Bacterial symbiosis maintenance in the asexually reproducing and regenerating flatworm Paracatenula galateia

Regeneration in the absence of canonical neoblasts in an early branching flatworm

The remarkable regenerative abilities of flatworms are closely linked to neoblasts - adult pluripotent stem cells that are the only division-competent cell type outside of the reproductive system. Although the presence of neoblast-like cells and whole-body regeneration in other animals has led to the idea that these features may represent the ancestral metazoan state, the evolutionary origin of both remains unclear. Here we show that the catenulid Stenostomum brevipharyngium, a member of the earliest-branching flatworm lineage, lacks conventional neoblasts despite being capable of whole-body regeneration and asexual reproduction. Using a combination of single-nuclei transcriptomics, in situ gene expression analysis, and functional experiments, we find that cell divisions are not restricted to a single cell type and are associated with multiple fully differentiated somatic tissues. Furthermore, the cohort of germline multipotency genes, which are considered canonical neoblast markers, are not expressed in dividing cells, but in the germline instead, and we experimentally show that they are neither necessary for proliferation nor regeneration. Overall, our results challenge the notion that canonical neoblasts are necessary for flatworm regeneration and open up the possibility that neoblast-like cells may have evolved convergently in different animals, independent of their regenerative capacity.

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.

Evolution and ontogeny of bacteriocytes in insects

The ontogenetic origins of the bacteriocytes, which are cells that harbour bacterial intracellular endosymbionts in multicellular animals, are unknown. During embryonic development, a series of morphological and transcriptional changes determine the fate of distinct cell types. The ontogeny of bacteriocytes is intimately linked with the evolutionary transition of endosymbionts from an extracellular to an intracellular environment, which in turn is linked to the diet of the host insect. Here we review the evolution and development of bacteriocytes in insects. We first classify the endosymbiotic occupants of bacteriocytes, highlighting the complex challenges they pose to the host. Then, we recall the historical account of the discovery of bacteriocytes. We then summarize the molecular interactions between the endosymbiont and the host. In addition, we illustrate the genetic contexts in which the bacteriocytes develop, with examples of the genetic changes in the hosts and endosymbionts, during specific endosymbiotic associations. We finally address the evolutionary origin as well as the putative ontogenetic or developmental source of bacteriocytes in insects.

Intracellular Bacterial Symbionts in Corals: Challenges and Future Directions

Corals are the main primary producers of coral reefs and build the three-dimensional reef structure that provides habitat to more than 25% of all marine eukaryotes. They harbor a complex consortium of microorganisms, including bacteria, archaea, fungi, viruses, and protists, which they rely on for their survival. The symbiosis between corals and bacteria is poorly studied, and their symbiotic relationships with intracellular bacteria are only just beginning to be acknowledged. In this review, we emphasize the importance of characterizing intracellular bacteria associated with corals and explore how successful approaches used to study such microorganisms in other systems could be adapted for research on corals. We propose a framework for the description, identification, and functional characterization of coral-associated intracellular bacterial symbionts. Finally, we highlight the possible value of intracellular bacteria in microbiome manipulation and mitigating coral bleaching.

Housing microbial symbionts: evolutionary origins and diversification of symbiotic organs in animals

In many animal hosts, microbial symbionts are housed within specialized structures known as symbiotic organs, but the evolutionary origins of these structures have rarely been investigated. Here, I adopt an evolutionary developmental (evo-devo) approach, specifically to apply knowledge of the development of symbiotic organs to gain insights into their evolutionary origins and diversification. In particular, host genetic changes associated with evolution of symbiotic organs can be inferred from studies to identify the host genes that orchestrate the development of symbiotic organs, recognizing that microbial products may also play a key role in triggering the developmental programme in some associations. These studies may also reveal whether higher animal taxonomic groups (order, class, phylum, etc.) possess a common genetic regulatory network for symbiosis that is latent in taxa lacking symbiotic organs, and activated at the origination of symbiosis in different host lineages. In this way, apparent instances of convergent evolution of symbiotic organs may be homologous in terms of a common genetic blueprint for symbiosis. Advances in genetic technologies, including reverse genetic tools and genome editing, will facilitate the application of evo-devo approaches to investigate the evolution of symbiotic organs in animals. This article is part of the theme issue 'The role of the microbiome in host evolution'.

