Dynamic interactions within the host-associated microbiota cause tumor formation in the basal metazoan Hydra

First evidence for the evolution of host manipulation by tumors during the long-term vertical transmission of tumor cells in Hydra oligactis

While host phenotypic manipulation by parasites is a widespread phenomenon, whether tumors, which can be likened to parasite entities, can also manipulate their hosts is not known. Theory predicts that this should nevertheless be the case, especially when tumors (neoplasms) are transmissible. We explored this hypothesis in a cnidarian Hydra model system, in which spontaneous tumors can occur in the lab, and lineages in which such neoplastic cells are vertically transmitted (through host budding) have been maintained for over 15 years. Remarkably, the hydras with long-term transmissible tumors show an unexpected increase in the number of their tentacles, allowing for the possibility that these neoplastic cells can manipulate the host. By experimentally transplanting healthy as well as neoplastic tissues derived from both recent and long-term transmissible tumors, we found that only the long-term transmissible tumors were able to trigger the growth of additional tentacles. Also, supernumerary tentacles, by permitting higher foraging efficiency for the host, were associated with an increased budding rate, thereby favoring the vertical transmission of tumors. To our knowledge, this is the first evidence that, like true parasites, transmissible tumors can evolve strategies to manipulate the phenotype of their host.

Ecology of vertical tumor transmission in the freshwater cnidarian Hydra oligactis

Transmissible tumors are increasingly regarded as a new form of parasitic life, but relatively little is known about the ecology and evolution of their interactions with their host. In this work, we provide new insights into transmission dynamics of vertically transmitted tumors in the freshwater cnidarian Hydra oligactis. First, we found tumoral hydra to be infectious at any age, regardless of whether they were in their asymptomatic or symptomatic phases, with the bacteriome composition remaining constant during both phases. Interestingly, tumor transmission increased with the number of tentacles, particularly for hydras with supernumerary tentacles. Additionally, tumors developed earlier in the offspring from parents with more advanced tumors. Furthermore, despite being direct descendants of tumoral polyps, some hydras never developed tumoral phenotype. The latter exhibited a distinct bacteriome composition, reduced lifespan and a lower tentacle number increase over time. Interestingly, the tumor phenotype expression in these hydras appears to be able to skip generations, as transmission occurred at any age from parents to offspring. We discuss these results in the context of current knowledge on the evolutionary ecology of host-transmissible tumor interactions as well as parasite-host interactions and suggest avenues for further research.

Environmental microbial reservoir influences the bacterial communities associated with Hydra oligactis

The objective to study the influence of microbiome on host fitness is frequently constrained by spatial and temporal variability of microbial communities. In particular, the environment serves as a dynamic reservoir of microbes that provides potential colonizers for animal microbiomes. In this study, we analyzed the microbiome of Hydra oligactis and corresponding water samples from 15 Hungarian lakes to reveal the contribution of environmental microbiota on host microbiome. Correlation analyses and neutral modeling revealed that differences in Hydra microbiota are associated with differences in environmental microbiota. To further investigate the influence of environmental bacterial community on the host microbiome, field-collected Hydra polyps from three populations were cultured in native water or foreign water. Our results show that lake water bacteria significantly contribute to Hydra microbial communities, but the compositional profile remained stable when cultured in different water sources. Longitudinal analysis of the in vitro experiment revealed a site-specific change in microbiome that correlated with the source water quality. Taken together, our findings demonstrate that while freshwater serves as a critical microbial reservoir, Hydra microbial communities exhibit remarkable resilience to environmental changes maintaining stability despite potential invasion. This dual approach highlights the complex interplay between environmental reservoirs and host microbiome integrity.

