Coral reefs are global resources of great ecological, economic, and aesthetic value. The success of corals in their typically nutrient-poor environments is due largely to their symbiosis with dinoflagellates of the genus Symbiodinium. These algae inhabit the symbiosome (a vacuole derived from the early endosome) in gastrodermal cells of the host [1–4] and transfer up to 95% of their photosynthetically fixed carbon to the host . Reef-building corals have recently declined worldwide, with pollution, disease, destructive fishing practices, increased sea-surface temperatures, and ocean acidification all implicated as contributory factors. Some of these environmental changes affect the symbiotic relationship between algae and host and can lead to dramatic and potentially lethal “bleaching” events, during which the algae are lost and the host may die. Bleaching events have become more frequent over the past 20 years.
Much recent research in coral biology has focused on the effects of stresses – particularly high temperature and lowered pH – on the coral holobiont (the community of living organisms making up a healthy coral), as well as on which genetic and molecular factors of the host and alga lead to differential stress responses and resilience [6–13]. However, these efforts have been impeded by the lack of an experimentally tractable system for studies of the establishment, maintenance, and breakdown of the symbiosis. Corals themselves present major logistical difficulties for laboratory investigation. They grow slowly and are difficult and costly to maintain, their calcareous skeletons make many biochemical and cell biological techniques difficult, and it can be difficult to obtain sufficient biomass to do high-throughput experiments. In addition, samples collected from the wild can have heterogeneous genetic backgrounds, causing difficulties in the application and interpretation of gene-expression studies.
To circumvent these difficulties, we and others are developing the small sea anemone Aiptasia as a model system for studies of dinoflagellate-cnidarian symbiosis [14, 15]. Like corals, Aiptasia is an anthozoan (a Class in the Phylum Cnidaria) and maintains intracellular symbiotic dinoflagellates closely related to those in corals. However, unlike corals, Aiptasia is extremely hardy, grows and reproduces rapidly via asexual reproduction in the laboratory (allowing the generation of large clonal populations), and lacks a calcareous skeleton. The lack of skeletal deposition makes Aiptasia an unsuitable model for this aspect of coral biology but greatly facilitates other studies of cell biology and biochemistry. Additionally, Aiptasia can exist in an aposymbiotic (dinoflagellate-free) state or host a variety of Symbiodinium types (although not all), allowing facile studies of symbiosis specificity [14, 16, 17]. We have recently developed a protocol for the year-round induction of spawning and larvae production in laboratory-raised Aiptasia, which should free a variety of studies from dependence on the seasonal coral reproductive cycle and potentially open the door to genetic analysis.
Studies of the dinoflagellate-cnidarian symbiosis can take advantage of genomics approaches. For example, gene-expression studies should help to elucidate how symbiotic cnidarians respond to various stressors, whereas comparative genomics approaches using sequence data from cnidarians that are not symbiotic with dinoflagellates should help us understand how these symbioses evolved. Genomic and transcriptomic resources for cnidarians are beginning to accumulate rapidly, thanks to the advent of new sequencing technologies. Recently, the genome of Acropora digitifera, a common Indo-Pacific coral, was sequenced and assembled . In addition, the genomes of two non-symbiotic cnidarians, the anemone Nematostella vectensis (an anthozoan) and the more distantly related Hydra magnipapillata (in Class Hydrozoa), have been sequenced [20, 21]. Small, Sanger-sequenced EST datasets are available for several species of corals and anemones [15, 22, 23], as are larger 454-sequenced datasets for several corals [24–26].
As a step in the development of Aiptasia as a model system, we have performed a detailed analysis of the transcriptome of the aposymbiotic animals. Unlike previous transcriptomes in the field of symbiotic cnidarian biology, these data are derived from a clonal and easily distributed strain of anemone, greatly facilitating a straightforward comparison of experimental results between different laboratories.