Christmas Trees Have Trouble Seeing The Light

Biology concepts – photoprotection, photosynthesis, non-photochemical quenching, reaction center, yule, evergreen, chlorophyll

Yule was/is a pagan celebration in midwinter. Krampus

was the spirit who came during Yule to punish children

who had misbehaved. Yule celebrations used evergreens

(note his headdress) and this has continued in the

modern Christmas celebration, but the Krampus became

paired with good Saint Nicholas, so they kind of went

the other way with that one.

Christmas trees are pagan holdovers from when early Christianity adopted December 25^th as the date of the holiday. Pagan religions used evergreens as a reminder that the Earth would bloom with life again even in the winter when the sun was scarce. The joke is on them though – biology shows us that evergreens have to protect themselves from the life giving sun.

The Romans decorated their houses with evergreen boughs in December, and in particular, the Germanic and Norse pagans celebrated the jexla or jol (respectively) festivals in late December until early January. In English, this became Yule, or Yuletide (yule time).

Evergreens, as the name suggests, stay green all winter; they don’t drop the majority of their leaves (needles) and they keep chlorophyll in their leaves all year round. Chlorophyll is green, so there you have it - evergreen.

Chlorophyll is energetically costly to make and maintain, so if there isn’t enough sunlight to make photosynthesis a net gain (once you subtract the energy needed to make and maintain leaves and chlorophyll); a good strategy might be to just drop them and start over again in the spring.

So one reason for being an evergreen would be that there is enough sun and moisture in a certain location to warrant chlorophyll production and maintenance year round. But there might be a more pressing reason for keeping leaves all year round - nutrients.

When deciduous trees lose their leaves, there are letting a lot of nutrients blow away with the wind – usually into my yard. This is costly, especially if you grow in nutrient poor soil like many evergreens do. In cold climates, leaf litter doesn’t decay fast enough to allow nutrients back into the soil, so holding on to your nitrogen is a good idea. Conifer needles tend to be lower in nitrogen than deciduous leaves, so they hold on to their nutrients better.

Conifer forests are often quite bare on the ground. This is

for a few reasons. One, the ground is usually nutrient poor

and fewer things can grow there. Two, the needles cover

the ground and reduce the light for seedlings. And three,

needles are acidic and make the ground acidic, making it

even harder for other things to grow. Fewer competitors

makes it good for the conifers.

In addition, being able to grow in acidic soil or soil that has less nitrogen and phosphorous is an evolutionary strategy for the evergreens; if fewer plants can grow there, than then will have fewer competitors for what nutrients there are and for sunlight. Some evergreen litter is designed to make soil acidic, so that fewer competitors will try and put down roots.

The result of all this is that evergreens, conifer trees and some bushes, stay green year round. But they’re just fighting to stay alive, not growing year round. It turns out that evergreens spend a lot of energy to avoid sun damage caused by the very mechanisms that allow them to gather sunlight light year round. In winter: chlorophyll + sunlight = death. Just like vampires, they have to protect themselves from the suns rays, or they burn up.

The truth is, evergreen trees in cold climates do very little photosynthesis in the winter, even if there is sunshine. They'll make carbohydrates from sunlight, water, and CO₂ (the three ingredients needed for photosynthesis) when they’re each available, but available is a relative term.

Air is always available, so CO₂ isn’t problem. The sun is at a lower angle in the winter, but it isn’t cloudy and gray every winter day; it just seems that way. So sunshine is available at least part of the time - usually daytime.

The large diameter tubes are vessels, while the narrower

ones are tracheids. Conifers only have tracheids, no vessels.

This is good for growing in cold weather environments.

When water freezes in the vessels, the width promotes gas

bubble formation. This will lock the vessel and no water can

ever be transported again. The narrow tracheids of conifers

prevent gas bubble formation.

The problem is water. Sure it’s there, but it may be solid. If the weather is cold enough to freeze the water on and in the shallow soil, it may also be cold enough to freeze the water in the tree trunk and leaves. Any photosynthetic plant, including trees, needs to split water into hydrogen and oxygen during photosynthesis. If they don’t have a source of water, then they can’t perform that little miracle that is the source of all life on Earth.

Even temperatures near freezing can slow down water movement in plants, and since cold air holds less water (humidity), more water can be lost from leaves - even the wax covered, thick leaves of conifers. The end result is that photosynthesis is just not feasible during most points of the winter.

Yet the evergreens don’t drop their leaves and keep their chlorophyll in their chloroplasts. That means that every time the sun comes out, some of the energy of the rays are caught and transferred to the photosynthetic reaction centers, including chlorophyll. If photosynthesis can’t be completed because of a lack of available water, then what happens to all that energy? It is free to bounce around and damage plant tissues, usually in the form of reactive oxygen species (ROS, see this post). Enough damage and the plant will lose its ability to function and die.

To avoid the irony and embarrassment of becoming a dead midwinter symbol of life, evolution has provided plants with certain photoprotective mechanisms. Not just evergreen plants, but all plants. It turns out that photosynthesis pathways are saturable, only so much sunlight can be used to produce energy and then carbohydrates.