The widespread vulnerability of Hydra oligactis to tumourigenesis confirms its value as a model for studying the effects of tumoural processes on the ecology and evolution of species

Tumoural processes, ubiquitous phenomena in multicellular organisms, influence evolutionary trajectories of all species. To gain a holistic understanding of their impact on species' biology, suitable laboratory models are required. Such models are characterised by a widespread availability, ease of cultivation, and reproducible tumour induction. It is especially important to explore, through experimental approaches, how tumoural processes alter ecosystem functioning. The cnidarian Hydra oligactis is currently emerging as a promising model due to its development of both transmissible and non-transmissible tumours and the wide breadth of experiments that can be conducted with this species (at the individual, population, mechanistic, and evolutionary levels). However, tumoural hydras are, so far, only documented in Europe, and it is not clear if the phenomenon is local or widespread. In this study we demonstrate that Australian hydras from two independent river networks develop tumours in the laboratory consisting of interstitial stem cells and display phenotypic alterations (supernumerary tentacles) akin to European counterparts. This finding confirms the value of this model for ecological and evolutionary research on host-tumour interactions.

Discovery of deep-sea coral symbionts from a novel clade of marine bacteria with severely reduced genomes

Microbes perform critical functions in corals, yet most knowledge is derived from the photic zone. Here, we discover two mollicutes that dominate the microbiome of the deep-sea octocoral, Callogorgia delta, and likely reside in the mesoglea. These symbionts are abundant across the host's range, absent in the water, and appear to be rare in sediments. Unlike other mollicutes, they lack all known fermentative capabilities, including glycolysis, and can only generate energy from arginine provided by the coral host. Their genomes feature several mechanisms to interact with foreign DNA, including extensive CRISPR arrays and restriction-modification systems, which may indicate their role in symbiosis. We propose the novel family Oceanoplasmataceae which includes these symbionts and others associated with five marine invertebrate phyla. Its exceptionally broad host range suggests that the diversity of this enigmatic family remains largely undiscovered. Oceanoplasmataceae genomes are the most highly reduced among mollicutes, providing new insight into their reductive evolution and the roles of coral symbionts.

Cancer and One Health: tumor-bearing individuals can act as super spreaders of symbionts in communities

Recent theoretical advances in the One Health approach have suggested that cancer pathologies should be given greater consideration, as cancers often render their hosts more vulnerable to infectious agents, which could turn them into super spreaders within ecosystems. Although biologically plausible, this hypothesis has not yet been validated experimentally. Using a community of cnidarians of the Hydra genus (Hydra oligactis, Hydra viridissima, Hydra vulgaris) and a commensal ciliate species (Kerona pediculus) that colonizes them, we tested whether tumoral polyps of H. oligactis, compared to healthy ones, played an amplifying role in the number of ciliates, potentially resulting in a higher likelihood of infection for other community members through spillovers. Our results indicate that K. pediculus has a higher proliferation rate on tumoral polyps of H. oligactis than on healthy ones, which results in the infestation of other hydras. However, the magnitude of the spillover differed between recipient species. This study provides to our knowledge the first elements of proof of concept that tumoral individuals in communities could act as super spreaders of symbionts within and between species, and thus affect biotic interactions and dynamics in ecosystems.

De novo evolution of transmissible tumours in hydra

While most cancers are not transmissible, there are rare cases where cancer cells can spread between individuals and even across species, leading to epidemics. Despite their significance, the origins of such cancers remain elusive due to late detection in host populations. Using Hydra oligactis, which exhibits spontaneous tumour development that in some strains became vertically transmitted, this study presents the first experimental observation of the evolution of a transmissible tumour. Specifically, we assessed the initial vertical transmission rate of spontaneous tumours and explored the potential for optimizing this rate through artificial selection. One of the hydra strains, which evolved transmissible tumours over five generations, was characterized by analysis of cell type and bacteriome, and assessment of life-history traits. Our findings indicate that tumour transmission can be immediate for some strains and can be enhanced by selection. The resulting tumours are characterized by overproliferation of large interstitial stem cells and are not associated with a specific bacteriome. Furthermore, despite only five generations of transmission, these tumours induced notable alterations in host life-history traits, hinting at a compensatory response. This work, therefore, makes the first contribution to understanding the conditions of transmissible cancer emergence and their short-term consequences for the host.

