Sea Anemone Aiptasia Pallida Research Articles

Response of bleached and symbiotic sea anemones to plastic microfiber exposure

Microplastics are emerging contaminants in the marine environment. They enter the ocean in a variety of sizes and shapes, with plastic microfiber being the prevalent form in seawater and in the guts of biota. Most of the laboratory experiments on microplastics has been performed with spheres, so knowledge on the interactions of microfibers and marine organisms is limited. In this study we examined the ingestion of microfibers by the sea anemone Aiptasia pallida using three different types of polymers: nylon, polyester and polypropylene. The polymers were offered to both symbiotic (with algal symbionts) and bleached (without algal symbionts) anemones. The polymers were introduced either alone or mixed with brine shrimp homogenate. We observed a higher percentage of nylon ingestion compared to the other polymers when plastic was offered in the absence of shrimp. In contrast, we observed over 80% of the anemones taking up all types of polymers when the plastics were offered in the presence of shrimp. Retention time differed significantly between symbiotic and bleached anemones with faster egestion in symbiotic anemones. Our results suggest that ingestion of microfibers by sea anemones is dependent both on the type of polymers and on the presence of chemical cues of prey in seawater. The decreased ability of bleached anemones to reject plastic microfiber indicates that the susceptibility of anthozoans to plastic pollution is exacerbated by previous exposure to other stressors. This is particularly concerning given that coral reef ecosystems are facing increases in the frequency and intensity of bleaching events due to ocean warming.

Sphingolipid Metabolism of a Sea Anemone Is Altered by the Presence of Dinoflagellate Symbionts.

In host-microbe interactions, signaling lipids function in interpartner communication during both the establishment and maintenance of associations. Previous evidence suggests that sphingolipids play a role in the mutualistic cnidarian-Symbiodinium symbiosis. Exogenously applied sphingolipids have been shown to alter this partnership, though endogenous host regulation of sphingolipids by the sphingosine rheostat under different symbiotic conditions has not been characterized. The rheostat regulates levels of pro-survival sphingosine-1-phosphate (S1P) and pro-apoptotic sphingosine (Sph) through catalytic activities of sphingosine kinase (SPHK) and S1P phosphatase (SGPP). The role of the rheostat in recognition and establishment of cnidarian-Symbiodinium symbiosis was investigated in the sea anemone Aiptasia pallida by measuring gene expression, protein levels, and sphingolipid metabolites in symbiotic, aposymbiotic, and newly recolonized anemones. Comparison of two host populations showed that symbiotic animals from one population had lower SGPP gene expression and Sph lipid concentrations compared to aposymbiotic animals, while the other population had higher S1P concentrations than their aposymbiotic counterparts. In both populations, the host rheostat trended toward host cell survival in the presence of symbionts. Furthermore, upregulation of both rheostat enzymes on the first day of host recolonization by symbionts suggests a role for the rheostat in host-symbiont recognition during symbiosis onset. Collectively, these data suggest a regulatory role of sphingolipid signaling in cnidarian-Symbiodinium symbiosis and symbiont uptake.

The scavenger receptor repertoire in six cnidarian species and its putative role in cnidarian-dinoflagellate symbiosis.

