Phagocytosis of algal chloroplasts by digestive gland cells in the photosynthesis-capable slug Elysia timida (Mollusca, Opisthobranchia, Sacoglossa)
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Parapodia of the sacoglossan slug Elysia timida were preserved by high-pressure cryofixation during feeding experiments and investigated with transmission electron microscopy. This slug has been known for its long-term retention of active chloroplasts and photosynthesis. We observed different stages of phagocytosis of chloroplast components from ingested algal food by slug digestive gland cells. Thylakoid stacks and stroma of chloroplasts were engulfed by the slug cells. In the slug cells thylakoids were surrounded by one membrane only. This membrane is interpreted as having been generated by the mollusk during phagocytosis. It is inferred to be eukaryotic in origin and unlikely, therefore, to be endowed with the translocons system ordinarily regulating import of algal gene-encoded plastid preproteins. Our structural findings suggest that chloroplast components in the slug cells are thylakoid stacks with chloroplast stroma only.
KeywordsKleptoplasty Mollusk cells Membranes High-pressure cryofixation Ultrastructure
We would like to thank Eberhard Schmid and Dr. Katharina Höhn for assistance in high-pressure cryofixation and freeze substitution of the samples. Dipl. biol. Valerie Schmitt supplied living specimens from Banyuls-sur-mer (France) under various conditions, after fasting and light exposition, or freshly fed and helped to prepare tissues for cryofixation. Prof. Edward Koenig (Buffalo, USA) critically read and revised the manuscript.
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