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We’re told that we can insert a gene to confer sterility and this trait would race like wildfire through Aedes aegypti. Why this species? Because it’s the vector of the Zika virus—along with the dengue and yellow fever viruses. The problem is that A. aegypti isn’t the only culprit. It’s just one of a dozen or more bloodsuckers that will also have to be wiped out. After we’ve driven these species to extinction, we’ll presumably move on to the Anopheles species that transmit malaria.
Dinosaurs, literally meaning 'terrible lizards', were first recognized by science, and named by Sir Richard Owen (who preferred the translation ‘fearfully great’), in the 1840's. In the intervening 170 years our knowledge of dinosaurs, including whether they all really died out 65 million years ago, has changed dramatically. Take a crash course on the history of the dinosaurs with our infographic.
Many say now is the century of biology, the study of life. Genomics is therefore “front-and-centre”, as DNA, is the software of life. From staring at stars, we are now staring at DNA. We can’t use our eyes, like we do in star gazing, but just as telescopes show us the far reaches of the Universe, DNA sequencing machines are reading out our genomes at an astonishing pace.
Many of us know that some birds trick other host parents from a different species into rearing their young. Best known is the common cuckoo in the UK and much of mainland Europe, However, this type of deception is not only the forte of birds – many insects ‘brood parasites’ too, especially ants, wasps, and bees.
Announced on January 13th by President Obama in his eighth and final State of the Union Address, the multi-billion dollar project will be led by US Vice President, Joe Biden, who has a vested interest in seeing new cures for cancer. Using genomics to cure cancer is being held on par with JFK’s desire in 1961 to land men on the moon.
2016 is here. The New Year is a time for renewal and resolution. It is also a time for dieting. Peak enrolment and attendance times at gyms occur after sumptuous holiday indulgences in December and again when beach wear is cracked out of cold storage in summer. As the obesity epidemic reaches across the globe we need new solutions. We need better ways to live healthy lifestyles.
As the days get shorter, the Netherlands, a low lying waterlogged country, becomes a safe haven for approximately five million waders, gulls, ducks, and geese, which spend the winter here resting and foraging in fresh water lakes, wetlands, and along rivers. Many of these birds travel to the Netherlands from their breeding ranges in the Arctic.
Knowledge that we all have DNA and what this means is getting around. The informed public is well aware that our cells run on DNA software called the genome. This software is passed from parent to child, in the long line of evolutionary history that dates back billions of years – in fact, research published this year pushes back the origin of life on Earth another 300 million years.
Charles Darwin was widely known as a travel writer and natural historian in the twenty years before On the Origin of Species appeared in 1859. The Voyage of the Beagle was a great popular success in the 1830s. But the radical theories developed in the Origin had been developed more or less in secret during those intervening twenty years.
It is hard to quantify the impact of ‘role-model’ celebrities on the acceptance and uptake of genetic testing and bio-literacy, but it is surely significant. Angelina Jolie is an Oscar-winning actress, Brad Pitt’s other half, mother, humanitarian, and now a “DNA celebrity”. She propelled the topic of familial breast cancer, female prophylactic surgery, and DNA testing to the fore.
News broke in July 2015 that the Rosetta mission’s Philae lander had discovered 16 ‘carbon and nitrogen-rich’ organic compounds on Comet 67P/Churyumov-Gerasimenko. The news sparked renewed debates about whether the ‘prebiotic’ chemicals required for producing amino acids and nucleotides – the essential building blocks of all life forms – may have been delivered to Earth by cometary impacts.
Two other major and largely unsolved problems in evolution, at the opposite extremes of the history of life, are the origin of the basic features of living cells and the origin of human consciousness. In contrast to the questions we have just been discussing, these are unique events in the history of life.
Society owes a debt to Henrietta Lacks. Modern life benefits from long-term access to a small sample of her cells that contained incredibly unusual DNA. As Rebecca Skloot reports in her best-selling book, “The Immortal Life of Henrietta Lacks”, the story that unfolded after Lacks died at the age of 31 is one of injustice, tragedy, bravery, innovation and scientific discovery.