Many cnidarians engage in a mutualism with endosymbiotic photosynthetic dinoflagellates that forms the basis of the coral reef ecosystem. Interpartner interaction and regulation includes involvement of the host innate immune system. Basal metazoans, including cnidarians have diverse and complex innate immune repertoires that are just beginning to be described. Scavenger receptors (SR) are a diverse superfamily of innate immunity genes that recognize a broad array of microbial ligands and participate in phagocytosis of invading microbes. The superfamily includes subclades named SR-A through SR-I that are categorized based on the arrangement of sequence domains including the scavenger receptor cysteine rich (SRCR), the C-type lectin (CTLD) and the CD36 domains. Previous functional and gene expression studies on cnidarian-dinoflagellate symbiosis have implicated SR-like proteins in interpartner communication and regulation. In this study, we characterized the SR repertoire from a combination of genomic and transcriptomic resources from six cnidarian species in the Class Anthozoa. We combined these bioinformatic analyses with functional experiments using the SR inhibitor fucoidan to explore a role for SRs in cnidarian symbiosis and immunity. Bioinformatic searches revealed a large diversity of SR-like genes that resembled SR-As, SR-Bs, SR-Es and SR-Is. SRCRs, CTLDs and CD36 domains were identified in multiple sequences in combinations that were highly homologous to vertebrate SRs as well as in proteins with novel domain combinations. Phylogenetic analyses of CD36 domains of the SR-B-like sequences from a diversity of metazoans grouped cnidarian with bilaterian sequences separate from other basal metazoans. All cnidarian sequences grouped together with moderate support in a subclade separately from bilaterian sequences. Functional experiments were carried out on the sea anemone Aiptasia pallida that engages in a symbiosis with Symbiodinium minutum (clade B1). Experimental blocking of the SR ligand binding site with the inhibitor fucoidan reduced the ability of S. minutum to colonize A. pallida suggesting that host SRs play a role in host-symbiont recognition. In addition, incubation of symbiotic anemones with fucoidan elicited an immune response, indicating that host SRs function in immune modulation that results in host tolerance of the symbionts.

The Role of Complement in Cnidarian-Dinoflagellate Symbiosis and Immune Challenge in the Sea Anemone Aiptasia pallida.

The complement system is an innate immune pathway that in vertebrates, is responsible for initial recognition and ultimately phagocytosis and destruction of microbes. Several complement molecules including C3, Factor B, and mannose binding lectin associated serine proteases (MASP) have been characterized in invertebrates and while most studies have focused on their conserved role in defense against pathogens, little is known about their role in managing beneficial microbes. The purpose of this study was to (1) characterize complement pathway genes in the symbiotic sea anemone Aiptasia pallida, (2) investigate the evolution of complement genes in invertebrates, and (3) examine the potential dual role of complement genes Factor B and MASP in the onset and maintenance of cnidarian-dinoflagellate symbiosis and immune challenge using qPCR based studies. The results demonstrate that A. pallida has multiple Factor B genes (Ap_Bf-1, Ap_Bf-2a, and Ap_Bf-2b) and one MASP gene (Ap_MASP). Phylogenetic analysis indicates that the evolutionary history of complement genes is complex, and there have been many gene duplications or gene loss events, even within members of the same phylum. Gene expression analyses revealed a potential role for complement in both onset and maintenance of cnidarian-dinoflagellate symbiosis and immune challenge. Specifically, Ap_Bf-1 and Ap_MASP are significantly upregulated in the light at the onset of symbiosis and in response to challenge with the pathogen Serratia marcescens suggesting that they play a role in the initial recognition of both beneficial and harmful microbes. Ap_Bf-2b in contrast, was generally downregulated during the onset and maintenance of symbiosis and in response to challenge with S. marcescens. Therefore, the exact role of Ap_Bf-2b in response to microbes remains unclear, but the results suggest that the presence of microbes leads to repressed expression. Together, these results indicate functional divergence between Ap_Bf-1 and Ap_Bf-2b, and that Ap_Bf-1 and Ap_MASP may be functioning together in an ancestral hybrid of the lectin and alternative complement pathways. Overall, this study provides information on the role of the complement system in a basal metazoan and its role in host-microbe interactions.

Impairment of symbiont photosynthesis increases host cell proliferation in the epidermis of the sea anemone Aiptasia pallida

Corals exposed to environmental stresses need to engage appropriate physiological strategies to survive. Here, we examined tissue modifications following algal dysfunction. Aiptasia pallida was exposed during 1 week to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), the herbicide called diuron. DCMU treatment produced a drastic loss in photosynthetic efficiency and a subsequent diminution of algae density over the following days. Cell proliferation evaluated by measuring the number of cells labeled with a thymidine analogue (EdU) revealed a significant increase in EdU+ cells in the epidermis after 1 week of DCMU incubation and in the gastrodermis at 4 weeks. TUNEL histology showed that the extent of cell death was, however, similar in the epidermis of control and treated specimens. In addition, we noticed a significant effect of DCMU treatment on the density of epidermal mucocytes after 1, 2 and 4 weeks. These results show that inhibition of Symbiodinium photosynthesis in the absence of any known direct effect of DCMU on host cells can induce an increase in epidermal host cell proliferation in both the epidermis and the gastrodermis. While new host gastrodermal cells are likely to promote tissue regeneration in order to recruit new algae, the new host epidermal cells may contribute to tissue adaptation following a decrease in energy income. Some of these new epidermal cells, such as mucocytes, may contribute to an eventual increase in the host heterotrophic ability until restoration of algal autotrophic contribution.

