G·A·B·B·A

Symbiosis between animals and microorganisms is widespread throughout the tree of life, often providing ecological advantages to the partners. One striking example is the endosymbiosis between corals and photosynthetic dinoflagellate algae (genus Symbiodinium), which provide essential nutrients to their coral host. This nutrient transfer is key for the productivity and biodiversity of coral reef ecosystems. Most corals establish symbiosis anew each generation during larval stages and take up algae through phagocytosis into the endodermal cells. However, corals are unsuitable as model system and sexually reproduce only once a year severely limiting access to larvae for dissecting the molecular mechanisms underlying symbiosis establishment. During my thesis, I established critical resources for Aiptasia, a small tropical marine sea anemone as a model system for coral symbiosis. First, I contributed to developing a robust spawning induction protocol for Aiptasia that allows unlimited access to larvae for experimentation. I then compared symbiosis specificity between Aiptasia larvae and two coral species using several defined symbiont types. I found that specificity patterns during symbiosis establishment are similar suggesting that the underlying mechanisms are conserved between Aiptasia and corals further strengthening Aiptasia as a model system. I refined an assay to quantitatively assess symbiosis establishment efficiency in Aiptasia during development and, importantly, established confocal microcopy as a powerful means to visualize the intracellular localization of symbionts in the larvae’s endodermal cells. Building up on these tools and techniques, I asked how symbionts escape digestion by the host. I used various lysosomal markers to test if symbionts reside in lysosomal-like organelles. I found evidence that symbionts co-localize with multiple lysosomal markers indicating that symbionts withstand acidic and proteolytic environments as a prerequisite to avoid lysosomal digestion by the host. Together, my work was fundamental to further develop Aiptasia as a powerful platform for analyzing the cellular mechanisms of coral symbiosis, a phenomenon of immense importance for coral reef ecosystems worldwide.