Exploring the Microbial Landscape of Neonatal Skin Flora: A Comprehensive Review

This comprehensive review explores the intricate landscape of the neonatal skin microbiome, shedding light on its dynamic composition, developmental nuances, and influential factors. The neonatal period represents a critical window during which microbial colonization significantly impacts local skin health and the foundational development of the immune system. Factors such as mode of delivery and gestational age underscore the vulnerability of neonates to disruptions in microbial establishment. Key findings emphasize the broader systemic implications of the neonatal skin microbiome, extending beyond immediate health outcomes to influence susceptibility to infections, allergies, and immune-related disorders. This review advocates for a paradigm shift in neonatal care, proposing strategies to preserve and promote a healthy skin microbiome for long-term health benefits. The implications of this research extend to public health, where interventions targeting the neonatal skin microbiome could potentially mitigate diseases originating in early life. As we navigate the intersection of research and practical applications, bridging the gap between knowledge and implementation becomes imperative for translating these findings into evidence-based practices and improving neonatal well-being on a broader scale.

Identifying disease-related microbes based on multi-scale variational graph autoencoder embedding Wasserstein distance

BACKGROUND

Enormous clinical and biomedical researches have demonstrated that microbes are crucial to human health. Identifying associations between microbes and diseases can not only reveal potential disease mechanisms, but also facilitate early diagnosis and promote precision medicine. Due to the data perturbation and unsatisfactory latent representation, there is a significant room for improvement.

RESULTS

In this work, we proposed a novel framework, Multi-scale Variational Graph AutoEncoder embedding Wasserstein distance (MVGAEW) to predict disease-related microbes, which had the ability to resist data perturbation and effectively generate latent representations for both microbes and diseases from the perspective of distribution. First, we calculated multiple similarities and integrated them through similarity network confusion. Subsequently, we obtained node latent representations by improved variational graph autoencoder. Ultimately, XGBoost classifier was employed to predict potential disease-related microbes. We also introduced multi-order node embedding reconstruction to enhance the representation capacity. We also performed ablation studies to evaluate the contribution of each section of our model. Moreover, we conducted experiments on common drugs and case studies, including Alzheimer's disease, Crohn's disease, and colorectal neoplasms, to validate the effectiveness of our framework.

CONCLUSIONS

Significantly, our model exceeded other currently state-of-the-art methods, exhibiting a great improvement on the HMDAD database.

The impact of food availability on tumorigenesis is evolutionarily conserved

The inability to control cell proliferation results in the formation of tumors in many multicellular lineages. Nonetheless, little is known about the extent of conservation of the biological traits and ecological factors that promote or inhibit tumorigenesis across the metazoan tree. Particularly, changes in food availability have been linked to increased cancer incidence in humans, as an outcome of evolutionary mismatch. Here, we apply evolutionary oncology principles to test whether food availability, regardless of the multicellular lineage considered, has an impact on tumorigenesis. We used two phylogenetically unrelated model systems, the cnidarian Hydra oligactis and the fish Danio rerio, to investigate the impact of resource availability on tumor occurrence and progression. Individuals from healthy and tumor-prone lines were placed on four diets that differed in feeding frequency and quantity. For both models, frequent overfeeding favored tumor emergence, while lean diets appeared more protective. In terms of tumor progression, high food availability promoted it, whereas low resources controlled it, but without having a curative effect. We discuss our results in light of current ideas about the possible conservation of basic processes governing cancer in metazoans (including ancestral life history trade-offs at the cell level) and in the framework of evolutionary medicine.

