Chloroplast acquisition without the gene transfer in kleptoplastic sea slugs, Plakobranchus ocellatus

Some sea slugs sequester chloroplasts from algal food in their intestinal cells and photosynthesize for months. This phenomenon, kleptoplasty, poses a question of how the chloroplast retains its activity without the algal nucleus, and there have been debates on the horizontal transfer of algal genes to the animal nucleus. To settle the arguments, we report the genome of a kleptoplastic sea slug Plakobranchus ocellatus and found no evidence that photosynthetic genes are encoded on the nucleus. Nevertheless, we confirmed that photosynthesis prolongs the life of mollusk under starvation. The data present a paradigm that a complex adaptive trait, as typified by photosynthesis, can be transferred between eukaryotic kingdoms by a unique organelle transmission without nuclear gene transfer. Our phylogenomic and transcriptomic analysis showed that genes for proteolysis and immunity underwent gene expansion and are upregulated in the chloroplast-enriched tissue, suggesting that these molluscan genes are involved in this DNA-independent transformation.