title: Apoptosis and the selective survival of host animals following thermal bleaching in zooxanthellate corals.
abstract: During the past several decades, numerous reports from disparate geographical areas have documented an increased frequency of "bleaching" in reef-forming corals. The phenomenon, triggered by increased sea surface temperatures, occurs when the cnidarian hosts digest and/or expel their intracellular, photosynthetic dinoflagellate symbionts ("zooxanthellae" in the genus Symbiodinium). Although coral bleaching is often followed by the death of the animal hosts, in some cases, the animal survives and can be repopulated with viable zooxanthellae. The physiological factors determining the ability of the coral to survive bleaching events are poorly understood. In this study, we experimentally established that bleaching and death of the host animal involve a caspase-mediated apoptotic cascade induced by reactive oxygen species produced primarily by the algal symbionts. In addition, we demonstrate that, although some corals naturally suppress caspase activity and significantly reduce caspase concentration under high temperatures as a mechanism to prevent colony death from apoptosis, even sensitive corals can be prevented from dying by application of exogenous inhibitors of caspases. Our results indicate that variability in response to thermal stress in corals is determined by a four-element, combinatorial genetic matrix intrinsic to the specific symbiotic association. Based on our experimental data, we present a working model in which the phenotypic expression of this symbiont/host relationship places a selective pressure on the symbiotic association. The model predicts the survival of the host animals in which the caspase-mediated apoptotic cascade is down-regulated.
authors.affiliation: Marine Biology Department, Leon H Charney School of Marine Sciences, University of Haifa, Haifa 31905, Israel. dtchernov@gmail.com
authors.forename: Dan
authors.initials: D
authors.lastname: Tchernov
authors.forename: Hagit
authors.initials: H
authors.lastname: Kvitt
authors.forename: Liti
authors.initials: L
authors.lastname: Haramaty
authors.forename: Thomas S
authors.initials: TS
authors.lastname: Bibby
authors.forename: Maxim Y
authors.initials: MY
authors.lastname: Gorbunov
authors.forename: Hanna
authors.initials: H
authors.lastname: Rosenfeld
authors.forename: Paul G
authors.initials: PG
authors.lastname: Falkowski
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D037421:Anthozoa / Q000201:enzymology* / Q000235:genetics / Q000469:parasitology; D017209:Apoptosis; D015153:Blotting, Western; D020169:Caspases / Q000235:genetics / Q000378:metabolism; D002462:Cell Membrane / Q000648:ultrastructure; D002843:Chromatin / Q000648:ultrastructure; D053938:DNA Fragmentation; D004141:Dinoflagellida / Q000378:metabolism* / Q000502:physiology; D017753:Ecosystem; D006790:Host-Parasite Interactions; D046529:Microscopy, Electron, Transmission; D008969:Molecular Sequence Data; D011156:Population Density; D011157:Population Dynamics; D017382:Reactive Oxygen Species / Q000378:metabolism*; D012623:Seawater; D015398:Signal Transduction; D013559:Symbiosis; D013696:Temperature
chemical_list: D002843:Chromatin; D017382:Reactive Oxygen Species; D020169:Caspases