Spatial and morphological reorganization of endosymbiosis during metamorphosis accommodates adult metabolic requirements in a weevil

Bacterial intracellular symbiosis (endosymbiosis) is widespread in nature and impacts many biological processes. In holometabolous symbiotic insects, metamorphosis entails a complete and abrupt internal reorganization that creates a constraint for endosymbiont transmission from larvae to adults. To assess how endosymbiosis copes-and potentially evolves-throughout this major host-tissue reorganization, we used the association between the cereal weevil Sitophilus oryzae and the bacterium Sodalis pierantonius as a model system. S. pierantonius are contained inside specialized host cells, the bacteriocytes, that group into an organ, the bacteriome. Cereal weevils require metabolic inputs from their endosymbiont, particularly during adult cuticle synthesis, when endosymbiont load increases dramatically. By combining dual RNA-sequencing analyses and cell imaging, we show that the larval bacteriome dissociates at the onset of metamorphosis and releases bacteriocytes that undergo endosymbiosis-dependent transcriptomic changes affecting cell motility, cell adhesion, and cytoskeleton organization. Remarkably, bacteriocytes turn into spindle cells and migrate along the midgut epithelium, thereby conveying endosymbionts to midgut sites where future mesenteric caeca will develop. Concomitantly, endosymbiont genes encoding a type III secretion system and a flagellum apparatus are transiently up-regulated while endosymbionts infect putative stem cells and enter their nuclei. Infected cells then turn into new differentiated bacteriocytes and form multiple new bacteriomes in adults. These findings show that endosymbiosis reorganization in a holometabolous insect relies on a synchronized host-symbiont molecular and cellular "choreography" and illustrates an adaptive feature that promotes bacteriome multiplication to match increased metabolic requirements in emerging adults.

The curious and neglected soft-bodied meiofauna: Rouphozoa (Gastrotricha and Platyhelminthes)

Gastrotricha and Platyhelminthes form a clade called Rouphozoa. Representatives of both taxa are main components of meiofaunal communities, but their role in the trophic ecology of marine and freshwater communities is not sufficiently studied. Traditional collection methods for meiofauna are optimized for Ecdysozoa, and include the use of fixatives or flotation techniques that are unsuitable for the preservation and identification of soft-bodied meiofauna. As a result, rouphozoans are usually underestimated in conventional biodiversity surveys and ecological studies. Here, we give an updated outline of their diversity and taxonomy, with some phylogenetic considerations. We describe successfully tested techniques for their recovery and study, and emphasize current knowledge on the ecology, distribution and dispersal of freshwater gastrotrichs and microturbellarians. We also discuss the opportunities and pitfalls of (meta)barcoding studies as a means of overcoming the taxonomic impediment. Finally, we discuss the importance of rouphozoans in aquatic ecosystems and provide future research directions to fill in crucial gaps in the biology of these organisms needed for understanding their basic role in the ecology of benthos and their place in the trophic networks linking micro-, meio- and macrofauna of freshwater ecosystems.

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.

Chemosynthetic symbiont with a drastically reduced genome serves as primary energy storage in the marine flatworm Paracatenula

Animals typically store their primary energy reserves in specialized cells. Here, we show that in the small marine flatworm Paracatenula, this function is performed by its bacterial chemosynthetic symbiont. The intracellular symbiont occupies half of the biomass in the symbiosis and has a highly reduced genome but efficiently stocks up and maintains carbon and energy, particularly sugars. The host rarely digests the symbiont cells to access these stocks. Instead, the symbionts appear to provide the bulk nutrition by secreting outer-membrane vesicles. This is in contrast to all other described chemosynthetic symbioses, where the hosts continuously digest full cells of a small and ideally growing symbiont population that cannot provide a long-term buffering capacity during nutrient limitation.

