An Exploration into the Photosynthetic Symbioses Between Sacoglossan Sea Slugs and their Algal Prey
Wendy Ouriel
Department of Biological Science, California State University, Fullerton
800 N. State College Blvd, Fullerton, CA 92835
Published
August 15, 2015
Abstract
Animals such as sea sponges, coral, and sea slugs have evolved mechanisms to obtain the byproducts of photosynthesis through a symbiotic relationship with chloroplast-containing algae. However, sea slugs, particularly sacoglossans are of particular interest due to their ability to sequester and maintain functional chloroplasts (kleptoplasts). Some species of sacoglossan, such as Elysia chlorotica are capable of maintaining functional kleptoplasts for periods of ten months or more. The exact biological mechanism to allow this phenomenon has been the subject of much debate and recent research, however many studies suggest that horizontal gene transfer is occurring. The direct benefit to a symbiotic relationship with photosynthetic algae has also been a focus of study and controversy, with some research leaning towards enhanced growth as a direct result of ingestion of functional chloroplasts, while other studies offer conflicting results.
Introduction
Humankind’s curiosity of the autotrophic nature of plants was first documented over two millennia ago with the Greek philosopher and scientist, Aristotle. Through observation, he concluded that plants are anchored to the soil of the earth because they lack the traditional digestive system seen in humans and other animals. It was reasoned that the soil acts as the stomach, providing water and atmospheric nutrients essential for survival (Gotthelf, 2013, p. 169). Although this view is overly simplistic and incomplete, the basic premise is not far from the truth. Plants require water from soil, and atmospheric carbon dioxide (obtained through stomata), but not as a direct food source as originally hypothesized. Instead, plants require water and carbon dioxide because they are essential components of the photosynthetic process (Maurino and Weber, 2013).
Photosynthesis is the conversion of light energy into usable chemical energy to fuel biological processes. It is the mechanism that permits plants to feed themselves, and is considered a defining quality that separates the plant and animal kingdoms. However it took nearly two thousand years from Aristotle’s first observations of plant autotrophy for photosynthesis to be discovered. In 1771, Joseph Priestley published the first work to demonstrate that the gases released by plants (oxygen) are different than the gases exhaled by animals (carbon dioxide). He described this process as a “method of restoring air which has been injured by the burning of candles.” (Bogorad, 1981). Then in 1888, Gottlieb Haberlandt, an Austrian botanist, determined that the chloroplast (plastid), a green organelle found in plant cells, is directly associated with oxygen production in plants (Bogorad, 1981).
Further study in the late 19th century by botanist Andreas Schimper led to two key discoveries: the first was that chloroplasts are capable of reproducing independently from the rest of the cell, and do so in a way that is reminiscent of prokaryotic fission. The second was that the chloroplast strongly resembled free-living cyanobacteria (Vargas-Parada, 2010). In 1905, the Russian botanist, Konstantin Mereschkowsky, further expanded upon these discoveries by proposing that chloroplasts originated from symbiotic cyanobacteria (O’Malley 2015).
Mereschkowsky’s theory, now known as endosymbiotic theory, evolved since its inception in the early 20th century. It is now accepted that over one billion years ago, a single endosymbiotic event occurred in which a eukaryotic cell engulfed, but did not digest, a photosynthetic cyanobacterium (Shih et al, 2013). The cyanobacterium became incorporated permanently into the cell as a double-membrane bound organelle to become the chloroplast we see in plants and algae today (Okazaki et al. 2010, Price et al., 2012). Endosymbiosis between the eukaryotic cell and cyanobacterium proved to be a landmark event in evolutionary history. The newly acquired organelle conferred a great advantage upon the eukaryote: the ability to obtain food autotrophically through sunlight (Nakayama et al., 2014). This ability was assumed to be reserved only for plants and algae until work by Parke and Manton in 1967 found that some marine turbellarians, such as Tetraselmis form a symbiosis with photosynthetic algae (Venn et al., 2008).