Kuwait is changing the playing field. In early July, just days after the June 26th deadly Imam Sadiq mosque bombing claimed by ISIS, Kuwait ruled to instate mandatory DNA-testing for all permanent residents. This is the first use of DNA testing at the national-level for security reasons, specifically as a counter-terrorism measure. An initial $400 million dollars is set aside for collecting the DNA profiles of all 1.3 million citizens and 2.9 million foreign residents
Did you learn about Mrs Gren at school? She was a useful person to know when you wanted to remember that Movement, Respiration, Sensation, Growth, Reproduction, Excretion, and Nutrition were the defining signs of life. But did you ever wonder how accurate this classroom mnemonic really is, or where it comes from?
Another ‘Awareness Day’, International Kissing Day, is coming up on July 6. It might not seem obvious but kissing, like most subjects can now be easily linked to the science of DNA. Thus, there could be no more perfect opener for my Double Helix column, given the elegance and beauty of a kiss. To start, there is the obvious biological link between kissing and DNA: propagation of the species. Kissing is not only pleasurable but seems to be a solid way to assess the quality and suitability of a mate.
The latest incarnation (I chose that word advisedly!) of the Jurassic Park franchise has been breaking box-office records and garnering mixed reviews from the critics. On the positive side the film is regarded as scary, entertaining, and a bit comedic at times (isn't that what most movies are supposed to be?). On the negative side the plot is described as rather 'thin', the human characters two-dimensional, and the scientific content (prehistoric animals) unreliable, inaccurate, or lacking entirely in credibility.
“In the spring a young man's fancy lightly turns to thoughts of love” (Alfred, Lord Tennyson), but he could have said the same for insects too. Male insects will be following the scent of females, looking for a partner, but not every female is what she seems to be. It might look like the orchid is getting some unwanted attention in the video below, but it’s actually the bee that’s the victim. The orchid has released complex scents to fool the bee into thinking it’s meeting a female.
One of the most fun and exciting sources of information available for free on the Internet are the videos found on the Technology, Entertainment and Design (TED) website. TED is a hub of stories about innovation, achievement and change, each artfully packaged into a short, highly accessible talk by an outstanding speaker. As of April 2015, the TED website boasts 1900+ videos from some of the most imminent individuals in the world. Selected speakers range from Bill Clinton and Al Gore to Bono and other global celebrities to a range of academics experts.
DNA is the foundation of life. It codes the instructions for the creation of all life on Earth. Scientists are now reading the autobiographies of organisms across the Tree of Life and writing new words, paragraphs, chapters, and even books as synthetic genomics gains steam. Quite astonishingly, the beautiful design and special properties of DNA makes it capable of many other amazing feats. Here are five man-made functions of DNA, all of which are contributing to the growing “industrial-DNA” phenomenon.
On 25th March 2015, 530 years after his death, King Richard III of England will be interred in Leicester Cathedral. This remarkable ceremony is only taking place because of the success of DNA analysis in identifying his skeletal remains. So what sort of genes might a king be expected to have? Or, more prosaically, how do you identify a long dead corpse from its DNA? Several methods were used, and in particular the deduction of the skeleton’s probable hair and eye colour raises some interesting questions about future trends in forensic DNA analysis.
Richard III is one of England’s best known kings, largely due to the famous play of William Shakespeare in which he is portrayed as an evil villain. He only reigned for two years and was killed at the age of 32 at the battle of Bosworth in 1485. According to the historical records he was unceremoniously buried at Greyfriars Friary in Leicester. At some stage knowledge of the exact location of Richard’s burial was lost. But in 2012 excavations under a car park at the probable site of the former friary yielded “skeleton 1″. Suspicion of his royal identity was excited by the fact that the skeleton had a severely bent spine causing the right shoulder to be higher than the left. This well-known deformity of Richard was mentioned in a contemporary source, as well as by Shakespeare. Furthermore, the skeleton was male, the age was about right, it had evidently been killed in battle, and the radiocarbon date was consistent with death in 1485.