Hosts of chemoautotrophic bacteria typically have much higher biomass than their symbionts and consume symbiont cells for nutrition. In contrast to this, chemoautotrophic Candidatus Riegeria symbionts in mouthless Paracatenula flatworms comprise up to half of the biomass of the consortium. Each species of Paracatenula harbors a specific Ca. Riegeria, and the endosymbionts have been vertically transmitted for at least 500 million years. Such prolonged strict vertical transmission leads to streamlining of symbiont genomes, and the retained physiological capacities reveal the functions the symbionts provide to their hosts. Here, we studied a species of Paracatenula from Sant’Andrea, Elba, Italy, using genomics, gene expression, imaging analyses, as well as targeted and untargeted MS. We show that its symbiont, Ca. R. santandreae has a drastically smaller genome (1.34 Mb) than the symbiont´s free-living relatives (4.29–4.97 Mb) but retains a versatile and energy-efficient metabolism. It encodes and expresses a complete intermediary carbon metabolism and enhanced carbon fixation through anaplerosis and accumulates massive intracellular inclusions such as sulfur, polyhydroxyalkanoates, and carbohydrates. Compared with symbiotic and free-living chemoautotrophs, Ca. R. santandreae’s versatility in energy storage is unparalleled in chemoautotrophs with such compact genomes. Transmission EM as well as host and symbiont expression data suggest that Ca. R. santandreae largely provisions its host via outer-membrane vesicle secretion. With its high share of biomass in the symbiosis and large standing stocks of carbon and energy reserves, it has a unique role for bacterial symbionts—serving as the primary energy storage for its animal host.

Cellular dynamics during regeneration of the flatworm Monocelis sp. (Proseriata, Platyhelminthes)

BACKGROUND

Proseriates (Proseriata, Platyhelminthes) are free-living, mostly marine, flatworms measuring at most a few millimetres. In common with many flatworms, they are known to be capable of regeneration; however, few studies have been done on the details of regeneration in proseriates, and none cover cellular dynamics. We have tested the regeneration capacity of the proseriate Monocelis sp. by pre-pharyngeal amputation and provide the first comprehensive picture of the F-actin musculature, serotonergic nervous system and proliferating cells (S-phase in pulse and pulse-chase experiments and mitoses) in control animals and in regenerates.

RESULTS

F-actin staining revealed a strong body wall, pharynx and dorsoventral musculature, while labelling of the serotonergic nervous system showed an orthogonal pattern and a well developed subepidermal plexus. Proliferating cells were distributed in two broad lateral bands along the anteroposterior axis and their anterior extension was delimited by the brain. No proliferating cells were detected in the pharynx or epidermis. Monocelis sp. was able to regenerate the pharynx and adhesive organs at the tip of the tail plate within 2 or 3 days of amputation, and genital organs within 8 to 10 days. Posterior pieces were not able to regenerate a head. The posterior regeneration blastema was found to be a centre of cell proliferation, whereas within the pharynx primordium, little or no proliferation was detected. The pharynx regenerated outside of the blastema and was largely, but not solely formed by cells that were proliferating at the time of amputation.

CONCLUSIONS

Our findings suggest that proliferating cells or their offspring migrated to the place of organ differentiation and then stopped proliferating at that site. This mode of rebuilding organs resembles the mode of regeneration of the genital organs in another flatworm, Macrostomum lignano. Pharynx regeneration resembles embryonic development in Monocelis fusca and hints at the vertically directed pharynx being plesiomorphic in proseriates. Proliferation within the regeneration blastema has been detected in anterior and posterior blastemas of other flatworms, but is notably missing in triclads. The phylogenetic relationships of the flatworms studied indicate that proliferation within the blastema is the plesiomorphic condition in Platyhelminthes.

