE Sap-feeding insects critically rely on one or more bacteria or fungi to provide essential nutrients that are not available at sufficient levels in their diets. These microbes are passed between insect generations when the mother places a small packet of microbes into each of her eggs before it is laid. We have previously described an unusual lineage fragmentation process in a nutritional endosymbiotic bacterium of cicadas called Hodgkinia In some cicadas, a single Hodgkinia lineage has split into numerous related lineages, each performing a subset of original function and therefore each required for normal host function. Here we test how this splitting process affects symbiont transmission to eggs. We find that cicadas dramatically increase the titer of Hodgkinia cells passed to each egg in response to lineage fragmentation, and we hypothesize that this increase in bacterial cell count is one of the major costs associated with endosymbiont fragmentation.