Dysbiosis of intestinal homeostasis contribute to Whitmania pigra edema disease

Whitmania pigra is widely used in traditional Chinese medicine. However, W. pigra is being threatened by an edema disease with unknown causes (WPE). In this study, a comprehensive exploration of virome, microbiome, and metabolome aberrations in the intestine of W. pigra was performed to address the aetiology of WPE. Virome analysis indicated that eukaryotic viruses did not contribute to WPE, whereas an expansion of Caudovirales was observed in WPE. Compared to the control, the microbial richness and diversity in diseased W. pigra decreased remarkably. Nine genera, including Aeromonas, Anaerotruncus, Vibrio, Proteocatella, Acinetobacter, and Brachyspira were overrepresented in WPE, whereas eleven genera, including Bifidobacterium, Phascolarctobacterium, Lactobacillus, Bacillus and AF12, were enriched in healthy individuals. Furthermore, certain metabolites, especially amino acids, short-chain fatty acids, and bile acids, were found to be linked to intestinal microbiota alterations in WPE. An integration of the microbiome and metabolome in WPE found that dysbiosis of the gut microbiota or metabolites caused WPE. Notably, W. pigra accepted intestinal microbiota transplantation from WPE donors developed WPE clinical signs eventually, and the dysbiotic intestinal microbiota can be recharacterized in this recipient W. pigra. Strikingly, pathological features of metanephridium and uraemic toxin enrichment in the gut indicated a putative interconnection between the gut and metanephridium in WPE, which represents the prototype of the gut-kidney axis in mammals. These finding exemplify the conservation of "microecological Koch's postulates" from annelids to insects and other vertebrates, which provides a direction of prevention and treatment for WPE and opens a new insight into the pathogenesis of aquatic animal diseases from an ecological perspective.

Spontaneously occurring tumors in different wild-derived strains of hydra

Hydras are freshwater cnidarians widely used as a biological model to study different questions such as senescence or phenotypic plasticity but also tumoral development. The spontaneous tumors found in these organisms have been so far described in two female lab strains domesticated years ago (Hydra oligactis and Pelmatohydra robusta) and the extent to which these tumors can be representative of tumors within the diversity of wild hydras is completely unknown. In this study, we examined individuals isolated from recently sampled wild strains of different sex and geographical origin, which have developed outgrowths looking like tumors. These tumefactions have common features with the tumors previously described in lab strains: are composed of an accumulation of abnormal cells, resulting in a similar enlargement of the tissue layers. However, we also found diversity within these new types of tumors. Indeed, not only females, but also males seem prone to form these tumors. Finally, the microbiota associated to these tumors is different from the one involved in the previous lineages exhibiting tumors. We found that tumorous individuals hosted yet undescribed Chlamydiales vacuoles. This study brings new insights into the understanding of tumor susceptibility and diversity in brown hydras from different origins.

Tumors alter life-history traits in the freshwater cnidarian, Hydra oligactis

Although tumors can occur during the lifetime of most multicellular organisms and have the potential to influence health, how they alter life-history traits in tumor-bearing individuals remains poorly documented. This question was explored using the freshwater cnidarian Hydra oligactis, a species sometimes affected by vertically transmitted tumors. We found that tumorous polyps have a reduced survival compared to healthy ones. However, they also displayed higher asexual reproductive effort, by producing more often multiple buds than healthy ones. A similar acceleration is observed for the sexual reproduction (estimated through gamete production). Because tumoral cells are not transmitted through this reproductive mode, this finding suggests that hosts may adaptively respond to tumors, compensating the expected fitness losses by increasing their immediate reproductive effort. This study supports the hypothesis that tumorigenesis has the potential to influence the biology, ecology, and evolution of multicellular species, and thus should be considered more by evolutionary ecologists.

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Vertically transmitted tumors influence the life history traits of hydras

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Tumor-bearing hydras have a reduced survival rate

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Tumorous hydras show increased early reproductive effort (asexual and sexual)

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Changes in sexual reproduction pattern can be a compensatory response of the host

Beyond Lynn Margulis’ green hydra

Lynn Margulis has made it clear that in nature partnerships are the predominant form of life; that life processes can only be understood in terms of the interactions of such partnerships; and that their inherent complexity can only be understood by taking a holistic approach. Here I attempt to relate Lynn Margulis´ observations on the freshwater polyp hydra to the perceptions and problems of today’s Hydra research. To accomplish this, I will synthesize our current understanding of how symbionts influence the phenotype and fitness of hydra. Based on this new findings, a fundamental paradigm shift and a new era is emerging in the way that we consider organisms such as hydra as multi-organismic metaorganisms, just as Lynn Margulis may have thought about it.

