Cytologia Focus
A Photosynthetic Animal: A Sacoglossan Sea Slug that Steals Chloroplasts
2021 年 86 巻 2 号 p. 103-107
詳細
2021 年 86 巻 2 号 p. 103-107
Sacoglossan sea slugs are able to steal chloroplasts from their algal prey and acquire photosynthetic capacity (termed kleptoplasty). These ‘stolen’ plastids provide sea slugs with a long-term supply of organic carbon and energy. This augmented nutrient supply brings many benefits in terms of survival, body planning, reproductive traits, and body regeneration. However, the mechanisms of maintenance of chloroplasts and photosynthesis in sea slugs are poorly understood. Here, we introduce this mysterious phenomenon, including recent research findings, and consider its feasibility for synthetic biology, e.g., construction of artificial photosynthetic animal cells.
Research on algae has rapidly diversified into various fields, including cell biology (Mine et al. 2018, Takano et al. 2018, Miyamura et al. 2019, Kuroiwa et al. 2020, Yoshida et al. 2020), molecular biology (Uchida et al. 2018, Ota et al. 2019, Miyagishima and Fujiwara 2020), and biotechnology (Hayashi et al. 2018). Recent products derived from algae include bioethanol and so-called ‘functional foods’ (Hosokawa and Kawano 2020). Heterotrophic protists and some animals have symbiotic relationships with unicellular alga or cyanobacteria (Venn et al. 2008). Examples of such relationships are found in several phyla, e.g., Mollusca (giant clams and nudibranchs), Porifera (sponges), Cnidaria (corals, anemones, and hydra), Acoelomorpha (flatworms) (Van Steenkiste et al. 2019), and Chordata (ascidians). In these cases, the host obtains oxygen and organic carbon via their intercellular symbiont’s photosynthesis. Although a large number of studies have been made on symbioses between animals or heterotrophic protists and algae, little is known about their underlying molecular mechanisms.
A more mysterious and somewhat controversial symbiotic phenomenon is observed in several sacoglossan molluscs (sea slugs) within the Plakobranchoidea, particularly in the genus Elysia (Rumpho et al. 2000, 2011). These slugs have evolved mechanisms for the capture of algae prey and selective retention of functional chloroplasts (called kleptoplasty). They are, thereby, able to photosynthesise using these ‘stolen’ plastids and assimilate photosynthate for periods ranging from a few days to several months. A particular puzzle is that the sea slugs are able to maintain photosynthetic activity despite the algal nuclei having been digested. Because some photosynthetic proteins are encoded in the algal nuclear genome, photosynthesis should be impaired. Here, we introduce this mysterious phenomenon about sea slags.
Kleptoplasty was first noted in Elysia viridis (De Negri and De Negri 1876) and has subsequently been observed in the closely related species, E. atroviridis (Kawaguchi et al. 1965) and E. chlorotica (Trench et al. 1969). There have been several physiological studies of kleptoplasty (Hinde and Smith 1972, Jensen 1986, Clark et al. 1990). Elysia chlorotica can be grown in a laboratory and the kleptoplasts are retained for a long period (10–12 months). It feeds on particular species of algae, including Vaucheria vaucheria, V. litorea, and V. compacta, which belong to a coenocytic heterokont genus (West 1979). The sea slug is unable to complete metamorphosis and development to the adult without plastid uptake from the algal prey (Rumpho et al. 2011). Vaucheria species are coenocytic filamentous algae, i.e., they consist of a single multinucleate cell (Pelletreau et al. 2011). Sea slugs use their radular teeth to break the cell wall and suck out the cell contents, including plastids. The plastids are then incorporated into the tubular cells of their digestive diverticula, where they carry out photosynthesis (Rumpho et al. 2011) (Fig. 1).
Two main hypotheses are proposed to explain this phenomenon. First, horizontal gene transfer (HGT) accompanies kleptoplasty. Alternatively, the mechanism is intrinsic to the chloroplasts themselves. In the first hypothesis, after the digestion of the algal nuclei, the algal genome should be directly incorporated into the host cell nucleus without being decomposed. However, no nuclear-encoded algal-derived HGT to the germline was observed in E. chlorotica (Wägele et al. 2011, Pierce et al. 2012, Bhattacharya et al. 2013). Similarly, there was no evidence of HGT between E. timida and Plakobranchus ocellatus. In several transcriptomic analyses of sea slugs, algal nuclear-encoded mRNA was not detected (Wägele 2011, Pierce et al. 2012). These studies tend to refute the first hypothesis. The second hypothesis implies that the chloroplasts of Vaucheria species are less dependent on the algal genome than in other algae and embryophytes (Rumpho et al. 2006); therefore, the kleptoplasts may be genetically autonomous. Approximately 60% of chloroplasts isolated from V. litorea continued to evolve oxygen for 2 days, whereas fewer than 30% of chloroplasts isolated from spinach did so after 1 day (Green et al. 2005). In addition, RuBisCo protein continued to be synthesised in kleptoplasts, 3 days after extraction, as in the wild chloroplasts. Moreover, it is known that V. litorea chloroplasts are resistant to osmotic stress (Gallop et al. 1980, Green et al. 2005). These observations imply that kleptoplasty relies on specific properties of the chloroplasts of Vaucheria species.
