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.

Symbiotic associations are ubiquitous in terrestrial and marine environments and are of great ecological importance. The onset of a symbiotic relationship differs among associations. Symbionts can be vertically transmitted from host parent to offspring or they can be acquired horizontally from the surrounding environment with each new host generation. Cnidarian-algal symbioses, the subject of our study, exhibit both strategies. We investigated the clade identity of symbionts in soft coral hosts (Eilat, Red Sea) in relation to their hosts' mode of symbiont acquisition. We found for the first time that all hosts using horizontal transmission harbored symbionts belonging to Clade C while those with vertical transmission uniquely harbored symbionts from Clade A. The latter, capable of coping with a wide array of environmental conditions, evolved to be optimal vertically transmitted symbionts. The limitation of Clade A symbionts to hosts with vertical transmission suggests a coevolution of the hosts and symbionts. Clade C symbionts, characterized by large sub-clade variability, are found in corals with horizontal transmission and, most probably, each of its genotypes exhibits a more specialized set of physiological capabilities. KEY WORDS: Symbiont acquisition · Octocorals · Zooxanthellae · Red SeaResale or republication not permitted without written consent of the publisher

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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.

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