Dysbiosis of the Gut Microbiome Is Associated With Histopathology of Lung Cancer

Lung cancer is a malignancy with high incidence and mortality worldwide. Previous studies have shown that the gut microbiome plays an important role in the development and progression of metabolic cancers. However, data on the characteristics of the gut microbiome with different histopathology types of lung cancer remain scant. We collected stool samples from 28 healthy people (HP) and 61 lung cancer patients. The lung cancer patients were classified into three types according to their histopathology: Atypical Adenomatous Hyperplasia/Adenocarcinoma in situ (AAH/AIS), Minimally Invasive Adenocarcinoma (MIA), and Invasive Adenocarcinoma (IA). In addition, we employed 16S rRNA gene amplicon sequencing to analyze the characteristics of the gut microbiome in these patients. Our analysis revealed that the categorized cancer patients had unique intestinal flora characteristics, and had lower density and flora diversity compared to healthy people. Besides, the structure of the flora families and genera was more complex, and each group presented specific pathogenic microbiota. The patients in the AAH/AIS group and HP group had relatively similar flora structure compared with the IA and MIA groups. In addition, we identified several flora markers that showed significant changes with the development of lung cancer. Lung cancer gut microbiota showed a decrease in short-chain fatty acids (SCFAs) producing and anti-inflammatory bacteria compared to healthy people, while some pathogenic bacteria such as proinflammatory or tumor-promoting bacteria were more abundant in lung cancer patients. On the other hand, the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Clusters of Orthologous Group (COG) annotation demonstrated suppression of some dominant metabolism-related pathways in lung cancer. These findings provide new biomarkers for the diagnosis and prognostic assessment of lung cancer and lay the basis for novel targeted therapeutic strategies for the prevention and treatment of lung cancer.

Cancer Susceptibility as a Cost of Reproduction and Contributor to Life History Evolution

Reproduction is one of the most energetically demanding life-history stages. As a result, breeding individuals often experience trade-offs, where energy is diverted away from maintenance (cell repair, immune function) toward reproduction. While it is increasingly acknowledged that oncogenic processes are omnipresent, evolving and opportunistic entities in the bodies of metazoans, the associations among reproductive activities, energy expenditure, and the dynamics of malignant cells have rarely been studied. Here, we review the diverse ways in which age-specific reproductive performance (e.g., reproductive aging patterns) and cancer risks throughout the life course may be linked via trade-offs or other mechanisms, as well as discuss situations where trade-offs may not exist. We argue that the interactions between host–oncogenic processes should play a significant role in life-history theory, and suggest some avenues for future research.

Hydra's Lasting Partnership with Microbes: The Key for Escaping Senescence?

Aging results from a complex interplay between genetic endowment and environmental exposures during lifetime. As our understanding of the aging process progresses, so does the need for experimental animal models that allow a mechanistic understanding of the genetic and environmental factors involved. One such well-studied animal model is the freshwater polyp Hydra. Hydra are remarkable because they are non-senescent. Much of this non-senescence can be ascribed to a tissue consisting of stem cells with continuous self-renewal capacity. Another important fact is that Hydra's ectodermal epithelial surface is densely colonized by a stable multispecies bacterial community. The symbiotic partnership is driven by interactions among the microbiota and the host. Here, we review key advances over the last decade that are deepening our understanding of the genetic and environmental factors contributing to Hydra's non-senescent lifestyle. We conclude that the microbiome prevents pathobiont invasion (colonization resistance) and stabilizes the patterning mechanisms, and that microbiome malfunction negatively affects Hydra's continuous self-renewal capacity.