Interactions between the tropical sea anemone Aiptasia pallida and Serratia marcescens, an opportunistic pathogen of corals.

Coral reefs are under increasing stress caused by global and local environmental changes, which are thought to increase the susceptibility of corals to opportunistic pathogens. In the absence of an easily culturable model animal, the understanding of the mechanisms of disease progression in corals remains fairly limited. In the present study, we tested the susceptibility of the tropical sea anemone Aiptasia pallida to an opportunistic coral pathogen (Serratia marcescens). A. pallida was susceptible to S. marcescens PDL100 and responded to this opportunistic coral pathogen with darkening of the tissues and retraction of tentacles, followed by complete disintegration of polyp tissues. Histological observations revealed loss of zooxanthellae and structural changes in eosinophilic granular cells in response to pathogen infection. A screen of S. marcescens mutants identified a motility and tetrathionate reductase mutants as defective in virulence in the A. pallida infection model. In co-infections with the wild-type strain, the tetrathionate reductase mutant was less fit within the surface mucopolysaccharide layer of the host coral Acropora palmata.

Outcomes of Infections of Sea Anemone Aiptasia pallida with Vibrio spp. Pathogenic to Corals

Incidents of coral disease are on the rise. However, in the absence of a surrogate animal host, understanding of the interactions between coral pathogens and their hosts remains relatively limited, compared to other pathosystems of similar global importance. A tropical sea anemone, Aiptasia pallida, has been investigated as a surrogate model to study certain aspects of coral biology. Therefore, to test whether the utility of this surrogate model can be extended to study coral diseases, in the present study, we tested its susceptibility to common coral pathogens (Vibrio coralliilyticus and Vibrio shiloi) as well as polymicrobial consortia recovered from the Caribbean Yellow Band Disease (CYBD) lesions. A. pallida was susceptible to each of the tested pathogens. A. pallida responded to the pathogens with darkening of the tissues (associated with an increased melanization) and retraction of tentacles, followed by complete disintegration of polyp tissues. Loss of zooxanthellae was not observed; however, the disease progression pattern is consistent with the behavior of necrotizing pathogens. Virulence of some coral pathogens in Aiptasia was paralleled with their glycosidase activities.

Cover

Loss (bleaching) of symbiotic dinoflagellates (genus Symbiodinium) from the sea anemone Aiptasia pallida caused by elevated temperatures or disruption of symbiont photosynthesis can be restored through exposure to axenic Symbiodinium cultures (top part of figure; Symbiodinium appear red due to chlorophyll autofluorescence under blue light). [Vol. 49, No. 3, pp.447–458]

Increased Cell Proliferation and Mucocyte Density in the Sea Anemone Aiptasia pallida Recovering from Bleaching

Recovery of coral after bleaching episodes is a critical period for the health of the reef ecosystem. While events such as symbiont (genus Symbiodinium) shifting/shuffling or tissue apoptosis have been demonstrated to occur following bleaching, little is known concerning tissue recovery or cell proliferation. Here, we studied the sea anemone Aiptasia pallida exposed to a transient elevation of water temperature combined with high illumination (33°C and 1900 µmolphotons.m−2.s−1 for 30h). Following such treatment bleached anemones showed a significant reduction of their Symbiodinium density. Cell proliferation in the ectodermis and gastrodermis was determined by assessing the densities of cells labeled with a thymidine analogue (EdU). Cell proliferation significantly increased during the first day following stress in both tissue types. This increased cell proliferation returned to pre-stress values after one week. Although cell proliferation was higher in the ectodermis in absence of stress, it was relatively more pronounced in the gastrodermis of stressed anemones. In addition, the ratio of ectodermal mucocytes significantly increased three weeks after induced stress. These results suggest that thermal/photic stress coupled with the loss of the symbionts is able to enhance cell proliferation in both gastrodermis and ectodermis of cnidarians. While new cells formed in the gastrodermis are likely to host new Symbiodinium, the fate of new cells in the ectodermis was only partially revealed. Some new ectodermal cells may, in part, contribute to the increased number of mucocytes which could eventually help strengthen the heterotrophic state until restoration of the symbiosis.