Parke and Manton’s discovery was the first of many that revealed a multitude of photosynthetic symbioses in animals, although the majority of animal hosts are found within two phyla, Porifera and Cnidaria (Venn et. al., 2008). However, recent studies have discovered animals belonging to a wider array of taxonomic groups that are in an endosymbiotic relationship with a photosynthetic organism. These animals include the spotted salamander (Graham et. al, 2012), the oriental hornet (Plotkin et al., 2010), and the sacoglossan mollusk, known commonly as the sea slug. This review, however, will only focus on the sacoglossan mollusk by first discussing how these sea slugs, especially Elysia chlorotica, obtain and maintain chloroplasts. There will then be a discussion on the possible benefits gained by sacoglossans for having symbiosis with an algal partner.
Obtaining the Chloroplast
Many animals have evolved mechanisms to obtain the byproducts of photosynthesis through a symbiotic relationship with chloroplast-containing algae (Rumpho et al. 2010). The majority of hosts maintain their algal symbiont within a symbiosomal membrane (e.g. cnidarians), or on a specific body region (e.g. sea anemone tentacles) (Venn et al., 2008). Having an endosymbiotic relationship permits a non-chloroplast containing animal to utilize its prey’s chloroplast to reap the nutritional benefits. The problem with this relationship, however, is that the animal cannot synthesize chloroplasts of its own, and once the symbiont exits the body, the host animal can no longer photosynthesize. Sacoglossans, on the other hand, are an extraordinary exception. These sea slugs, particularly those of the genus Elysia, have the ability to maintain functional plastids from their algal symbiont by retaining the organelle intracellularly. This process, known as kleptoplasty, has been discovered in over 150 species of sea slug and is unique to sacoglossans (Christa et al., 2013).
Kleptoplasty is a form of phagocytosis exclusive to sacoglossans (Cruz et al. 2013) wherein the host ingests, sequesters, and utilizes algal chloroplasts (Rumpho et al., 2011, Schwartz et al, 2014). The ingested chloroplasts, or kleptoplasts, can maintain photosynthetic activity within the host cell for a time ranging from hours to months, depending on the species (Handeler et al., 2009, Pelletreau et al., 2014), and act as an energy source for the slug. Since sacoglossans neither inherit nor renew chloroplasts, unlike their algal symbiont, the ability to maintain the acquired organelle is paramount for retaining photosynthetic capabilities.
Maintaining the Chloroplast and the Horizontal Gene Transfer Debate
Sacoglossans that maintain a photosymbiotic relationship are grouped into three categories based on the duration for which they can retain a photosynthetically active plastid: long-term retention species, short-term retention species, and non-retention species. Based on a kleptoplasty survey conducted on 29 species of sacoglossan, it was concluded that the majority of slugs fall under the non-retention or short-term retention category (Handeler et al., 2009). In this study, pulse amplitude modulation measured photosystem II activity of sea slugs over a period of 75 days to determine how long plastids remained photosynthetically active. Of the 29 species tested, 14 exhibited a loss of photosynthetic ability immediately and were considered non-retention species, 12 were considered short-term retention species due to a loss after two weeks, and three were considered long-term retention species for their ability to maintain their photosynthetic capacity for over four weeks. Elysia chlorotica, Elysia timida, and Elysia clarki were all cited for their ability to retain functional plastids for a month or longer during a period of starvation.
Although multiple species that have been noted for their ability to retain photosynthetic plastids for a timespan of over one month, the majority of information on the various mechanisms for long-term chloroplast retention has come from studies on E. chlorotica. This sacoglossan mollusk, found primarily in marshy regions along the east coast of North America, is known to participate in photosynthetic endosymbiosis with the chloroplasts obtained through consumption of its algal prey, V. litorea. This symbiotic relationship is defined by the ability of mature E.chlorotica to alter its metabolism from a state of heterotrophism to photoautotrophism (Devine et al., 2012).
