Department of Molecular and Biomedical Sciences, 5735 Hitchner Hall, University of Maine, Orono, ME 04469, USA.
J Exp Biol. 2011 Jan 15;214(Pt 2):303-11. doi: 10.1242/jeb.046540.
Symbiotic animals containing green photobionts challenge the common perception that only plants are capable of capturing the sun's rays and converting them into biological energy through photoautotrophic CO(2) fixation (photosynthesis). 'Solar-powered' sacoglossan molluscs, or sea slugs, have taken this type of symbiotic association one step further by solely harboring the photosynthetic organelle, the plastid (=chloroplast). One such sea slug, Elysia chlorotica, lives as a 'plant' when provided with only light and air as a result of acquiring plastids during feeding on its algal prey Vaucheria litorea. The captured plastids (kleptoplasts) are retained intracellularly in cells lining the digestive diverticula of the sea slug, a phenomenon sometimes referred to as kleptoplasty. Photosynthesis by the plastids provides E. chlorotica with energy and fixed carbon for its entire lifespan of ~10 months. The plastids are not transmitted vertically (i.e. are absent in eggs) and do not undergo division in the sea slug. However, de novo protein synthesis continues, including plastid- and nuclear-encoded plastid-targeted proteins, despite the apparent absence of algal nuclei. Here we discuss current data and provide hypotheses to explain how long-term photosynthetic activity is maintained by the kleptoplasts. This fascinating 'green animal' provides a unique model to study the evolution of photosynthesis in a multicellular heterotrophic organism.
共生动物体内含有绿色光合生物，这挑战了一个普遍的认知，即只有植物能够通过光合作用（photosynthesis）将太阳光转化为生物能量并固定 CO2。“太阳能”驱动的有肺目软体动物，或海蛞蝓，通过仅拥有光合作用细胞器——质体（=叶绿体），将这种共生关系推进了一步。有一种海蛞蝓，Elysia chlorotica，当它通过摄食 Vaucheria litorea 藻类获得质体时，只需要光和空气就能像“植物”一样生活。捕获的质体（kleptoplasts）被保留在海蛞蝓消化盲囊的细胞内，这种现象有时被称为盗食质体现象（kleptoplasty）。质体的光合作用为 E. chlorotica 提供了能量和固定碳，足以维持其大约 10 个月的整个生命周期。质体不会垂直传递（即不存在于卵中），也不会在海蛞蝓中分裂。然而，尽管没有藻类核，新的蛋白质合成仍在继续，包括质体和核编码的质体靶向蛋白。在这里，我们讨论了当前的数据，并提出了假设来解释 kleptoplasts 如何长期维持光合作用。这个迷人的“绿色动物”为研究光合作用在多细胞异养生物中的进化提供了一个独特的模型。
