Variable retention of kleptoplast membranes in cells of sacoglossan sea slugs: plastids with extended, shortened and non-retained durations

Abstract

Six species of sacoglossan sea slugs engulf and store chloroplasts from their algal food (kleptoplasts) in digestive gland cells for weeks. The question is unresolved as to why kleptoplasts are retained only in certain species, while in most others they are digested. We recently showed phagocytosis of algal chloroplasts by digestive cells in the long-term retention form Elysia timida. Chloroplasts with functional thylakoids and stroma, but devoid of inner and outer envelopes, had been taken up. The stored plastids in the slug cells had only a phagosome membrane. In the present study we report that in cells of another long-term retention species, Plakobranchus ocellatus, only one membrane is present. In contrast, chloroplasts in digestive cells of the non-retention form Thuridilla hopei were enveloped by inner and outer chloroplast membranes, as well as a phagosome membrane. On the other hand, Elysia viridis, which has been considered to be a short-term retention form, had some chloroplasts with and without chloroplast envelopes. The hypothesis is proposed that the absence of chloroplast envelopes in long-term retention forms help to avoid digestion.