Tracking transmission of apicomplexan symbionts in diverse Caribbean corals

Symbionts in each generation are transmitted to new host individuals either vertically (parent to offspring), horizontally (from exogenous sources), or a combination of both. Scleractinian corals make an excellent study system for understanding patterns of symbiont transmission since they harbor diverse symbionts and possess distinct reproductive modes of either internal brooding or external broadcast spawning that generally correlate with vertical or horizontal transmission, respectively. Here, we focused on the under-recognized, but apparently widespread, coral-associated apicomplexans (Protista: Alveolata) to determine if symbiont transmission depends on host reproductive mode. Specifically, a PCR-based assay was utilized towards identifying whether planula larvae and reproductive adults from brooding and broadcast spawning scleractinian coral species in Florida and Belize harbored apicomplexan DNA. Nearly all (85.5%; n = 85/89) examined planulae of five brooding species (Porites astreoides, Agaricia tenuifolia, Agaricia agaricites, Favia fragum, Mycetophyllia ferox) and adults of P. astreoides were positive for apicomplexan DNA. In contrast, no (n = 0/10) apicomplexan DNA was detected from planulae of four broadcast spawning species (Acropora cervicornis, Acropora palmata, Pseudodiploria strigosa, and Orbicella faveolata) and rarely in gametes (8.9%; n = 5/56) of these species sampled from the same geographical range as the brooding species. In contrast, tissue samples from nearly all (92.0%; n = 81/88) adults of the broadcast spawning species A. cervicornis, A. palmata and O. faveolata harbored apicomplexan DNA, including colonies whose gametes and planulae tested negative for these symbionts. Taken together, these data suggest apicomplexans are transmitted vertically in these brooding scleractinian coral species while the broadcast spawning scleractinian species examined here acquire these symbionts horizontally. Notably, these transmission patterns are consistent with those of other scleractinian coral symbionts. While this study furthers knowledge regarding these symbionts, numerous questions remain to be addressed, particularly in regard to the specific interaction(s) between these apicomplexans and their hosts.

Proliferation pattern during rostrum regeneration of the symbiotic flatworm Paracatenula galateia: a pulse-chase-pulse analysis

The remarkable totipotent stem-cell-based regeneration capacities of the Platyhelminthes have brought them into the focus of stem cell and regeneration research. Although selected platyhelminth groups are among the best-studied invertebrates, our data provide new insights into regenerative processes in the most basally branching group of the Platyhelminthes, the Catenulida. The mouth- and gutless free-living catenulid flatworm Paracatenula galateia harbors intracellular bacterial symbionts in its posterior body region, the trophosome region, accounting for up to 50% of the volume. Following decapitation of this flatworm, we have analyzed the behavior of the amputated fragments and any anterior and posterior regeneration. Using an EdU-pulse-chase/BrdU-pulse thymidine analog double-labeling approach combined with immunohistochemistry, we show that neoblasts are the main drivers of the regeneration processes. During anterior (rostrum) regeneration, EdU-pulse-chase-labeled cells aggregate inside the regenerating rostrum, whereas BrdU pulse-labeling before fixation indicates clusters of S-phase neoblasts at the same position. In parallel, serotonergic nerves reorganize and the brain regenerates. In completely regenerated animals, the original condition with S-phase neoblasts being restricted to the body region posterior to the brain is restored. In contrast, no posterior regeneration or growth of the trophosome region in anterior fragments cut a short distance posterior to the brain has been observed. Our data thus reveal interesting aspects of the cellular processes underlying the regeneration of the emerging catenulid-bacteria symbiosis model P. galateia and show that a neoblast stem cell system is indeed a plesiomorphic feature of basal platyhelminths.