Hydra and the hair follicle - An unconventional comparative biology approach to exploring the human holobiont

The microbiome of human hair follicles (HFs) has emerged as an important player in different HF and skin pathologies, yet awaits in-depth exploration. This raises questions regarding the tightly linked interactions between host environment, nutrient dependency of host-associated microbes, microbial metabolism, microbe-microbe interactions and host immunity. The use of simple model systems facilitates addressing generally important questions and testing overarching, therapeutically relevant principles that likely transcend obvious interspecies differences. Here, we evaluate the potential of the freshwater polyp Hydra, to dissect fundamental principles of microbiome regulation by the host, that is the human HF. In particular, we focus on therapeutically targetable host-microbiome interactions, such as nutrient dependency, microbial interactions and host defence. Offering a new lens into the study of HF - microbiota interactions, we argue that general principles of how Hydra manages its microbiota can inform the development of novel, microbiome-targeting therapeutic interventions in human skin disease.

Population Differences and Host Species Predict Variation in the Diversity of Host-Associated Microbes in Hydra

Most animals co-exist with diverse host-associated microbial organisms that often form complex communities varying between individuals, habitats, species and higher taxonomic levels. Factors driving variation in the diversity of host-associated microbes are complex and still poorly understood. Here, we describe the bacterial composition of field-collected Hydra, a freshwater cnidarian that forms stable associations with microbial species in the laboratory and displays complex interactions with components of the microbiota. We sampled Hydra polyps from 21 Central European water bodies and identified bacterial taxa through 16S rRNA sequencing. We asked whether diversity and taxonomic composition of host-associated bacteria depends on sampling location, habitat type, host species or host reproductive mode (sexual vs. asexual). Bacterial diversity was most strongly explained by sampling location, suggesting that the source environment plays an important role in the assembly of bacterial communities associated with Hydra polyps. We also found significant differences between host species in their bacterial composition that partly mirrored variations observed in lab strains. Furthermore, we detected a minor effect of host reproductive mode on bacterial diversity. Overall, our results suggest that extrinsic (habitat identity) factors predict the diversity of host-associated bacterial communities more strongly than intrinsic (species identity) factors, however, only a combination of both factors determines microbiota composition in Hydra.

Transmissible Cancer Evolution: The Under-Estimated Role of Environmental Factors in the “Perfect Storm” Theory

Although the true prevalence of transmissible cancers is not known, these atypical malignancies are likely rare in the wild. The reasons behind this rarity are only partially understood, but the “Perfect Storm hypothesis” suggests that transmissible cancers are infrequent because a precise confluence of tumor and host traits is required for their emergence. This explanation is plausible as transmissible cancers, like all emerging pathogens, will need specific biotic and abiotic conditions to be able to not only emerge, but to spread to detectable levels. Because those conditions would be rarely met, transmissible cancers would rarely spread, and thus most of the time disappear, even though they would regularly appear. Thus, further research is needed to identify the most important factors that can facilitate or block the emergence of transmissible cancers and influence their evolution. Such investigations are particularly relevant given that human activities are increasingly encroaching into wild areas, altering ecosystems and their processes, which can influence the conditions needed for the emergence and spread of transmissible cell lines.

Coral-microbe interactions: their importance to reef function and survival

Many different microorganisms associate with the coral host in a single entity known as the holobiont, and their interactions with the host contribute to coral health, thereby making them a fundamental part of reef function, survival, and conservation. As corals continue to be susceptible to bleaching due to environmental stress, coral-associated bacteria may have a potential role in alleviating bleaching. This review provides a synthesis of the various roles bacteria have in coral physiology and development, and explores the possibility that changes in the microbiome with environmental stress could have major implications in how corals acclimatize and survive. Recent studies on the interactions between the coral's algal and bacterial symbionts elucidate how bacteria may stabilize algal health and, therefore, mitigate bleaching. A summary of the innovative tools and experiments to examine host-microbe interactions in other cnidarians (a temperate coral, a jellyfish, two anemones, and a freshwater hydroid) is offered in this review to delineate our current knowledge of mechanisms underlying microbial establishment and maintenance in the animal host. A better understanding of these mechanisms may enhance the success of maintaining probiotics long-term in corals as a conservation strategy.