Comparative Lipid Profiling of the Cnidarian Aiptasia pallida and Its Dinoflagellate Symbiont

Corals and other cnidarians house photosynthetic dinoflagellate symbionts within membrane-bound compartments inside gastrodermal cells. Nutritional interchanges between the partners produce carbohydrates and lipids for metabolism, growth, energy stores, and cellular structures. Although lipids play a central role in the both the energetics and the structural/morphological features of the symbiosis, previous research has primarily focused on the fatty acid and neutral lipid composition of the host and symbiont. In this study we conducted a mass spectrometry-based survey of the lipidomic changes associated with symbiosis in the sea anemone Aiptasia pallida, an important model system for coral symbiosis. Lipid extracts from A. pallida in and out of symbiosis with its symbiont Symbiodinium were prepared and analyzed using negative-ion electrospray ionization quadrupole time-of-flight mass spectrometry. Through this analysis we have identified, by exact mass and collision-induced dissociation mass spectrometry (MS/MS), several classes of glycerophospholipids in A. pallida. Several molecular species of di-acyl phosphatidylinositol and phosphatidylserine as well as 1-alkyl, 2-acyl phosphatidylethanolamine (PE) and phosphatidycholine were identified. The 1-alkyl, 2-acyl PEs are acid sensitive suggestive that they are plasmalogen PEs possessing a double bond at the 1-position of the alkyl linked chain. In addition, we identified several molecular species of phosphonosphingolipids called ceramide aminoethylphosphonates in anemone lipid extracts by the release of a characteristic negative product ion at m/z 124.014 during MS/MS analysis. Sulfoquinovosyldiacylglycerol (SQDG), an anionic lipid often found in photosynthetic organisms, was identified as a prominent component of Symbiodinium lipid extracts. A comparison of anemone lipid profiles revealed a subset of lipids that show dramatic differences in abundance when anemones are in the symbiotic state as compared to the non-symbiotic state. The data generated in this analysis will serve as a resource to further investigate the role of lipids in symbiosis between Symbiodinium and A. pallida.

Tracking losses of brevetoxins on exposure to phytoplankton competitors: Ecological impacts

The frequent occurrence of devastating blooms of the harmful dinoflagellate Karenia brevis in the Gulf of Mexico has motivated research into bloom dynamics and potential mitigation strategies. The use of competing phytoplankton to lower waterborne concentrations of the most abundant and toxic brevetoxins produced during these blooms has been proposed. However the ecological impacts of using such biocontrol agents have not been addressed. This study investigated the impact on marine invertebrates of lowered brevetoxin concentrations due to the presence of competing phytoplankton. Even at low brevetoxin concentrations, the presence of the common diatom Skeletonema grethae ameliorated harmful toxic effects of brevetoxins upon the brine shrimp, Artemia salina, and reduced the incidence of negative physiological and morphological responses of the sea anemone Aiptasia pallida. In addition, brevetoxin biotransformation products formed by competing phytoplankton appear to be non-toxic or do not trigger the same physiological responses as brevetoxins in the model organisms used. These findings may impact the interpretation of ecotoxicological data gathered during bloom events, since the presence of phytoplankton competitors in Karenia blooms is likely to reduce the harmful effects seen on many marine invertebrates.