E. Chlorotica is of particular interest due to its ability to retain functional plastids for periods greater than ten months during starvation (Christa et al, 2013; Schwartz et al, 2014, 2010; de Vries et al, 2009; Pelletreau et al, 2012). How E.chlorotica is capable of such long-term plastid retention has been the focus of much debate and research in recent years. One common explanation is horizontal gene transfer, which occurs when one organism’s genes are transferred to another through means other than reproduction. Many researchers tend to favor this explanation because sacoglossans do not sequester algal nuclei, but require the importation of proteins to maintain photosynthetically active plastids. Horizontal gene transfer may be responsible for providing the essential genes required for plastid maintenance, and has thus been the focus of extensive research (Christa et al., 2013).
Schwartz et al. (2010), reasoned that the extreme longevity of the ingested V. litorea chloroplast within E. chlorotica must be due to some portion of the algal genome being transferred to the host cell. To test this hypothesis, the cDNA and genomic DNA of V. litorea and E.chlorotica was amplified via PCR, sequenced, and analyzed. It was determined that three genes encoded within the nucleus of V. litorea, fcp, a gene that encodes for a chlorophyll binding protein, Lhcv1 and Lhcv2, genes that encode for light harvesting complex proteins, are also found in the E.chlorotica’s genomic DNA. These findings indicate an integration of the genes necessary for photosynthesis into the slug’s genome by being actively transcribed by its cellular machinery, which may be the key to maintaining long-term functional plastids.
Wagele et. al’s (2011) findings appear to contradict Schwartz’s conclusion of horizontal gene transfer occurring in E.chlorotica. This study sequenced the expressed sequence tags of E.chlorotica, and looked for transcripts of PsbO, among many other V.litorea-derived nuclear genes. No transcripts from PsbO or any other V.litorea genes were found, and it was therefore concluded that horizontal gene transfer is not occurring between the two.
Criticizing Wagele’s study for using too small of a sample size to gather mRNA specimens, and for a possible incorrect interpretation of negative results, which may have arisen from problematic data, Pierce et al (2012), performed a follow up study. In this study, genomic DNA was extracted from V.litorea to create a transcriptome set for analysis. Next, total RNA of both V. litorea was extracted, and its genome sequenced. The same was repeated for E.chlorotica. It was determined that there exist, although rare, some transcripts within the transcriptome of E.chlorotica, that originated from the transcriptome of V. litorea. Interestingly, the authors discovered that these transcripts are of both chloroplast-encoded and nuclear-encoded origin. This suggests that the chloroplast’s genome is transcriptionally active within E.chlorotica’s cells, and produces transcripts to encode many chloroplast proteins. In particular, there were segments of E.chlorotica’s transcriptome that coded for D1, D2, and CP43 proteins, which are components of the PSII and PSI reaction centers. In total, the study determined that there were 101 chloroplast-encoded gene sequences found in E.chlorotica that came from its symbiotic partner, V.litorea. From these findings, the authors concluded that E.chlorotica is capable of maintaining functional chloroplasts within its cells for such long periods of time due to horizontal gene transfer.
This conclusion may appear to be incorrect to some researchers. For example, Pierce et al. sequenced about 100 million E.chlorotica transcripts, and found only 101 that could imply horizontal gene transfer. Furthermore, of these 101, only a few segments were discovered that are known to code for proteins that have photosynthetic functions. Photosynthesis is a complex process that necessitates the expression of thousands of genes, so it is reasonable to question the validity of research that only found trace support of horizontal gene transfer in a pool of several million transcripts. It is especially suspect that transcripts for RuBisCO and the light harvesting complex (LHC) were not detected. Especially when considering that these two together make up approximately 20% of the total transcripts for Arabidopsis leaves, and play a central role in photosynthesis overall (Christa et al., 2013).
Bhattacharya et al.’s (2013) study was motivated by Pierce’s seemingly questionable results, and by skepticism for the horizontal gene transfer hypothesis in general. To address the issues of previous studies, the researchers took a different approach by examining E. chlorotica’s egg DNA and looking for algae-derived transcripts within the genome. The author’s of this study argued that the presence of transcripts (which were less than 90bp in length) in the host’s cells is not strong enough evidence to support the conclusion that integration of algal genes into the host’s genome is occurring. It was also argued that using E.chlorotica egg DNA in particular is a better sample to study given that this DNA does not come in contact with the algal prey. Like the previous studies (Pierce et al., 2012; Wagele et al., 2011; Schwartz et al., 2010), Bhattacharya sequenced the V.littorea and E.chlorotica genomes, and used PCR to analyze genomic DNA and cDNA, but this study also utilized the Basic Local Alignment Search Tool (BLAST) to find regions of similarity. The results of the BLAST analysis did not find regions of alignment that were greater than 46 nucleotides, indicating an absence of algal derived genes within the egg genomes. To explain the difference between these findings, and those of Pierce (2012) and Schwartz (2010), Bhattacharya argued that these prior studies, whose findings support horizontal gene transfer, may have been dealing with contaminated algal DNA and cDNA.