Tumors (re)shape biotic interactions within ecosystems: Experimental evidence from the freshwater cnidarian Hydra

While it is often assumed that oncogenic processes in metazoans can influence species interactions, empirical evidence is lacking. Here, we use the cnidarian Hydra oligactis to experimentally explore the consequences of tumor associated phenotypic alterations for its predation ability, relationship with commensal ciliates and vulnerability to predators. Unexpectedly, hydra's predation ability was higher in tumorous polyps compared to non-tumorous ones. Commensal ciliates colonized preferentially tumorous hydras than non-tumorous ones, and had a higher replication rate on the former. Finally, in a choice experiment, tumorous hydras were preferentially eaten by a fish predator. This study, for the first time, provides evidence that neoplastic growth has the potential, through effect(s) on host phenotype, to alter biotic interactions within ecosystems and should thus be taken into account by ecologists.

Culturomics revealed the bacterial constituents of the microbiota of a 10-year-old laboratory culture of planarian species S. mediterranea

The planarian species Schmidtea mediterranea is a flatworm living in freshwater that is used in the research laboratory as a model to study developmental and regeneration mechanisms, as well as antibacterial mechanisms. However, the cultivable microbial repertoire of the microbes comprising its microbiota remains unknown. Here, we characterized the bacterial constituents of a 10-year-old laboratory culture of planarian species S. mediterranea via culturomics analysis. We isolated 40 cultivable bacterial species, including 1 unidentifiable species. The predominant phylum is Proteobacteria, and the most common genus is Pseudomonas. We discovered that parts of the bacterial flora of the planarian S. mediterranea can be classified as fish pathogens and opportunistic human pathogens.

Linking pollution and cancer in aquatic environments: A review

Due to the interconnectedness of aquatic ecosystems through the highly effective marine and atmospheric transport routes, all aquatic ecosystems are potentially vulnerable to pollution. Whilst links between pollution and increased mortality of wild animals have now been firmly established, the next steps should be to focus on specific physiological pathways and pathologies that link pollution to wildlife health deterioration. One of the pollution-induced pathologies that should be at the centre of attention in ecological and evolutionary research is cancer, as anthropogenic contamination has resulted in a rapid increase of oncogenic substances in natural habitats. Whilst wildlife cancer research is an emerging research topic, systematic reviews of the many case studies published over the recent decades are scarce. This research direction would (1) provide a better understanding of the physiological mechanisms connecting anthropogenic pollution to oncogenic processes in non-model organisms (reducing the current bias towards human and lab-animal studies in cancer research), and (2) allow us to better predict the vulnerability of different wild populations to oncogenic contamination. This article combines the information available within the scientific literature about cancer occurrences in aquatic and semi-aquatic species. For the first aim, we use available knowledge from aquatic species to suggest physiological mechanisms that link pollution and cancer, including main metabolic detoxification pathways, oxidative damage effects, infections, and changes to the microbiome. For the second aim, we determine which types of aquatic animals are more vulnerable to pollution-induced cancer, which types of pollution are mainly associated with cancer in aquatic ecosystems, and which types of cancer pollution causes. We also discuss the role of migration in exposing aquatic and semi-aquatic animals to different oncogenic pollutants. Finally, we suggest novel research avenues, including experimental approaches, analysis of the effects of pollutant cocktails and long-term chronic exposure to lower levels of pollutants, and the use of already published databases of gene expression levels in animals from differently polluted habitats.

Animal development in the microbial world: The power of experimental model systems

The development of powerful model systems has been a critical strategy for understanding the mechanisms underlying the progression of an animal through its ontogeny. Here we provide two examples that allow deep and mechanistic insight into the development of specific animal systems. Species of the cnidarian genus Hydra have provided excellent models for studying host-microbe interactions and how metaorganisms function in vivo. Studies of the Hawaiian bobtail squid Euprymna scolopes and its luminous bacterial partner Vibrio fischeri have been used for over 30 years to understand the impact of a broad array of levels, from ecology to genomics, on the development and persistence of symbiosis. These examples provide an integrated perspective of how developmental processes work and evolve within the context of a microbial world, a new view that opens vast horizons for developmental biology research. The Hydra and the squid systems also lend an example of how profound insights can be discovered by taking advantage of the "experiments" that evolution had done in shaping conserved developmental processes.