This was all very suggestive, but it was the DNA analysis that really proved the case. The work was led by a team at the University of Leicester, with participation by many other UK and European centres. It is important to note that this was not the normal type of forensic DNA identification, which relies on comparing a set of highly variable DNA markers to a database. Such analysis is fine so long as your suspect is in the database, but it is no use for identifying a long dead individual who is not in any database.
By far the best evidence for the identity of Richard III comes from the analysis of his mitochondrial DNA. Mitochondria are bodies found in every cell, responsible for the production of energy. They have their own DNA which is passed down the generations only through the female line. Barring the occasional new mutation, the DNA sequence of mitochondrial DNA should be identical from mother to daughter down a particular female line of descent. Like their sisters, males also carry the mitochondrial DNA of their mothers, but they do not pass it down to their own offspring.
Richard will have shared mitochondrial DNA with his sister, Anne of York. Two complete female lines of descent were traced back to Anne of York, one of 17 generations down to Michael Ibsen, a resident of London, and the other of 19 generations down to Wendy Duldig, formerly of New Zealand. Complete sequencing of their mitochondrial DNA showed a 100% match between skeleton 1 and of Michael Ibsen, and a single base change compared to Wendy Duldig. One change over this period of time is quite likely to be a new mutation. The sequence family (haplogroup) to which the mitochondrial DNA sequence belongs is a fairly rare one, so few other people in England in 1485 would have shared it and in fact the team has systematically ruled out all the other males of the period who might have shared it because of a common female lineage with Richard III. So this match is highly significant and is the best piece of evidence that the “skeleton 1″ is indeed King Richard.
By Bdna. gif: Spiffistan derivative work: Jahobr (Bdna.gif). Public domain via Wikimedia Commons
Also applied was a newer method which is a technique for predicting the hair and eye colour of someone from their DNA. The most important variants affecting hair colour are mutations of a gene called MC1R, which encodes a cell surface receptor for a hormone. Individuals carrying variants of the MC1R gene with reduced function are likely to have red or blond hair rather than the normal dark hair. The pigmentation of the iris of the eye depends significantly on a gene called OCA2, encoding a protein which transports tyrosine into cells. Again variants of reduced function give less pigmented eyes, meaning that the colour is blueish rather than brownish. Recently a Dutch group created a forensic test based on variants at 24 genetic loci, of which 11 are in the MC2R gene and the rest in 12 other positions including the OCA2 gene. Identification of these 24 variants yields a fairly accurate prediction of hair and eye colour, and in the case of skeleton 1 the prediction was for blue eyes and blond hair. The existing portraits of Richard III all date from some time after his death but the older ones do indeed show light-coloured eyes and reddish-brown hair, an appearance which is consistent with the prediction.
These two types of analysis indicate two rather different senses in which we use the word “gene”. The sequence variants of the mitochondrial DNA, like those used in normal forensic identification, do not, in general, affect the characteristics of the individuals carrying them. The DNA changes often lie outside actual genes, in the regions of DNA between genes. They are better described as “markers” than as “genes”. But the hair and eye colour analysis is based at least partly on actual gene variants that might be expected to generate those visible characteristics.
How much further might this kind of analysis be pushed? Could the height, facial features or skin colour of a crime suspect be deduced from their DNA? The essential issue is the number of gene variants in the population that affect a feature. If it is relatively small, as with hair and eye colour, then prediction is possible. If it is very large, as for height, then it is not possible, because most of the variants affecting height have too small effects to be detectable. Most of the human characteristics that have been studied in this way have turned out to depend on a very large number of variants of small effect. So, contrary to popular perception, there are real limits to what is possible in terms of prediction of bodily features from DNA data. There will doubtless be some other features that are predictable, and these may eventually include skin colour. But unless a completely new approach is invented, it is unlikely that we shall ever see an identikit picture of a suspect generated from DNA at the crime scene.
