A host organelle integrates stolen chloroplasts for animal photosynthesis

Abstract

Eukaryotic life evolved over a billion years ago when ancient cells engulfed and integrated prokaryotes to become modern mitochondria and chloroplasts. Sacoglossan "solar-powered" sea slugs possess the ability to acquire organelles within a single lifetime by selectively retaining consumed chloroplasts that remain photosynthetically active for nearly a year. The mechanism for this "animal photosynthesis" remains unknown. Here, we discovered that foreign chloroplasts are housed within novel, host-derived organelles we term "kleptosomes." Kleptosomes use ATP-sensitive ion channels to maintain a luminal environment that supports chloroplast photosynthesis and longevity. Upon slug starvation, kleptosomes digest stored chloroplasts for additional nutrients, thereby serving as a food source. We leveraged this discovery to find that organellar retention and digestion of photosynthetic cargo has convergently evolved in other photosynthetic animals, including corals and anemones. Thus, our study reveals mechanisms underlying the long-term acquisition and evolutionary incorporation of intracellular symbionts into organelles that support complex cellular function.

Keywords: cell biology; endosymbiosis; evolution; kleptoplasty; organellar ion channels; photosynthetic animal.