Characterisation of Nitric Oxide Synthase in Three Cnidarian-Dinoflagellate Symbioses

BackgroundNitric oxide synthase (NOS) is an enzyme catalysing the conversion of L-arginine to L-citrulline and nitric oxide (NO), the latter being an essential messenger molecule for a range of biological processes. Whilst its role in higher vertebrates is well understood little is known about the role of this enzyme in early metazoan groups. For instance, NOS-mediated signalling has been associated with Cnidaria-algal symbioses, however controversy remains about the contribution of enzyme activities by the individual partners of these mutualistic relationships.Methodology/Principal FindingsUsing a modified citrulline assay we successfully measured NOS activity in three cnidarian-algal symbioses: the sea anemone Aiptasia pallida, the hard coral Acropora millepora, and the soft coral Lobophytum pauciflorum, so demonstrating a wide distribution of this enzyme in the phylum Cnidaria. Further biochemical (citrulline assay) and histochemical (NADPH-diaphorase) investigations of NOS in the host tissue of L. pauciflorum revealed the cytosolic and calcium dependent nature of this enzyme and its in situ localisation within the coral's gastrodermal tissue, the innermost layer of the body wall bearing the symbiotic algae. Interestingly, enzyme activity could not be detected in symbionts freshly isolated from the cnidarians, or in cultured algal symbionts.Conclusions/SignificanceThese results suggest that NOS-mediated NO release may be host-derived, a finding that has the potential to further refine our understanding of signalling events in cnidarian-algal symbioses.

Generation and analysis of transcriptomic resources for a model system on the rise: the sea anemone Aiptasia pallida and its dinoflagellate endosymbiont

BackgroundThe most diverse marine ecosystems, coral reefs, depend upon a functional symbiosis between cnidarian hosts and unicellular dinoflagellate algae. The molecular mechanisms underlying the establishment, maintenance, and breakdown of the symbiotic partnership are, however, not well understood. Efforts to dissect these questions have been slow, as corals are notoriously difficult to work with. In order to expedite this field of research, we generated and analyzed a collection of expressed sequence tags (ESTs) from the sea anemone Aiptasia pallida and its dinoflagellate symbiont (Symbiodinium sp.), a system that is gaining popularity as a model to study cellular, molecular, and genomic questions related to cnidarian-dinoflagellate symbioses.ResultsA set of 4,925 unique sequences (UniSeqs) comprising 1,427 clusters of 2 or more ESTs (contigs) and 3,498 unclustered ESTs (singletons) was generated by analyzing 10,285 high-quality ESTs from a mixed host/symbiont cDNA library. Using a BLAST-based approach to predict which unique sequences derived from the host versus symbiont genomes, we found that the contribution of the symbiont genome to the transcriptome was surprisingly small (1.6–6.4%). This may reflect low levels of gene expression in the symbionts, low coverage of alveolate genes in the sequence databases, a small number of symbiont cells relative to the total cellular content of the anemones, or failure to adequately lyse symbiont cells. Furthermore, we were able to identify groups of genes that are known or likely to play a role in cnidarian-dinoflagellate symbioses, including oxidative stress pathways that emerged as a prominent biological feature of this transcriptome. All ESTs and UniSeqs along with annotation results and other tools have been made accessible through the implementation of a publicly accessible database named AiptasiaBase.ConclusionWe have established the first large-scale transcriptomic resource for Aiptasia pallida and its dinoflagellate symbiont. These data provide researchers with tools to study questions related to cnidarian-dinoflagellate symbioses on a molecular, cellular, and genomic level. This groundwork represents a crucial step towards the establishment of a tractable model system that can be utilized to better understand cnidarian-dinoflagellate symbioses. With the advent of next-generation sequencing methods, the transcriptomic inventory of A. pallida and its symbiont, and thus the extent of AiptasiaBase, should expand dramatically in the near future.

