Plastid longevity in sacoglossan sea slugs

质体的内共生起源在真核生物中产生了可遗传的光合作用。它需要一个异养宿主吸收和稳定整合一个古老的蓝藻。但在真核生物中也有光合作用通过共生关系发生的情况，这种共生关系是不可遗传的。海蛞蝓栖息在沿海的海水中，以藻类为食。一些物种不加选择地进食并消化藻类，而另一些物种则对食物来源有选择性，只隔离质体，然后将其保持在光合作用活跃状态数月（kleptoplasts）。值得注意的是，在没有藻核的鼻涕虫消化细胞的细胞质中，这种窃液质体仍然具有光合活性。由于高等植物质体的光合作用严格依赖于核编码的基因产物，因此长期以来人们认为，蛞蝓通过横向基因转移（LGT）从藻类食物中获得基因，从而支撑质体的寿命。但最近发表的报告（从我们自己的报告开始）驳斥了LGT假说。在新的初步工作中，我们发现了我们现在认为是几十年来被忽视的关键：质体保持活性，但蛞蝓不是自养的。它们在光明和黑暗中存活的时间一样长。这就引出了两个非常新的问题：(1)蛞蝓从窃体中获得了什么好处，我们现在认为窃体是一种食物储备；(2)在与藻类核基因分离的几个月里，蛞蝓和窃体对维持窃体的光合活性有什么贡献？

The endosymbiotic origin of plastids gave rise to heritable photosynthesis in eukaryotes. It entailed the uptake and stable integration of an ancient cyanobacterium by a heterotrophic host. But there are cases where photosynthesis occurs in eukaryotes via symbiotic associations that are not heritable. Sacoglossan sea slugs inhabit marine coastal waters and feed upon algae. Some species feed indiscriminately and digest the algae, others however are selective about their food source and sequester only the plastids, which they then retain in a photosynthetically active state for months to come (kleptoplasts). Notably, such kleptoplasts remain photosynthetically active in the cytosol of slug digestive cells that are devoid of algal nuclei. Because photosynthesis in higher plant plastids strictly depends upon nuclear-encoded gene products, it was long thought that slugs obtained genes from their algal food through lateral gene transfer (LGT) so as to underpin plastid longevity. But recent published reports (starting with our own) refute the LGT hypothesis. In new preliminary work, we have uncovered what we now believe to be the key - overlooked for decades - to understanding sacoclossan kleptoplasty: The plastids remain active, but the slugs are not autotrophic. They survive equally long in light vs. dark. That opens up two very new questions: (i) what benefit do slugs reap from kleptoplasts, which we now think are a food reserve, and (ii) what are the slugs and kleptoplasts contributions to maintaining kleptoplasts photosynthetically active for months of separation from algal nuclear genes.