Competition between Chlorellae in chimeric infections of Hydra viridis: the evolution of a stable symbiosis

Thermal plasticity of a freshwater cnidarian holobiont: detection of trans-generational effects in asexually reproducing hosts and symbionts

Understanding factors affecting the susceptibility of organisms to thermal stress is of enormous interest in light of our rapidly changing climate. When adaptation is limited, thermal acclimation and deacclimation abilities of organisms are critical for population persistence through a period of thermal stress. Holobionts (hosts plus associated symbionts) are key components of various ecosystems, such as coral reefs, yet the contributions of their two partners to holobiont thermal plasticity are poorly understood. Here, we tested thermal plasticity of the freshwater cnidarian Hydra viridissima (green hydra) using individual behavior and population responses. We found that algal presence initially reduced hydra thermal tolerance. Hydra with algae (symbiotic hydra) had comparable acclimation rates, deacclimation rates, and thermal tolerance after acclimation to those without algae (aposymbiotic hydra) but they had higher acclimation capacity. Acclimation of the host (hydra) and/or symbiont (algae) to elevated temperatures increased holobiont thermal tolerance and these effects persisted for multiple asexual generations. In addition, acclimated algae presence enhanced hydra fitness under prolonged sublethal thermal stress, especially when food was limited. Our study indicates while less intense but sublethal stress may favor symbiotic organisms by allowing them to acclimate, sudden large, potentially lethal fluctuations in climate stress likely favor aposymbiotic organisms. It also suggests that thermally stressed colonies of holobionts could disperse acclimated hosts and/or symbionts to other colonies, thereby reducing their vulnerability to climate change.

What hydra has to say about the role and origin of symbiotic interactions

The Hydra holobiont involves at least three types of organisms that all share a long coevolutionary history and appear to depend on each other. Here I review how symbiotic algae and stably associated bacteria interact with the Hydra host and where in the tissue they are located. In particular I discuss the role of Toll-like receptor (TLR) signaling in maintaining Hydra's species-specific microbiota. I also discuss studies in Hydra viridis and its symbiotic Chlorella algae which indicate that the symbiotic algae are critically involved in the control of sexual differentiation in green Hydra. Finally, I review the state of "omics" in this tripartite association and the fact that the functioning of this holobiont is also a tale of several genomes.

Common trends in mutualism revealed by model associations between invertebrates and bacteria

Mutually beneficial interactions between microorganisms and animals are a conserved and ubiquitous feature of biotic systems. In many instances animals, including humans, are dependent on their microbial associates for nutrition, defense, or development. To maintain these vital relationships, animals have evolved processes that ensure faithful transmission of specific microbial symbionts between generations. Elucidating mechanisms of transmission and symbiont specificity has been aided by the study of experimentally tractable invertebrate animals with diverse and highly evolved associations with microorganisms. Here, we review several invertebrate model systems that contribute to our current understanding of symbiont transmission, recognition, and specificity. Although the details of transmission and symbiont selection vary among associations, comparisons of diverse mutualistic associations are revealing a number of common themes, including restriction of symbiont diversity during transmission and glycan-lectin interactions during partner selection and recruitment.

SymbioticHydraexpress a plant-like peroxidase gene during oogenesis

Symbiotic associations accompanied by gene exchange between the symbionts form the phylogenetic origin of eukaryotic cells and, therefore, had significant impact on species diversity and evolutionary novelty. Among the phylogenetically oldest metazoan animals known to form symbiotic relationships are the Cnidaria. In the Cnidarian Hydra viridis, symbiotic algae of the genus Chlorella are located in endodermal epithelial cells and impact sexual differentiation. When screening for Hydra viridis genes that are differentially expressed during symbiosis, we found a gene, HvAPX1, coding for a plant-related ascorbate peroxidase. HvAPX1 is expressed exclusively during oogenesis and in contrast to all known ascorbate peroxidase genes in plants does not contain introns. No member of this gene family has previously been identified from a member of the animal kingdom. We discuss the origin of the HvAPX1 gene and propose that it may have been transferred horizontally following an endosymbiotic event early in evolution of the Hydra lineage as an RNA or cDNA intermediate.

Coral bleaching: a potential biomarker of environmental stress

Coral bleaching refers to the loss of symbiotic algae by host corals, or to the loss of pigmentation by the algae themselves, causing corals to appear white or "bleached." Some corals may regain algae or pigmentation and survive, but when bleaching is severe the host coral dies. Coral bleaching events have increased dramatically in the last two decades, and coral reefs throughout the world have been extensively degraded as a result. This article reviews coral bleaching for investigators working in the field of toxicology and environmental health, a group of scientists not normally exposed to this issue. Several environmental stressors have been correlated with bleaching, including fluctuations in sea surface temperatures and salinity, increased sedimentation, increased solar radiation, and contaminants such as oil and herbicides. Molecular mechanisms of bleaching are only beginning to be investigated and are thus far poorly understood. Toxicologists have the potential to make significant contributions toward understanding anthropogenic aspects of coral bleaching and elucidating molecular mechanisms of this important environmental problem.

The Hydra viridis / Chlorella symbiosis. Growth and sexual differentiation in polyps without symbionts

To investigate interactions between the basal metazoan Hydra viridis and its symbiotic Chlorella algae, we generated aposymbiotic hydra lacking algae and compared them to symbiotic ones with regard to growth and sexual differentiation. Under standard feeding conditions aposymbiotic polyps proliferated similarly to symbiotic polyps. Under moderate and low feeding conditions asexual growth was reduced in polyps lacking algae, indicating that the symbionts supply nutrients to their hosts. In addition, the Chlorella symbionts had a strong influence on the sexual reproduction of Hydra viridis: in most cases female gonads were produced only when symbiotic algae were present. Spermatogenesis proceeded similarly in symbiotic and aposymbiotic polyps. Since during oogenesis symbionts are actively transferred from endodermal epithelial cells to the ectodermal oocytes, this oogenesis promoting role could indicate that the symbionts are critically involved in the control of sexual differentiation in green hydra.