Cell biology of the chloroplast symbiosis in sacoglossan sea slugs

Abstract

Chloroplasts removed from their species of origin may survive for various periods and even photosynthesize in foreign cells. One of the best studied and impressively long, naturally occurring examples of chloroplast persistence, and function inside foreign cells are the algal chloroplasts taken up by specialized cells of certain sacoglossan sea slugs, a phenomenon called chloroplast symbiosis or kleptoplasty. Among sacoglossan species, kleptoplastic associations vary widely in length and function, with some animals immediately digesting chloroplasts, while others maintain functional plastids for over 10 months. Kleptoplasty is a complex process in long-term associations, and research on this topic has focused on a variety of aspects including plastid uptake and digestive physiology of the sea slugs, the longevity and maintenance of symbiotic associations, biochemical interactions between captured algal plastids and slug cells, and the role of horizontal gene transfers between the sea slug and algal food sources. Although the biochemistry underlying chloroplast symbiosis has been extensively examined in only a few slug species, it is obvious that the mechanisms vary from species to species. In this chapter, we examine those mechanisms from early discoveries to the most current research.