Featured image credit: Stained glass, by VeteranMP. CC-BY-SA 3.0 via Wikimedia Commons
Watching the field of genomics evolve over the past 20 years, it is intriguing to notice the word ‘genome’ cozying up to the word ‘million’. Genomics is moving beyond 1k, 10k and 100k genome projects. A new courtship is blossoming.
The Obama Administration has just announced a Million Genomes Project – and it’s not even the first.
Now both Craig Venter and Francis Collins, leads of the private and public versions of the Human Genome Project, are working on their million-omes.
The company 23andMe might be the first ‘million-ome-aire’. By 2014, the company founded by Ann Wojcicki processed upwards of 800,000 customer samples. Pundit Eric Topol suggests in his article “Who Owns Your DNA” that without the skirmish with the FDA, 23andMe would already have millions.
In 2011, China’s BGI, the world’s largest genomics research company, boldly announced a million human genomes project. Building on projects like the panda genome and the 3000 Rice Genomes project, the BGI is building new next-generation sequencing technologies to support its flagship project.
Also in 2011, the United States Veterans Affairs (VA) Research and Development program launched its Million Veteran Program (MVP) aiming to build the world’s largest database of genetic, military exposure, lifestyle, and health information. The “large, diverse, and altruistic patient population” of the VA puts it ahead of the others in collecting samples.
Venter’s path will be through his non-profit Human Longevity, Inc (HLI), launched in San Diego, California in 2014 with $70 million in investor funding. To support the company’s tagline — “It’s not just a long life we’re striving for, but one which is worth living” — Venter aims to sequence a million genomes by 2020.
At a price tag of $1000 dollars per genome, one million genomes could cost a billion US dollars. The original human genome project cost $3 billion only 13 years ago, but produced 1 trillion US dollars in economic impact.
The Collins’ ‘million-ome’ will pull together new and existing genomes, with an initial budget of $215 million dollars. This includes genomes from the MVP, which has already enrolled 300,000 veterans and sequenced 200,000. The focus will initially be on cancer but subjects will be healthy and ill, men and women, old and young; it is the foundation of a Precision Medicine Initiative.
In addition to these projects we will have millions anyway. ARC Investment Analysis suggested we could see 4 to 34 billion human genomes by 2024 at historical rates of sequencing – if current trends in dropping costs and demand continue.
How could we have more genomes than humans living on earth? Cancer genomics is in ‘gold rush’ phase. Steve Jobs was famously one of the first 20 people to have his genome sequenced. He paid $100k but did so to also have the genome of the cancer that killed him sequenced. He left a personal genomics legacy to the world, but his investment in DNA sequencing also serves as a reminder that a genome is not the same as a cure. Hopes are high, though, especially for cancer diagnostics. The International Cancer Genomics Consortium is already backed with a billion dollar budget and the field continues to explode.
Further, an adult human body consists of 37 trillion genomes all working together (plus the 100 trillion genomes of the microbial cells in our microbiome). There is mounting evidence we are all genomic mosaics, meaning we all have more than one genome (e.g. from pre-cancerous cells, transplants, and mothers who carry the genomes of past live-born babies).
It is good to cultivate a healthy skepticism and not be drawn into the hype. Critics exist, as always. At the other end of the continuum, Ken Weiss of The Mermaid’s Tale blog, a geneticist himself, has outlined reasons to put valuable research dollars elsewhere than a million genomes project or precision medicine, but given than they will happen, he also contemplates what should be done with resulting data.
Eric Topol said in response to the rise of ‘million-ome’ projects, that there are now many 100k projects and he “might rather have 100,000 people with ‘pan-oromic definition’ than 1 million with just native DNA”. By high definition he means all the mapping (sensors, anatomy, environmental quantified, gut microbiome, etc.) that belongs to his vision of a “Google medical map”.
There are huge differences between “projections,” “announcements,” and “hard (published) data.” Big projects can fall by the way-side. 23andMe hit a barrier with the FDA decision. The BGI is still tooling up. Obama hasn’t yet secured a budget. Venter is giving himself time. Everyone is starting to think about genomes inside the systems in which they exist in (cells, organs, organisms, ecosystems).
