Epigenetic transcriptional memory of thermal stress in the cnidarian model system Aiptasia

Abstract

Ocean warming is leading to increased occurrence of coral mass bleaching events, threatening the persistence of these ecosystems and the communities that rely on them. While reef recovery is possible, conservation approaches based purely on transplantation/coral-gardening will not suffice to maintain these ecosystems over the projected environmental changes. Assisted evolution approaches aim to boost acclimatization and adaptation processes. A potential approach could be to harness the naturally occurring mechanism of environmental memory that has been observed in corals and other organisms, where an organism remembers a priming stress event to allow a faster/stronger response when the stress re-occurs. In this thesis I aimed to investigate whether this mechanism exists and how it is regulated on a molecular level in the sea anemone Aiptasia. Aiptasia were primed to heat stress by exposing them to 32 °C water for several years, or for one week. After a recovery period of one week at 25 °C, a naïve and the primed treatments were exposed to lethal thermal stress at 34 °C for three days. Primed treatments performed better than the naïve treatment in survival, photosynthetic efficiency and symbiont density for two days, after which the priming advantage was lost. The difference between the primed treatments indicated that the priming dose may affect priming success. There were clear indications of an epigenetic transcriptional memory mechanism on a transcriptional level. I observed a pronounced difference between control and heat-stressed treatments, indicating that transcription returned to near baseline expression after cessation of the priming exposure. The functional categories of differentially expressed genes in heat stress relative to control were similar between naïve and primed treatments, with the main difference observed in a stronger up- and downregulation of stress response genes in the long-term primed treatment. I optimized a chromatin immunoprecipitation protocol for use with Aiptasia by adjusting fixation, sonication and immunoprecipitation conditions. The enrichment of H3K4me2/me3 and poised RNA Pol II in the promoters of stress response genes will be investigated next to elucidate the mechanism of the observed epigenetic transcriptional memory in Aiptasia, and to ultimately inform conservation strategies for coral reefs globally.