One important point to consider is Bhattacharya et al. (2013) ceded that although their results showed an absence of algal sequences, this data alone is not enough to prove that algal genes are completely absent within E. chlorotica’s genome. It was proposed that a mechanism other than horizontal gene transfer must have been occurring to allow for long-term chloroplast retention. One possibility suggested was that E.chlorotica takes up extrachromosomal DNA fragments through contact with V. litorea, and that this would account for the algal nuclear genes found within the adult slug. This would also explain why these genes are absent in the egg DNA, which does not come into contact with algae.
Schwartz et al. (2014) criticized Bhattacharya’s extrachromosomal DNA hypothesis on the grounds that prior studies have found algal sequences in larval DNA. To tackle this issue, and to address the potential problem of improper genomic analysis, fluorescence in situ hybridization (FISH) was used to examine the chromosomes of un-hatched E.chlorotica larvae (unexposed to algae) for the presence of algal gene sequences. The rationale for using FISH is that a gene specific to V. litorea can be chosen for a probe. Specifically, this study chose prk, a nuclear gene that encodes for phosphoribulokinase, a Calvin cycle enzyme with no known homolog in non-photosynthetic animals. Their results found that the prk probe was able to bind to some of E.chlorotica chromosomes, indicating a transfer and incorporation of algal genes into the slug’s chromosomes.
Although only a few studies were mentioned here in this review, there are many others that either support or refute horizontal gene transfer and incorporation as E.chlorotica’s method of maintaining a functional chloroplast. More research is necessary before a valid conclusion can be drawn in regards to long-term plastid maintenance in E.chlorotica.
What also remains an uncertainty is the benefit to the animal host for having functional plastids within their cells. As the next section will explore, the benefit to maintaining chloroplasts is largely unclear as well.
The Questionable Benefits of a Symbiotic Relationship
Sacoglossan sea slugs received the nicknames “leaves that crawl” and “solar powered leaves” in the 1970’s when researchers first became aware of their kleptoplastic nature. Long term retention species, specifically Elysia chlorotica, Elysia timida, Elysia crispate, and Plakobranchus ocellatus, were of particular interest for their ability to retain ingested plastids for six months or more (Christa et al., 2014). During this time, it was believed that plastid sequestering was the key to surviving prolonged starvation. However, recent studies now challenge this long-held view, and bring into question the actual role of kleptoplasts in sacoglossan sea slugs.
At first blush the benefits of a symbiotic relationship with algae may seem obvious and not in need of further investigation. Researchers in the 1970’s assumed that sequestering plastids meant that the slugs could survive starvation by using the acquired chloroplasts to carry out photosynthesis. However, one feature of starved plastid-bearing slugs is that they decrease in size the longer they are starved. The starved slugs also gradually lose their green color, and turn a pale pink as the starvation period progresses. These observations call into question the role of light, photosynthesis, and the plastid in sea slug survivability (Christa et al., 2013).
Christa et al. (2013) investigated the long-term survivability of starved E. timida and P.ocellatus in light versus dark conditions. The first portion of the experiment examined the ability of both species to fix CO2 in the presence and absence of light. CO2 fixation was observed in both species when in the presence of light, and occurred in a light-dependent manner. It was also found that CO2 fixation is non-existent in dark conditions. CO2 fixation is a hallmark of photosynthesis, but not necessarily indicative that the sea slugs are in fact photosynthetic. Therefore, the researchers blocked photosynthesis either by using monolinuron and culturing starved slugs in light, or keeping starved slugs in dark conditions. PAM analysis measured photosynthetic activity over a period of 88 days and found that specimens in light conditions showed the greatest decline in photosynthetic activity, but specimens kept in the dark showed a similar decline. The monolinuron slugs also exhibited a similar survival rate to control slugs. Additionally, control, monolinuron-exposed (in light) and slugs kept in the dark all showed a similar degree of weight loss by the end of the experiment. From these findings the authors concluded that light, and photosynthesis as a primary carbon source might not be essential for sea slug survival.