Regardless of trajectory, it is a foregone conclusion that, counting all sources, the number of sequenced genomes will pass one million in 2015, if it hasn’t already.
Google is imagining the day when researchers compute over millions of genomes and is building the infrastructure to support it; Google Genomics has launched offering $25/year pricing to hold your genome in the Cloud.
Why stop at millions? Jong Bhak is calling for billions. He is suggesting that “the genomics era hasn’t even started.” Bhak, a leader of the Korean Personal Genomes Project, a project to sequence the genomes of all 50 million Koreans, has outlined a vision for a Billion Genome Project.
The first to talk of ‘a genome for everyone’ was perhaps George Church, technologist and founder of the Personal Genome Project. He wrote 2005 a paper entitled “The Personal Genome Project.” In it he recalled talking with Wally Gilbert that “Six billion base pairs for six billion people had a nice ring to it”—back in 1976, soon after Gilbert invented DNA sequencing, for which he won a Nobel Prize.
The fact that more voices in global science are debating the pros and cons of ‘millions and billions of genomes’ is evidence that 2015 marks a shift towards a Practical Genomics Revolution. It is becoming practical to think big(ger).
The study of minerals is the most fundamental aspect of the Earth and environmental sciences. Minerals existed long before any forms of life. They have played an important role in the origin and evolution of life and interact with biological systems in ways we are only now beginning to understand.
One of the most rapidly developing areas in what is now called ‘geobiology’ concerns the role of microbes in processes both of mineral formation and destruction. For example, the ‘geobacter’ bacteria, shown in the accompanying picture taken in an electron microscope, are not just sitting on an iron oxide mineral surface but interacting with it because it is their method of ‘respiration’ (just as breathing oxygen is ours).
A transfer of electrons between the microbe and the mineral in this case brings about a change in the chemical state of the iron (its ‘reduction’) which also causes the mineral to dissolve. Interactions of this type are now known to play important roles in the release and movement of metals and other elements, including pollutants such as arsenic, at the Earth’s surface.
A very different story linking minerals and the living world concerns the ways that many organisms form minerals to fulfil a particular function, such as providing an external skeleton (shell).
For example, the chalk rock responsible for the ‘White Cliffs of Dover’ in the south of England are almost entirely composed of the remains of microscopic plates of calcite derived from a protective armour around unicellular planktonic algae (‘coccolithophorids’). In many cases the products of such processes of ‘biomineralisation’ are delicate structures of great beauty.
A good example is provided by the (as illustrated) ‘radiolaria’, free-floating single celled organisms found in the upper regions of the water column in the oceans, and which have skeletons of poorly crystalline (‘opaline’) silica.
Growth of Geobacter sulfurreducens on Poorly Crystalline Fe (III) Oxyhydroxide Coatings. Used with permission from David Vaughan.
Amongst the most remarkable examples of organisms producing a mineral to serve a specific function are the ‘magnetotactic bacteria’. Here the bacterium concerned produces a chain of perfect crystals (see illustration of magnetite crystals), most commonly of magnetite, which make use of the magnetic properties of that mineral. It seems that these organisms use magnetite to become aligned in relation to the Earth’s magnetic field and therefore in the most advantageous position in relation to the sediment-water interface.
One of the most challenging questions in all of science is: ‘How did life on Earth originate’? It is now widely believed that minerals played a key role as catalysts for biochemical reactions and templates for the emergence of the complex biomolecules needed for life. Many different routes have been proposed for the emergence of the first living organisms, almost all have major roles for minerals. These roles may have been in providing catalysts through biomolecule sized cavities in their crystal structures or weathered surfaces. Other routes involve clay minerals as substrates aiding in the formation of the first self-replicating genetic molecules, or look to the environments at, or near, mid-ocean ridges where hot fluids emerge releasing a stream of metal sulphide mineral particles. At the present day, both micro- and macro-organisms utilise chemical energy available in these environments for their metabolisms. Iron sulphide minerals are suggested as the key catalysts in these models.