Identification and Gene Expression Analysis of a Taxonomically Restricted Cysteine-Rich Protein Family in Reef-Building Corals

The amount of genomic sequence information continues to grow at an exponential rate, while the identification and characterization of genes without known homologs remains a major challenge. For non-model organisms with limited resources for manipulative studies, high-throughput transcriptomic data combined with bioinformatics methods provide a powerful approach to obtain initial insights into the function of unknown genes. In this study, we report the identification and characterization of a novel family of putatively secreted, small, cysteine-rich proteins herein named Small Cysteine-Rich Proteins (SCRiPs). Their discovery in expressed sequence tag (EST) libraries from the coral Montastraea faveolata required the performance of an iterative search strategy based on BLAST and Hidden-Markov-Model algorithms. While a discernible homolog could neither be identified in the genome of the sea anemone Nematostella vectensis, nor in a large EST dataset from the symbiotic sea anemone Aiptasia pallida, we identified SCRiP sequences in multiple scleractinian coral species. Therefore, we postulate that this gene family is an example of lineage-specific gene expansion in reef-building corals. Previously published gene expression microarray data suggest that a sub-group of SCRiPs is highly responsive to thermal stress. Furthermore, data from microarray experiments investigating developmental gene expression in the coral Acropora millepora suggest that different SCRiPs may play distinct roles in the development of corals. The function of these proteins remains to be elucidated, but our results from in silico, transcriptomic, and phylogenetic analyses provide initial insights into the evolution of SCRiPs, a novel, taxonomically restricted gene family that may be responsible for a lineage-specific trait in scleractinian corals.

Cyclophilin and the Regulation of Symbiosis inAiptasia pallida

The sea anemone Aiptasia pallida, symbiotic with intracellular dinoflagellates, expresses a peptydyl-prolyl cis-trans isomerase (PPIase) belonging to the conserved family of cytosolic cyclophilins (ApCypA). Protein extracts from A. pallida exhibited PPIase activity. Given the high degree of conservation of ApCypA and its known function in the cellular stress response, we hypothesized that it plays a similar role in the cnidarian-dinoflagellate symbiosis. To explore its role, we inhibited the activity of cyclophilin with cyclosporin A (CsA). CsA effectively inhibited the PPIase activity of protein extracts from symbiotic A. pallida. CsA also induced the dose-dependent release of symbiotic algae from host tissues (bleaching). Laser scanning confocal microscopy using superoxide and nitric oxide-sensitive fluorescent dyes on live specimens of A. pallida revealed that CsA strongly induced the production of these known mediators of bleaching. We tested whether the CsA-sensitive isomerase activity is important for maintaining the activity of the antioxidant enzyme superoxide dismutase (SOD). SOD activity of protein extracts was not affected by pre-incubation with CsA in vitro.

Hyperthermic stress-induced increase in the expression of glutamate-cysteine ligase and glutathione levels in the symbiotic sea anemone Aiptasia pallida

Hyperthermic stress is known to trigger the loss of unicellular algae from a number of symbiotic cnidarians, a phenomenon commonly referred to as bleaching. Oxidative and nitrosative stress have been suggested to play a major role during the process of bleaching, however the underlying molecular mechanisms are still poorly understood. In animals, the intracellular tripeptide glutathione (GSH) is involved in antioxidant defense, redox homeostasis and intracellular redox signaling. Therefore, we tested the hypothesis that hyperthermal stress-induced bleaching in Aiptasia pallida, a model for symbiotic cnidarians, results in increased levels of GSH synthesis. We report the cDNA sequence and functional analysis of the catalytic subunit of glutamate-cysteine ligase (GCLC), which catalyzes the rate-limiting step in GSH biosynthesis. In a time-series experiment, both GCLC gene expression and total GSH levels increased 4- and 1.5-fold, respectively, in response to hyperthermal stress. These results suggest that hyperthermal stress triggers adaptive increases in intracellular GSH biosynthesis in cnidarians as a protective response to oxidative/nitrosative stress. Our results show the conserved function of GCLC and GSH across animals while placing a new perspective on the role of GSH in redox signaling during cnidarian bleaching.

Knockdown of Actin and Caspase Gene Expression by RNA Interference in the Symbiotic AnemoneAiptasia pallida

Since the discovery of the ancient eukaryotic process of RNA-mediated gene silencing, the reverse-genetics technique RNA interference (RNAi) has increasingly been used to examine gene function in vertebrate and invertebrate systems. In this study, we report on the use of RNAi, adapted from studies on animal model systems, to manipulate gene expression in a symbiotic marine cnidarian. We describe gene knockdown of actin and of acasp--a cysteine protease, or caspase--in the symbiotic sea anemone Aiptasia pallida. Knockdown was assessed qualitatively with in situ hybridizations for both genes. Quantitative PCR and caspase activity assays were used as a quantitative measure of knockdown for acasp.