Christa’s findings certainly question sacoglossan’s title of “solar-powered” slug, but this study is worth challenging primarily due to statistical shortcomings that may have skewed the results. The first issue being a small sample size for both E.timida (n=4) and P.ocellatus (n=2) for each of the experiments, and the second being that the sample size for each experiment was not controlled (larger number of specimens for light conditions compared to those in dark conditions). Yamamoto et al. (2013) also examined the survivability of starved P.ocellatus under light and dark conditions. Contradicting Christa, Yamamoto found that P. ocellatus survivability and relative weight was higher when placed in light conditions, compared to dark conditions. The conflicting results of both research teams necessitate further study to determine the benefit, if any, of photosynthesis in sacoglossans.
One important factor that many studies overlook is that kleptoplasts often degrade in high light conditions, and are incapable of being replenished in starved specimens (Baumgartner et al., 2015, Jesus et al., 2010). The destruction of kleptoplasts during experimental protocol, followed by a failure to replenish them in starved individuals may be the reason why benefits to a symbiotic relationship have yet to be measured. Pelletreau et al. (2012) also noted in their work on E.chlorotica that plastids are not automatically stable within the host following ingestion, but rather “permanent kleptoplasty” is only obtained after feeding on their algal food source for a period of seven days or more.
Baumgartner (2015) took these considerations into account, and investigated the fitness benefits of photosynthetically active kleptoplasts in E.viridis. This particular species of sacoglossan has been found in prior studies to survive, but lose weight when starved and in light conditions. The ability to survive without food has been attributed to kleptoplasty retention, however, no physiological benefits, such as weight gain, due to photosynthesis by functional kleptoplasts have been found. This study fed E.viridis specimens one of two types of algae: Codium fragile, whose kleptoplasts are considered highly functional, or Cladophora rupestris, whose kleptoplasts are of low functionality. In doing so, the authors were determined to avoid the potential artifacts that may have arisen in previous studies due to starvation. Also, to circumvent the potential issue of kleptoplast destruction by irradiation, this study kept E.viridis under natural light conditions, alternating between high and low levels, over a period of four weeks. Those that fed on a diet of C. fragile saw a two-fold increase in growth efficiency in high light compared to low light, while those that fed on C. rupestris saw no difference in growth in either light conditions. Relative electron transport rates, a measure of photosynthetic efficiency, was higher for C.fragile- fed slugs kept in high light conditions compared to C.rupestris- fed slugs in the same light environment. Throughout this study, the researchers also measured consumption rates between the slugs in different light treatments to rule out any possibility that changes in growth may be due to differences in consumption. It was found that the consumption rates were similar enough to conclude that the measured increased growth efficiency of E.viridis was due to the retention of functional kleptoplasts
Akimoto et al. 2014 also studied the role of light and food on the growth of sacoglossans E.trisinuata and E.atroviridis. Similar to what was found in Baumgartner’s study, Akimoto’s results found that when fed and in light conditions, E.trisinuata exhibited significant increases in growth. However, the same was not observed in E.atroviridis. These findings indicate that E.trinsuata can acquire additional energy to sustain growth through photosynthesis when kleptoplasts are supplied through food. However, these findings also indicate that this is not true for all species. What remains to be determined is why some species, such as E. atroviridis saw no increase in growth despite being on the same diet that permitted growth in E. trinsuata. Further research is therefore needed to determine the function of kleptoplasts in endosymbiosis with sacoglossans, especially those species that do not exhibit an increase in growth or otherwise apparent indication of improved physiological fitness.
