DataSheet_1_The nudibranch Berghia stephanieae (Valdés, 2005) is not able to initiate a functional symbiosome-like environment to maintain Breviolum minutum (J.E.Parkinson & LaJeunesse 2018).csv

Photosymbiosis is found in different animal lineages and is best understood in cnidarians. A successful initiation and maintenance of the symbiosis between the animal hosts and the photosymbiotic partners is based on a recognition by specific host receptors. This triggers signaling cascades that promote the photobiont tolerance by the host, including an interpartner nutrient exchange and the ability of the host to cope with increased levels of reactive oxygen species (ROS) generated by the photobiont. Key to the successful symbiosis is the inhibition of the phagosomal maturation resulting in the formation of the symbiosome. In animals other than cnidarians, little is known about the photosymbiosis initiation and maintenance, for instance in sea slugs belonging to the Nudibranchia. Here, we investigated the gene expression profile of Berghia stephanieae, which is able to incorporate Breviolum minutum from its cnidarian prey Exaiptasia diaphana (Rapp, 1829) but is not able to maintain the algae for more than a couple of days during starvation. We show that the recognition of the algae is based on similar mechanisms present in cnidarians, and we identified some additional candidate genes that might be molluscan specific for photobiont recognition. Downstream, B. stephanieae responds to increased levels of ROS but is not able to stop the phagosomal maturation or decrease the immune response against B. minutum, which seem to be the key factors missing in B. stephanieae that accounts for the unstable symbiosis in this slug. Hence, B. stephanieae can be considered a transitional state toward a stable photosymbiosis and can help to elucidate general aspects of the evolutionary processes involved in establishing photosymbioses in animals.

共生现象存在于不同的动物谱系中，尤其在腔肠动物中表现得尤为明显。动物宿主与光合共生伙伴之间成功启动并维持共生关系，依赖于宿主受体对特定伙伴的识别。这种识别触发了信号级联反应，促进了宿主对光合生物的耐受性，包括伙伴间的营养交换以及宿主应对由光合生物产生的活性氧（ROS）增加的能力。成功共生的关键在于抑制吞噬体成熟，从而形成共生体。在腔肠动物之外的动物中，关于光合共生启动和维持的了解甚少，例如属于海蛞蝓的裸鳃动物。在本研究中，我们调查了贝氏贝蛤（Berghia stephanieae）的基因表达谱，该物种能够从其腔肠动物猎物——透明海葵（Exaiptasia diaphana Rapp, 1829）中摄取微小贝雷奥（Breviolum minutum），但在饥饿状态下无法维持藻类超过数日。我们发现，藻类的识别基于与腔肠动物中存在的类似机制，并鉴定了一些可能的候选基因，这些基因可能是贝蛤类特异性的光合生物识别基因。在下游过程中，贝氏贝蛤对ROS水平的增加做出反应，但无法阻止吞噬体成熟或降低对贝雷奥的免疫反应，这些因素似乎是贝氏贝蛤中缺失的关键，导致了这种海蛞蝓中不稳定的共生关系。因此，贝氏贝蛤可以被视为向稳定的光合共生过渡的状态，并有助于阐明动物中建立光合共生所涉及的进化过程的普遍方面。