kleptoplastid

A kleptoplastid is a plastid that has been stolen from a donor cell by a host organism through the process of kleptoplasty, allowing the host to temporarily retain photosynthetic capability. The phenomenon is best known from certain sacoglossan sea slugs, which ingest algal cells and sequester chloroplasts within their own tissues. These retained plastids can remain functional for varying periods, from days to months, depending on the species and conditions, and can contribute to the host’s energy budget during periods of fasting.

Occurrence and sources: Kleptoplastids have been most thoroughly studied in sacoglossan slugs such as Elysia chlorotica, Plakobranchus ocellatus, and Elysia timida. The chloroplasts (and sometimes other plastids) are commonly derived from green algae; in the classic case of E. chlorotica, the plastids are taken from Vaucheria litorea. The plastids are typically housed in specialized digestive or mantle-associated cells, where they can photosynthesize outside of the algal nucleus.

Genetics and maintenance: In many instances, the donor algal nucleus is not retained, so the host slug must rely on the plastid’s own genetic machinery to some extent. The extent to which host organisms provide nuclear-encoded, plastid-targeted proteins or acquire algal genes through horizontal transfer remains disputed, and no case of stable, long-term genomic integration of plastid function into the host has been established.

Significance: Kleptoplastidy provides a living model for studying organelle maintenance, inter-organismal interactions, and the evolutionary steps toward endosymbiosis. Ongoing research seeks to clarify the cellular mechanisms that sustain kleptoplastids and the ecological relevance of this strategy in natural environments.

See also: kleptoplasty, chloroplast, sacoglossan sea slug, endosymbiosis.