Examining the Retention and Digestion of Functional Kleptoplasts in Solar-Powered Sea Slugs

Abstract

Sacoglossan sea slugs are the only metazoans known to sequester and retain functional chloroplasts within their digestive gland tubules, a process known as functional kleptoplasty. This ability coincides with the capacity to withstand extended starvation periods – some species even achieving 3-12 months without access to food. Since these chloroplasts remain photosynthetically active and multiple different slug-derived compounds contain photosynthetically fixed carbon, the ability to sequester and retain plastids has been linked to the ability to withstand extended starvation periods. Despite the correlation between functional chloroplasts and starvation survival, almost nothing is known about how these plastids help the slug survive and when their photosynthates become available to the starving slug.<br /> In this study, I examined sequestered chloroplasts throughout the starvation periods of two sister species, one that can withstand extended starvation and one that cannot, finding that only the long-term plastid retaining species contains chloroplasts that produce starch. Starch, the main photosynthate produced in chlorophytes, accumulates in the sequestered chloroplasts for the first half of the starvation period and then disappears gradually, likely due to digestive processes (Chapters 2,3). Contrastingly, chloroplasts do not accumulate starch in the short-term surviving species and are observed in the animal’s excrement throughout the starvation period (Chapter 5). The ability to retain plastids without excreting them while they build up starch is likely the key to withstanding starvation in these species.<br /> The decrease in starch concentration is likely due to digestion, but previous studies on intracellular digestion in these animals are virtually non-existent. To examine digestive processes regarding the decrease in functional chloroplasts and decrease in starch within those chloroplasts, I tried a myriad of staining techniques finally finding success with acridine orange, a stain that reveals lysosomes in living tissues. Using acridine orange staining in multiple species, I present here a first look at intracellular digestion in these animals. I observed different digestive<br /> activity trends for a species capable of long-term plastid retention and multiple species unable to withstand extended starvation, providing evidence as to how some species regulate their own digestive activity to withstand starvation (Chapters 2,4).<br /> The last aspect of the slug/plastid interaction that I present here concerns the development of this ability in juveniles. Functional kleptoplasts are not passed from parents to offspring, rather each generation must acquire its own plastids. During early development, these slugs cannot sequester and retain plastids for extended periods, nor can they survive prolonged starvation. I examined the acquisition of functional chloroplasts by juveniles and their digestive activity throughout different developmental stages, to determine when functional kleptoplasty is established and the starvation capacity experienced at each developmental stage (Chapter 6).<br /> Overall, these studies provide new insight into how these remarkable animals maintain foreign organelles in their tissues and profit from the photosynthates produced to withstand extended starvation periods. Each investigation required the development of new methodologies that can be used in a variety of future endeavors. The conclusions presented here reshape our hypotheses about functional kleptoplasty in metazoans, and provide new theories as to how this amazing ability evolved in within the Sacoglossa.