The greatest benefit of the phenomenon of kleptoplasty is probably not related to the conferment of crypsis but to the supply of nutrients. Photosynthetic carbon is supplied to sea slugs by the kleptoplasts (Trench et al. 1974, Kopp et al. 2015, LeKieffre et al. 2018). Recent spatio-temporal analysis has demonstrated uptake of kleptoplasty-derived carbon and nitrogen into sea slugs (Cruz et al. 2020). Using radioisotopes, it was shown that the uptake of carbon and nitrogen reached the reproductive organs. Light was required not only for energy supply but also affected body size and the number of offspring in E. atroviridis (Shiroyama et al. 2020). That study showed that light intensity and food availability were correlated with shell height and the total number of eggs. These data suggest that kleptoplasty serves as a photosynthetic device that supplies nutrients that strengthen the individual, and also supports specific lifestyles, body-plans and reproductive traits.
An advantage of kleptoplasty is that, in the presence of light, sea slugs are able to survive without algal food for at least 10 months (Rumpho et al. 2006). Sea slugs starved in the dark, lost weight much more rapidly than those starved in the light (Hinde and Smith 1975). Additionally, survival and growth rate were greater in the light than in the dark for P. ocellatus (functional chloroplasts were retained for ＞17 days) (Yamamoto et al. 2013, Akimoto et al. 2014). Conversely, in a similar experiment using E. trisinuata, there were no significant differences in different light levels (functional chloroplasts were retained ＜4 days). These experiments demonstrated that dependence on photosynthesis is related to the duration of retention of chloroplasts. Nevertheless, some sea slugs with short-term functional kleptoplasty did obtain supplementary nutrition and energy via photosynthesis. Conversely, it has also been reported that photosynthesis may not be important for the survival of sea slugs (Christa et al. 2014). In that report, there were no significant differences in weight loss among animals that survived several months of starvation in complete darkness, or in the light in the presence of the photosynthesis inhibitor monolinuron, or in animals with no treatment. These results support the hypothesis that besides being a source of solar power, kleptoplasts can serve as a food reserve. Indeed, kleptoplasts are a source of lipids, and it is known that starved sea slugs decompose them as a nutrient source in the dark (Pelletreau et al. 2014, Cartaxana et al. 2017, Rey et al. 2017). Thus, there are two aspects of nutrient supply by kleptoplasts: as a direct source and via photosynthesis.
Reliance on photosynthesis also bring disadvantages. There is an optimal light intensity for sea slugs; too strong or too weak light both adversely affect the growth of sea slugs (Donohoo et al. 2020). Reactive oxygen species (ROS) formed by the addition of electrons to oxygen through photosynthesis can seriously damage animal membranes and intracellular proteins, which, ultimately may lead to cell death. However, in contrast to the chloroplasts of green algae, the kleptoplasts of E. timida possess a mechanism that preferentially accepts electrons; plastoquinone maintains an oxidised state that inhibits the formation of harmful ROS (Christa et al. 2018, Cartaxana et al. 2019, Havurinne and Tyystjärvi 2020). In this way, several sea slugs have apparently adapted and optimised their photosynthetic activity which seems like a very short-term evolution.
Are sea slugs genetically adapted to kleptoplasty? RNA-seq analysis was performed to identify genetic interaction between host and kleptoplast during E. chlorotica development (Chan et al. 2018). That research showed that during incorporation of V. litorea plastids, genes involved in microbe-associated molecular patterns and oxidative stress-response mechanisms were significantly up-regulated. This implies that functional kleptoplasty belongs to a category of animal–algal symbiotic interactions resembling that between corals and dinoflagellates.
Recently, it was reported that a sea slug’s detached head was able to survive and regenerate a whole new body (Mitoh and Yusa 2021). Although sacoglossan sea slugs are able to autotomize, known as regeneration of only a part of the body, such as the tail (Fleming et al. 2007), surprisingly, this study found that two species of sacoglossan sea slug, E. marginata and E. atroviridis were able to survive without the whole body including the heart and regenerate from the head but not from the body. In Elysia, since digestive cells containing chloroplasts are also present in the head, it is thought that kleptoplasty may have contributed to the survival of sea slugs with only the head and regeneration of complex body plans. These findings have provided us with new insights into the relationship between kleptoplasty and regeneration.
Finally, synthetic biology has attracted much attention. A particular tool, which is currently the focus of research and development, is the use of long-coding DNA, involving synthesis on the genome-scale, construction of artificial organelles, and their insertion into artificial cells. A few laboratories are attempting to create artificial photosynthetic animal cells based on the concept of symbiosis (Matsunaga 2018, Puri et al. 2021). We may be able to create photosynthetic animal cells that, like sea slugs, are capable of photosynthesis for several months using the following strategies: firstly, transfer of genes related to algal photosynthesis into the nucleus of cultured cells by HGT; secondly, use of microinjection technology to insert chloroplasts into cultured cells, ‘imposed’ chloroplasts. Conveniently, cultured cells can be genetically preserved and amplified into the next generation by cell division. The feasibility of constructing artificial photosynthetic animal cells is increased by using sequencing tools with high speed, low cost, and high throughput, and genome modification tools such as CRISPR-Cas9 systems. Artificial photosynthetic animal cells are expected to contribute to medical fields with applications in cancer treatment and photosynthetic therapy using oxygen evolution (Wang et al. 2019, Chávez et al. 2020).
Although kleptoplasty still leaves many open questions in relation to HGT, retention of kleptoplasts, and its relationship to reproduction and regeneration, it is a fascinating phenomenon in phylogenetics and evolution. Further elucidation could lead to a new understanding of the symbiotic phenomenon and create new possibilities from the viewpoint of synthetic biology.
This research was supported by grants from MXT/JSPS KAKENHI (19H03259 and 20H03297) and JST, CREST Grant Number JPMJCR20S6 to SM. We thank Harry Taylor, PhD, from Edanz Group (https://en-author-services.edanz.com/ac) for editing a draft of this manuscript.