The sun is unwilling to play the game; once the saturation limit is reached, it just keeps on shining. On bright summer days, just about any plant is susceptible to damage from excess energy absorption in chloroplasts. Some plants have elaborate mechanisms to change the angle of their leaves so that they receive less sunlight.

Some plants, like this Oxalis triagularis can quickly change the

angle of their leaves so that low levels of sun can be maximized

or that high levels of sun can be avoided. Evergreens can’t do t

his, so they need more mechanisms of photoprotection

Evergreens, especially conifers, can’t regulate the amount of light that shines on them, so they need additional photoprotective mechanisms. Plants of the genus Taxus, like English yew, move their chloroplasts (discussed in this post) instead of their needles.

According to a 2007 study from Japan, yew cell chloroplasts congregate in the center of the cell volume in response to low temperatures, whereas in summer they can be found along the edges closest to where the light comes in. In this way, many chloroplasts can be shielded from sunlight in the winter, so less energy will be harvested. Pretty smart.

For many evergreen plants, their chlorophyll is doomed to harvest sunlight all winter without being able to use it for photosynthesis, so what can they do with it - other than just let it damage them until they die?

It is important that the plant dissipates the light energy before it reaches the reaction center. Chlorophyll isn’t the molecule that changes sun energy to chemicoelectrical energy. Chlorophyll is the pigment that absorbs the energy. That energy can be transferred to an adjacent chlorophyll molecule and so on until it reaches the reaction center. Here, the accumulated the energy is used to split a water molecule and two electrons move into the electron transport chain. It's the reaction center that actually transduces (changes) the energy from light to chemical form.

The photosystem is made up of the reactions center and the

surrounding light harvesting complex (LHC). The LHC is made

up of many chlorophyll molecules that gather light energy and

bounce it around toward the reaction center. The reaction center

has many proteins that work together to transduce the light

energy into chemical energy by splitting water.

If the electrons are generated by the combined work of the protein complexes of the reaction center, then damage can be done because they can’t go on to fix CO2 and turn it into carbohydrate. Those electrons are free to attack any nearby proteins or lipids and break them down. In particular, they can attach themselves to oxygen and create ROS. These molecules are just itching to react with something, anything, and this leads to damage to many structures of the cell.

Plants can try to stop this by producing more antioxidants, which can absorb the electrons from ROS molecules. They might be destroyed in the process, but at least they aren’t allowing the ROS to damage something important. Evergreens ramp up antioxidant molecule production (especially glutathione and alpha-tocopherol) in the winter to prevent ROS damage.

Notice that we said above that the reaction center splits a water to generate the free electrons. Didn’t we also say that in winter, freezing conditions wouldn't allow for available water? This true, but damage to the reaction center and/or chlorophyll is possible BEFORE the point where water would be split. This energy has to be dealt with as well.

To dissipate the energy before it damages the chlorophyll or the reaction center, plants use a technique called non-photochemical quenching (NPQ). Demonstrated in a classic 1987 study, the physical positions of proteins in the photosystem (chlorophylls + reaction center) can be shifted during NPQ so that they create energy traps. In these traps, different pigments, called carotenoids (see this post for plant pigments), can accept the energy of the light.

When the energy hits a carotenoid pigment called violaxanthin, it converts it to another pigment, zeaxanthin. Zeaxanthin can’t passed energy along to the reaction center, but is good at giving it up as heat. When sunlight can be used for photosynthesis, zeaxanthin is turned back to violaxanthin and the photosystem redistributes itself so that light energy can be focused to the reaction center. This is called the xanthophyll cycle.

Cadmium is used in batteries, paint pigments, metal plating

and in the production of other metals, like copper and zinc.

Long-term exposure can lead to kidney damage, but a new

study shows the problem may be worse than that. The

experiments showed that cadmium can interrupt non-

photochemical quenching in barley. This can lead to damage

and reduced barley harvests. Barley is used in making beer –

something must be done!

The zeaxanthin NPQ mechanism (truthfully, it's much more detailed than we have talked about here) is best for quick changes in sunlight level, However, the cycle can be disconnected in winter so that zeaxanthin is maintained for a long time. In addition, a 1995 study showed that some evergreens will prevent damage by inactivated or down-regulating (making less of) proteins of the reaction center, so that the high energy electrons won’t be generated.

This would presumably create more oxygen radicals from the chlorophyll since the energy can’t be transferred to the reaction center, but since zeaxanthin is being kept all winter, that energy is dissipated too. It sounds like a lot of work, but it requires much less energy than dropping leaves in the Fall and then re-growing them in Spring. You wonder why all plants aren’t evergreens – because then we wouldn’t have a special symbol to cram presents during the holidays.

So instead of being a vampire that has to stay out of the sun, the evergreen is more like a superhero that can overcome the power of the sun – his power of photoprotection saves Christmas for us. But maybe not - next week we’ll talk about how many different ways your Christmas evergreens can kill you.

