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Many cnidarians rely on their dinoflagellate partners from the family Symbiodiniaceae for their ecological success. Symbiotic species of Symbiodiniaceae have two distinct life stages: inside the host, in hospite , and outside the host, ex hospite . Several aspects of cnidarian-algal symbiosis can be understood by comparing these two life stages. Most commonly, algae in culture are used in comparative studies to represent the ex hospite life stage, however, nutrition becomes a confounding variable for this comparison because algal culture media is nutrient rich, while algae in hospite are sampled from hosts maintained in oligotrophic seawater. In contrast to cultured algae, expelled algae may be a more robust representation of the ex hospite state, as the host and expelled algae are in the same seawater environment, removing differences in culture media as a confounding variable. Here, we studied the physiology of algae released from the sea anemone Exaiptasia diaphana (commonly called Aiptasia), a model system for the study of coral-algal symbiosis. In Aiptasia, algae are released in distinct pellets, referred to as egesta, and we explored its potential as an experimental system to represent Symbiodiniaceae in the ex hospite state. Observation under confocal and differential interference contrast microscopy revealed that egesta contained discharged nematocysts, host tissue, and were populated by a diversity of microbes, including protists and cyanobacteria. Further experiments revealed that egesta were released at night. In addition, algae in egesta had a higher mitotic index than algae in hospite , were photosynthetically viable for at least 48 hrs after expulsion, and could competently establish symbiosis with aposymbiotic Aiptasia. We then studied the gene expression of nutrient-related genes and studied their expression using qPCR. From the genes tested, we found that algae from egesta closely mirrored gene expression profiles of algae in hospite and were dissimilar to those of cultured algae, suggesting that algae from egesta are in a nutritional environment that is similar to their in hospite counterparts. Altogether, evidence is provided that algae from Aiptasia egesta are a robust representation of Symbiodiniaceae in the ex hospite state and their use in experiments can improve our understanding of cnidarian-algal symbiosis.
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Development of genetic tools for the sea anemone Aiptasia, a model system for coral biology
Abstract The reef-building corals can thrive in nutrient-poor waters because of the mutualistic symbiosis between the animal hosts and their photosynthetic dinoflagellate endosymbionts. This symbiosis is threatened by climate change and other anthropogenic stressors, so that a deeper mechanistic understanding of its function is not only of great basic biological interest but also crucial for developing rational approaches to coral conservation. The small sea anemone Aiptasia is an attractive model system for studies of this symbiosis but has been limited to date by a lack of effective genetic methods. Here, we describe the use of a simple electroporation protocol to introduce various genetic constructs [plasmid DNAs, mRNAs, and short-hairpin (sh) RNAs] into Aiptasia zygotes. Plasmid-based expression of reporter constructs in the resulting larvae was highly mosaic. In contrast, electroporation of mRNAs into zygotes resulted in uniform expression within the larvae, and success rates were similar when single or multiple mRNAs were introduced. The shRNAs were effective in knocking down expression of both coelectroporated mRNAs and endogenous genes. In this way, we could confirm the previously reported role of BRACHYURY in cnidarian embryonic development. In addition, we could show that knockdown of an Aiptasia homologue of the lysosomal-associated membrane protein 1 interfered with larval uptake and/or retention of a symbiosis-compatible algal strain. The ability to use Aiptasia larvae for such reverse-genetic studies should greatly enhance the power of this model system and serve as a starting point for further development of genetic tools in Aiptasia and other cnidarians.
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- Award ID(s):
- 2128072
- PAR ID:
- 10647125
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- GENETICS
- Volume:
- 231
- Issue:
- 3
- ISSN:
- 1943-2631
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Exaiptasia diaphana , a tropical sea anemone known as Aiptasia, is a tractable model system for studying the cellular, physiological, and ecological characteristics of cnidarian-dinoflagellate symbiosis. Aiptasia is widely used as a proxy for coral-algal symbiosis, since both Aiptasia and corals form a symbiosis with members of the family Symbiodiniaceae. Laboratory strains of Aiptasia can be maintained in both the symbiotic (Sym) and aposymbiotic (Apo, without algae) states. Apo Aiptasia allow for the study of the influence of symbiosis on different biological processes and how different environmental conditions impact symbiosis. A key feature of Aiptasia is the ease of propagating both Sym and Apo individuals in the laboratory through a process called pedal laceration. In this form of asexual reproduction, small pieces of tissue rip away from the pedal disc of a polyp, then these lacerates eventually develop tentacles and grow into new polyps. While pedal laceration has been described in the past, details of how tentacles are formed or how symbiotic and nutritional state influence this process are lacking. Here we describe the stages of development in both Sym and Apo pedal lacerates. Our results show that Apo lacerates develop tentacles earlier than Sym lacerates, while over the course of 20 days, Sym lacerates end up with a greater number of tentacles. We describe both tentacle and mesentery patterning during lacerate development and show that they form through a single pattern in early stages regardless of symbiotic state. In later stages of development, Apo lacerate tentacles and mesenteries progress through a single pattern, while variable patterns were observed in Sym lacerates. We discuss how Aiptasia lacerate mesentery and tentacle patterning differs from oral disc regeneration and how these patterning events compare to postembryonic development in Nematostella vectensis , another widely-used sea anemone model. In addition, we demonstrate that Apo lacerates supplemented with a putative nutrient source developed an intermediate number of tentacles between un-fed Apo and Sym lacerates. Based on these observations, we hypothesize that pedal lacerates progress through two different, putatively nutrient-dependent phases of development. In the early phase, the lacerate, regardless of symbiotic state, preferentially uses or relies on nutrients carried over from the adult polyp. These resources are sufficient for lacerates to develop into a functional polyp. In the late phase of development, continued growth and tentacle formation is supported by nutrients obtained from either symbionts and/or the environment through heterotrophic feeding. Finally, we advocate for the implementation of pedal lacerates as an additional resource in the Aiptasia model system toolkit for studies of cnidarian-dinoflagellate symbiosis.more » « less
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