A double chain of magnetite crystals in a magnetotactic bacterium. Courtesy of the Mineralogical Society of America. Used with permission.
There are challenging questions in all of these areas, whether it be understanding the electron transfer processes involved when bacteria interact with minerals, the mechanisms involved in biomineral formation, or the complex roles probably played by minerals in the emergence of life on Earth.
In these and many other cases, it is the processes at mineral surfaces which are critically important. Only in recent years has it been possible to study mineral surfaces at a molecular scale. Today, we are at the threshold of a new understanding of the processes taking place at the surface of the Earth which integrates the mineralogical, geochemical and biological realms at the molecular scale. Understanding what happens at surfaces and interfaces at scales from global to molecular is key to that understanding. Here, the emergent field of ‘molecular environmental science’ should provide new insights into the way our planet ‘works’ comparable to the revolutionary advances seen in human biology associated with the genetic code.
Featured image credit: Didimocrytus tetrathalamus, by Tim Evanson. CC-BY-SA-2.0 via Wikimedia Commons.
They might be short-lived — but between the time a bubble is born (Fig 1 and Fig 2a) and pops (Fig 2d-f), the bubble can interact with surrounding particles and microorganisms. The consequence of this interaction not only influences the performance of bioreactors, but also can disseminate the particles, minerals, and microorganisms throughout the atmosphere. The interaction between microorganism and bubbles has been appreciated in our civilizations for millennia, most notably in fermentation. During some of these metabolic processes, microorganisms create gas bubbles as a byproduct. Indeed the interplay of bubbles and microorganisms is captured in the origin of the word fermentation, which is derived from the Latin word ‘fervere’, or to boil. More recently, the importance of bubbles on the transfer of microorganisms has been appreciated. In the 1940s, scientists linked red tide syndrome to toxins aerosolized by bursting bubbles in the ocean. Other more deadly illnesses, such as Legionnaires’ disease have been linked since.
Figure 1: Bubble formation during wave breaking resulting in the white foam made of a myriad of bubbles of various sizes. (Walls, Bird, and Bourouiba, 2014, used with permission)
Bubbles are formed whenever gas is completely surrounded by an immiscible liquid. This encapsulation can occur when gas boils out of a liquid or when gas is injected or entrained from an external source, such as a breaking wave. The liquid molecules are attracted to each other more than they are to the gas molecules, and this difference in attraction leads to a surface tension at the gas-liquid interface. This surface tension minimizes surface area so that bubbles tend to be spherical when they rise and rapidly retract when they pop.
Figure 2: Schematic example of Bubble formation (a), rise (b), surfacing (c), rupture (d), film droplet formation (e), and finally jet droplet formation (f) illustrating the life of bubbles from birth to death. (Bird, 2014, used with permission)
When microorganisms are near a bubble, they can interact in several ways. First, a rising bubble can create a flow that can move, mix, and stress the surrounding cells. Second, some of the gas inside the bubble can dissolve into the surrounding fluid, which can be important for respiration and gas exchange. Microorganisms can likewise influence a bubble by modifying its surface properties. Certain microorganisms secrete surfactant molecules, which like soap move to the liquid-gas interface and can locally lower the surface tension. Microorganisms can also adhere and stick on this interface. Thus, a submerged bubble travelling through the bulk can scavenge surrounding particulates during its journey, and lift them to the surface.
When a bubble reaches a surface (Figure 2c), such as the air-sea interface, it creates a thin, curved film that drains and eventually pops. In Figure 3, a sequence of images shows a bubble before (Fig 3a), during, and after rupture (Fig 3b). The schematic diagrams displayed in Fig 2c-f complement this sequence. Once a hole nucleates in the bubble film (Fig 2d), surface tension causes the film to rapidly retract and centripetal acceleration acts to destabilize the rim so that it forms ligaments and droplets. For the bubble shown, this retraction process occurs over a time of 150 microseconds, where each microsecond is a millionth of a second. The last image of the time series shows film drops launching into the surrounding air. Any particulates that became encapsulated into these film droplets, including all those encountered by the bubble on its journey through the water column, can be transported throughout the atmosphere by air currents.
