Kleptoplasty: Getting away with stolen chloroplasts

Abstract

Kleptoplasty, the process by which a host organism sequesters and retains algal chloroplasts, is relatively common in protists. The origin of the plastid varies, as do the length of time it is retained in the host and the functionality of the association. In metazoa, the capacity for long-term (several weeks to months) maintenance of photosynthetically active chloroplasts is a unique characteristic of a handful of sacoglossan sea slugs. This capability has earned these slugs the epithets "crawling leaves" and "solar-powered sea slugs." This Unsolved Mystery explores the basis of chloroplast maintenance and function and attempts to clarify contradictory results in the published literature. We address some of the mysteries of this remarkable association. Why are functional chloroplasts retained? And how is the function of stolen chloroplasts maintained without the support of the algal nucleus?

Fig 1. Crypsis in sacoglossan sea slugs.

The sea slug Placida dendritica retains short-term, nonfunctional kleptoplasts that allow the sea slug to blend with the environment and avoid predation.

Fig 2. Photosynthesis and reproductive output.

Elysia viridis spawning an egg mass on the macroalga Codium tomentosum. Photosynthesis by long-term functional kleptoplasts can support the reproductive output of the host slug.

Fig 3. Repair and avoidance mechanisms involved in kleptoplast longevity.

Schematic representation of repair, photoprotection, and shielding mechanisms putatively involved in the long-term maintenance of photosynthetic active chloroplasts in sacoglossan sea slugs.