Nitric oxide and cnidarian bleaching: an eviction notice mediates breakdown of a symbiosis

Nitric oxide (NO) is a free radical implicated in numerous cell signaling, physiological and pathophysiological processes of eukaryotic cells. Here, we describe the production of NO as part of the cellular stress response of the symbiotic sea anemone Aiptasia pallida, which hosts dinoflagellates from the genus Symbiodinium. We show that exposure to elevated temperatures induces symbiotic anemones to produce high levels of NO, leading to the collapse of the symbiosis. These results shed light on the poorly understood cellular mechanism through which elevated seawater temperature causes the release of symbiotic algae from symbiotic cnidarians, a detrimental process known as coral (cnidarian) bleaching. The results presented here show that the host cell is a major source of NO during exposure to elevated temperatures and that this constitutes a cytotoxic response leading to bleaching. These results have important evolutionary implications as the observed NO production in these basal metazoans displays many parallels to the cytotoxic inflammatory response to pathogens, a well-understood process in mammalian model systems.

Highly Conserved Caspase and Bcl-2 Homologues from the Sea Anemone Aiptasia pallida: Lower Metazoans as Models for the Study of Apoptosis Evolution

Key insight into the complexities of apoptosis may be gained from the study of its evolution in lower metazoans. In this study we describe two genes from a cnidarian, Aiptasia pallida, that are homologous to key genes in the apoptotic pathway from vertebrates. The first is a novel ancient caspase, acasp, that displays attributes of both initiator and executioner caspases and includes a caspase recruitment domain (CARD). The second, a Bcl-2 family member, abhp, contains a BH1 and BH2 domain and shares structural characteristics and phylogenetic affinity with a group of antiapoptotic Bcl-2s including A1 and Bcl-2L10. The breadth of occurrence of other invertebrate homologues across the phylogenetic trees of both genes suggests that the complexity of apoptotic pathways is an ancient trait that predates the evolution of vertebrates and higher invertebrates such as nematodes and flies. This paves the way for establishing new lower metazoan model systems for the study of apoptosis.

Effect of short-term exposure to elevated temperatures and light levels on photosynthesis of different host-symbiont combinations in the Aiptasia pallida/Symbiodinium symbiosis

The physiology exhibited by symbioses between dinoflagellates (zooxanthellae) and hosts such as reef corals may be dictated by the host, the symbiont, or the synergistic effect of both partners. We compared the oxygen fluxes of two laboratory populations of the symbiotic sea anemone Aiptasia pallida, originally collected from Bermuda and Florida. A. pallida from Bermuda contained clade B zooxanthellae and A. pallida from Florida hosted clade A zooxanthellae. Both freshly isolated zooxanthellae and intact anemones were compared as a function of light intensity at culture (258C) and elevated (328C, 348C) temperatures. Zooxanthellae isolated from the Florida anemones had higher net photosynthetic rates at 328 C( P max: 6.2 6 0.9 mmol O2 h 21 mg chlorophyll a [Chl a] 21 vs. 0.7 6 3.6 for Bermuda algae), with the Bermuda algae exhibiting pronounced photoinhibition at higher irradiances. At 348C, the clade A (Florida) symbionts had lower net photosynthesis than at 328 C( P max: 3.6 6 2.2 mmol O2 h 21 mg Chl a 21 ), but the Bermuda symbionts had high rates of net O2 consumption at all irradiances. The intact Bermuda anemones, however, exhibited much less pronounced effects at elevated temperatures than did the isolated Bermuda zooxanthellae. Bermuda anemones containing the Florida symbionts had higher net oxygen fluxes than those with the Bermuda symbionts at elevated temperatures and irradiances. This study demonstrates that the physiology of the intact symbiosis is dictated by both partners and that studies of the intact symbiosis may not fully reveal the thermal liability of the algae.