Concluding Remarks
Our earth is both a beautiful and cruel place. The endless diversity among our planets inhabitants, the result of millions of years of evolution, makes each species a fascinating specimen to study. However, the traits that we find so intriguing are the physical manifestations of the most basic need of every living being: the need to survive. This most primal requirement has contributed to the planet’s seemingly infinite range of biological diversity. Symbiosis is of particular fascination because this phenomenon involves two organisms sharing their need for survival, and exploiting each other’s biological advantages to live another day. Sacoglossan sea slugs utilize the plastids obtained through consumption of their algal prey, and in doing so provide a fascinating opportunity for research into how the genetic and biochemical material from a photosynthetic organism can be transferred to a non-photosynthetic animal to give it photosynthetic functionality. Although there are disagreements as to how plastids are maintained, and their direct benefit to the animal, this only supports further research into understanding this truly unique symbiotic relationship.
References
Akimoto, A., Hirano, Y.M., Sakai, A., and Yusa, Y. (2014). Relative importance and interactive
effects of photosynthesis and food in two solar-powered sea slugs. International Journal on Life in Oceans and Coastal Waters, 1095-1102.
Baumgartner, F.A., Pavia, H., and Toth, G.B. (2015). Acquired Phototrophy through Retention of
Functional Chloroplasts Increases Growth Efficiency of the Sea Slug Elysia viridis. PLoS One 10, e0120874.
Bhattacharya, D., Pelletreau, K.N., Price, D.C., Sarver, K.E., and Rumpho, M.E. (2013).
Genome analysis of Elysia chlorotica Egg DNA provides no evidence for horizontal gene transfer into the germ line of this Kleptoplastic Mollusc. Mol Biol Evol 30, 1843-1852.
Bogorad, L. (1981). Chloroplasts. J Cell Biol 91, 256s-270s.
Christa, G., de Vries, J., Jahns, P., and Gould, S.B. (2014a). Switching off photosynthesis: The
dark side of sacoglossan slugs. Commun Integr Biol 7, e28029.
Christa, G., Zimorski, V., Woehle, C., Tielens, A.G., Wägele, H., Martin, W.F., and Gould, S.B.
(2014b). Plastid-bearing sea slugs fix CO2 in the light but do not require photosynthesis to survive. Proc Biol Sci 281, 20132493.
Cruz, S., Calado, R., Serôdio, J., and Cartaxana, P. (2013). Crawling leaves: photosynthesis in
sacoglossan sea slugs. J Exp Bot 64, 3999-4009.
de Vries, J., Christa, G., and Gould, S.B. (2014). Plastid survival in the cytosol of animal cells.
Trends Plant Sci 19, 347-350.
Devine, S.P., Pelletreau, K.N., and Rumpho, M.E. (2012). 16S rDNA-based metagenomic
analysis of bacterial diversity associated with two populations of the kleptoplastic sea slug Elysia chlorotica and its algal prey Vaucheria litorea. Biol Bull 223, 138-154.
Gotthelf, A. (2012). Teleology, First Principles, and Scientific Method in Aristotle's Biology, 1
edn (Oxford University Press).
Händeler, K., Grzymbowski, Y.P., Krug, P.J., and Wägele, H. (2009). Functional chloroplasts in
metazoan cells - a unique evolutionary strategy in animal life. Front Zool 6, 28.
Jesus, B., Venturaa, P., and Calado, G. (2010). Behaviour and a functional xanthophyll cycle
enhance photo-regulation mechanisms in the solar-powered sea slug Elysia timida (Risso, 1818). Journal of Experimental Marine Biology and Ecology 395, 98-105.
Maurino, V.G., and Weber, A.P. (2013). Engineering photosynthesis in plants and synthetic
microorganisms. J Exp Bot 64, 743-751.
Nakayama, T., Kamikawa, R., Tanifuji, G., Kashiyama, Y., Ohkouchi, N., Archibald, J.M., and
Inagaki, Y. (2014). Complete genome of a nonphotosynthetic cyanobacterium in a diatom reveals recent adaptations to an intracellular lifestyle. Proc Natl Acad Sci U S A 111, 11407-11412.
