Animals interact with a variety of microorganims and these interactions range from harmful to beneficial. One of the most important beneficial relationships is that between cnidarians (e.g., corals and sea anemones) and single-celled algae that live within cnidarian tissues. It is this relationship that enables coral reefs to thrive and to provide a number of ecosystem services. The stability of this relationship likely involves the interplay between the cnidarian immune response and the algal symbiont. However, we have a poor understanding of cnidarian immunity. The proposed research employs microarrays, a tool for looking at the expression of thousands of genes simultaneously, to characterize the immunological ?tool kit? of cnidarians and to determine how this ?tool kit? interacts with both harmful and beneficial microorganisms.
Because cnidarians are relatively simple animals, this study will provide insight into the development of animal immune systems and the subsequent changes that allowed for more complex responses. This study will leverage a vast literature on interactions between animal cells and parasitic protozoans, like those that cause malaria, to initiate a comparative exploration of the parallels and novelties of beneficial and harmful interactions. This research will also enable a better understanding of coral bleaching and disease, both of which are responsible for substantial coral reef decline. Finally, this research will support undergraduate training and a number of outreach programs catering to local middle-school students, undergraduates, and the general public.
Animals interact with a variety of microorganisms and these interactions range from harmful to beneficial. One of the most important beneficial relationships is that between cnidarians (e.g., corals and sea anemones) and single-celled algae that live within cnidarian cells. It is this relationship that enables coral reefs to thrive and to provide a number of ecosystem services. Cnidarians are also exposed to a variety of harmful pathogens that can lead to disease. Whether the relationship with microbes is beneficial or harmful, these interactions likely involve a complex interplay between the host immune system and the microorganism. The goal of this research was to gain a better understanding of cnidarian immunity by looking at what genes are turned on or off when cnidarians are in contact with harmful and/or beneficial microbes. We first looked at which genes were turned on/off when sea anemones were in a healthy, beneficial relationship with algae. We found many genes involved in this interaction. Since the algae provide the sea anemone with sugars via photosynthesis, as well as other metabolites (e.g. amino acids and fats), we found that genes involved in metabolism and transport (movement of the metabolites from the algae to the anemone) were turned on. We also found that genes that play important roles in the anemone immune response were turned on/off, suggesting that the algae must interact with anemone immune system. We were then interested in what genes were turned on/off when anemones were exposed to harmful bacteria. We found a large number of genes that were turned on/off, and many of these genes have been shown to play a role in immunity in other organisms. These include genes that send signals within cells, informing the organism that an intruder has been detected and giving instructions for how to get rid of the intruders. These genes also play a role in programmed cell death, or apoptosis, a process whereby a cell with commit suicide once it has been infected to prevent the harmful bacteria from spreading. Interestingly, many of these same genes are turned on/off in humans when they are exposed to harmful bacteria. This suggests that cnidarians have very complex immune systems that evolved very early in animal evolution, since anemones are very ancient animals. Lastly, we were interested in determining if the beneficial algae change how the anemone's immune system works. We compared anemones with and without algae that were exposed to harmful bacteria to see if their responses differed. We found that different numbers of genes were turned on/off in the two different types of anemones; there were more genes that were turned on/off in anemones without their algae. We also found that different genes were turned on/off in the two different types of anemones. In anemones without their algae, many of the genes that were turned on/off play a role in immunity (as described above). However, in anemones with their algae, many of the genes that were turned on/off play roles in other processes and there are not nearly as many genes with immune-related functions that are turned on/off. We conducted an additional experiment to see if these differences would lead to differences in how anemones physically respond to pathogens. We found that anemones with algae died more often than anemones without algae. Therefore, we conclude that anemones with algae may have a suppressed immune system, and this is how the beneficial relationship with the algae is maintained. This may seem counterintuitive, since the relationship with the algae is considered a beneficial relationship. However, the algae is still a foreign cell living in the anemone. If mechanisms were not put into place to prevent the algae cell from being recognized as foreign and and potentially eliminated, the relationship would not be successful/beneficial. Interestingly, the algae are closely related to harmful organisms that are very successful, inluding the parasites that cause malaria. These parasites are successful, partly because they are able to 'trick' the host immune system and prevent it from killing the parasite. Its possible that certain functions of the anemone immune system are similarly 'tricked' by the dinoflagellate, thus enabling the beneficial relationship to persist. Our research has provided a better understanding of how cnidarians interact with beneficial and harmful microorganisms. We have generated a list of genes that were turned on/off during these interactions that can be investigated in further detail. Coral reefs are declining at an alarming rate, in part because of coral disease and coral bleaching. Coral diseases are caused by harmful microorganisms, whereas bleaching is the breakdown of the beneficial relationship with the algae. Corals are closely related to sea anemones, so our results can be applied to coral biology and will hopefully lead to a better understanding of coral disease and bleaching and how/if we can help prevent these detrimental processes.