Figure 3: Photographs, before, during, and after bubble ruptures. The top panel illustrated the formation of small film droplets; the bottom panel illustrates the formation of larger jet drops. (Bird, 2014, used with permission)
Another source of droplets occurs after the bubble has ruptured (Fig 3b). The events occurring after the bubble ruptures is presented in the second time series of photographs. Here the time between photographs is one milliseconds, or 1/1000th of a second. After the film covering the bubble has popped, the resulting cavity rapidly closes to minimize surface area. The liquid filling the cavity overshoots, creating an upward jet that can break up into vertically propelled droplets. These jet drops can also transport any nearby particulates, also including those scavenged by the bubble on its journey to the surface. Although both film and jet drops can vary in size, jet drops tend to be bigger.
Whether it is for the best or the worst, bubbles are ubiquitous in our everyday life. They can expose us to diseases and harmful chemicals, or tickle our palate with fresh scents and yeast aromas, such as those distinctly characterizing a glass of champagne. Bubbles are the messenger that can connect the depth of the waters to the air we breathe and illustrate the inherent interdependence and connectivity that we have with our surrounding environment.
How can sacoglossan sea slugs perform photosynthesis – a process usually associated with plants?
Kleptoplasty describes a special type of endosymbiosis where a host organism retain photosynthetic organelles from their algal prey. Kleptoplasty is widespread in ciliates and foraminifera; however, within Metazoa animals (animals having the body composed of cells differentiated into tissues and organs, and usually a digestive cavity lined with specialized cells), sacoglossan sea slugs are the only known species to harbour functional plastids. This characteristic gives these sea slugs their very special feature.
The “stolen” chloroplasts are acquired by the ingestion of macro algal tissue and retention of undigested functional chloroplasts in special cells of their gut. These “stolen” chloroplasts (thereafter named kleptoplasts) continue to photosynthesize for varied periods of time, in some cases up to one year.
In our study, we analyzed the pigment profile of Elysia viridis in order to evaluate appropriate measures of photosynthetic activity.
The pigments siphonaxanthin, trans and cis-neoxanthin, violaxanthin, siphonaxanthin dodecenoate, chlorophyll (Chl) a and Chl b, ε,ε- and β,ε-carotenes, and an unidentified carotenoid were observed in all Elysia viridis. With the exception of the unidentified carotenoid, the same pigment profile was recorded for the macro algae C. tomentosum (its algal prey).
In general, carotenoids found in animals are either directly accumulated from food or partially modified through metabolic reactions. Therefore, the unidentified carotenoid was most likely a product modified by the sea slugs since it was not present in their food source.
Image credit: Lettuce sea slug, by Laszlo Ilyes. CC-BY-SA-2.0 via Flickr.
Pigments characteristic of other macro algae present in the sampling locations were not detected inthe sea slugs. These results suggest that these Elysia viridis retained chloroplasts exclusively from C. tomentosum.
In general, the carotenoids to Chl a ratios were significantly higher in Elysia viridis than in C. tomentosum. Further analysis using starved individuals suggests carotenoid retention over Chlorophylls during the digestion of kleptoplasts. It is important to note that, despite a loss of 80% of Chl a in Elysia viridis starved for two weeks, measurements of maximum capacity of performing photosynthesis indicated a decrease of only 5% of the photosynthetic capacity of kleptoplasts that remain functional.
This result clearly illustrates that measurement of photosynthetic activity using this approach can be misleading when evaluating the importance of kleptoplasts for the overall nutrition of the animal.
Finally, concentrations of violaxanthin were low in C. tomentosum and Elysia viridis and no detectable levels of antheraxanthin or zeaxanthin were observed in either organism. Therefore, the occurrence of a xanthophyll cycle as a photoregulatory mechanism, crucial for most photosynthetic organisms, seems unlikely to occur in C. tomentosum and Elysia viridis but requires further research.