O'Malley, M.A. (2015). Endosymbiosis and its implications for evolutionary theory. Proc Natl
Acad Sci U S A.
Okazaki, K., Kabeya, Y., and Miyagishima, S.Y. (2010). The evolution of the regulatory
mechanism of chloroplast division. Plant Signal Behav 5, 164-167.
Pelletreau, K.N., Bhattacharya, D., Price, D.C., Worful, J.M., Moustafa, A., and Rumpho, M.E.
(2011). Sea slug kleptoplasty and plastid maintenance in a metazoan. Plant Physiol 155, 1561-1565.
Pelletreau, K.N., Weber, A.P., Weber, K.L., and Rumpho, M.E. (2014). Lipid accumulation
during the establishment of kleptoplasty in Elysia chlorotica. PLoS One 9, e97477.
Pelletreau, K.N., Worful, J.M., Sarver, K.E., and Rumpho, M.E. (2012). Laboratory culturing of
Elysia chlorotica reveals a shift from transient to permanent kleptoplasty. Symbiosis 58, 221-232.
Pierce, S.K., Fang, X., Schwartz, J.A., Jiang, X., Zhao, W., Curtis, N.E., Kocot, K.M., Yang, B.,
and Wang, J. (2012). Transcriptomic evidence for the expression of horizontally transferred algal nuclear genes in the photosynthetic sea slug, Elysia chlorotica. Mol Biol Evol 29, 1545-1556.
Plotkin, M., Hod, I., Zaban, A., Boden, S.A., Bagnall, D.M., Galushko, D., and Bergman, D.J.
(2010). Solar energy harvesting in the epicuticle of the oriental hornet (Vespa orientalis). Naturwissenschaften 97, 1067-1076.
Price, D.C., Chan, C.X., Yoon, H.S., Yang, E.C., Qiu, H., Weber, A.P., Schwacke, R., Gross, J.,
Blouin, N.A., Lane, C., et al. (2012). Cyanophora paradoxa genome elucidates origin of photosynthesis in algae and plants. Science 335, 843-847.
Rumpho, M.E., Pelletreau, K.N., Moustafa, A., and Bhattacharya, D. (2011). The making of a
photosynthetic animal. J Exp Biol 214, 303-311.
Schwartz, J.A., Curtis, N.E., and Pierce, S.K. (2010). Using Algal Transcriptome Sequences to
Identify Transferred Genes in the Sea Slug, Elysia chlorotica. Evolutionary Biology 37, 27-37.
Schwartz, J.A., Curtis, N.E., and Pierce, S.K. (2014). FISH labeling reveals a horizontally
transferred algal (Vaucheria litorea) nuclear gene on a sea slug (Elysia chlorotica) chromosome. Biol Bull 227, 300-312.
Shih, P.M., Wu, D., Latifi, A., Axen, S.D., Fewer, D.P., Talla, E., Calteau, A., Cai, F., Tandeau
de Marsac, N., Rippka, R., et al. (2013). Improving the coverage of the cyanobacterial phylum using diversity-driven genome sequencing. Proc Natl Acad Sci U S A 110, 1053-1058.
Vargas-Parada, L. (2010). Mitochondria and the Immune Response (Nature Education), pp. 15.
Venn, A.A., Loram, J.E., and Douglas, A.E. (2008). Photosynthetic symbioses in animals. J Exp
Bot 59, 1069-1080.
Wägele, H., Deusch, O., Händeler, K., Martin, R., Schmitt, V., Christa, G., Pinzger, B., Gould,
S.B., Dagan, T., Klussmann-Kolb, A., et al. (2011). Transcriptomic evidence that longevity of acquired plastids in the photosynthetic slugs Elysia timida and Plakobranchus ocellatus does not entail lateral transfer of algal nuclear genes. Mol Biol Evol 28, 699-706.
Yamamoto S, H.Y., Hirano YJ, Trowbridge CD, Akimoto A, Sakai A, Yusa Y (2013). Effects of
photosynthesis on the survival and weight retention of two kleptoplastic sacoglossan opisthobranchs. J Mar Biol Ass UK, 209–215.