Lysenko EA, Klaus AA, Pshybytko NL, & Kusnetsov VV (2014). Cadmium accumulation in chloroplasts and its impact on chloroplastic processes in barley and maize. Photosynthesis research PMID: 25315190

Demmig, B., Winter, K., Kruger, A., & Czygan, F. (1987). Photoinhibition and Zeaxanthin Formation in Intact Leaves : A Possible Role of the Xanthophyll Cycle in the Dissipation of Excess Light Energy PLANT PHYSIOLOGY, 84 (2), 218-224 DOI: 10.1104/pp.84.2.218

Ottander C, Campbell D, Öquist G (1995). Seasonal changes in photosystem II organization and pigment composition in Pinus sylvestris. Planta, 197, 176-183.

The Colors of Alien Plants

Biology concepts – photosynthesis, chlorophyll, pigmentation, astrobiology, exoplanet, dormancy

The King Crimson Norway Maple in our front yard is

at least 50 ft. tall. It isn’t a rare tree, but I like it a lot.

In fact, it is invasive and native only in Asia. You can’t

plant on in several eastern states in the US, they are

taking over in some deciduous forests.

There is a large King Crimson Norway Maple (Acer platanoides 'King Crimson’) in our front yard. Healthy and round, it is a fine showpiece. We are also blessed with a 15-foot tall burning bush (Euonymus alata 'Compacta') not more than thirty feet from the maple. The burning bush straddles the property line with our neighbor, so when it needs work, its theirs, and when it is beautiful in autumn, it’s ours. Together, they make our landscaping come alive with color and provide ample shade.

They’re autotrophs (auto = self, and troph = feed) as are most plants. They make their own carbohydrates from sunlight, carbon dioxide, and water. Our sun radiates light energy that can be captured and transduced to chemical energy, but not all stars are the same and not all plants are green, so…..

Question of the Day: How can starlight support non-green plants and could it might it be different elsewhere?

Chlorophyll is one of several plant pigments, and chlorophyll itself comes in several flavors, but the primary plant chlorophylls are a and b. The “a” version is the major pigment for photosynthesis, absorbing light at the two ends of the visible spectrum – blues and reds (see picture). Green and yellow light get reflected, and this is what we see. Chlorophyll probably evolved to use red and blue because blue is high energy and red is abundant.

Chlorophyll b is an accessory pigment that plants use in smaller amounts. The “b” version absorbs light from near the same wavelengths as chlorophyll a, but they pass the energy on to the “a” version for use in photosynthesis. The two chlorophylls differ at only one of their 55 carbon atoms.

Green light is higher energy than red light, but less abundant

in our atmosphere. Blue light is much higher energy, so it

can power a lot of photosynthesis even if it isn’t that abundant.

Therefore, is it surprising that our plants appear green,

chorophylls absorb and use red light because it is abundant, and

blue light because it is high energy. Green isn’t worth bothering

with and is reflected.

There are also chlorophylls c, d, and f. Chlorophyll c is also an accessory pigment which transfers energy to chlorophyll a, but it is very different structurally. Chlorophyll c is found only in some marine algae, and actually comes in three similar structures; c1, c2, and c3.

Chlorophyll f absorbs in the near infrared (NIR, not visible but close to red) range. Discovered in 2010 as the major chlorophyll in stromatolites of Australia, it is the first new chlorophyll identified in the last 60 years. However, its usefulness in photosynthesis has not yet been confirmed.

Chlorophyll d, on the other hand, is found to be the primary chlorophyll in cyanobacteria. A recent study showed that this chlorophyll absorbs NIR light as well. Though lower energy than red light, but the 2012 paper shows that the cyanobacteria are just as efficient at photosynthesis as plants with chlorophyll a. This works out well since in water, the higher energy wavelengths are absorbed near the surface and the only light that penetrates to the cyanobacteria is the NIR.

This is important for the science of astrobiology, predicting what life might look like on other planets and trying to identify which planets might hold life. Knowing that low energy light can still power photosynthesis tells us that we should not discount the planets around red dwarf stars. These stars have light of different wavelengths than our sun. Autotrophs from planets around red dwarfs may use NIR chlorophylls exclusively; therefore they might reflect all light and appear almost white.

On the other hand, light from different stars might drive evolution of different chlorophylls, so plants on other planets might not be green at all, but could reflect just lower energy light and appear red, or reflect just higher energy waves and be blue – blue plants, cool!

Current possible habitable exoplanets have been numbered and are

under investigation. Scientists look for planets in the habitable zone,

meaning they are of a temperature to have liquid water. They also look

for rocky planets that are about the same size as Earth to provide the

same amount of gravity. They also look for planets around stars with the

same kind of light as our sun – maybe they shouldn’t limit it to stars like

ours. Those with “Kepler” in their name come from the orbiting Kepler

telescope, which is now in danger of never working again.

Based on the light reflected from exoplanets (planets outside our solar system), a 2007 study in the journal, Astrobiology, says we might be able to predict the color of their possible plants and the wavelengths they might use. Furthermore, a study in 2012 stated that in binary systems that have two stars, each giving off different wavelengths of light, might force the evolution of dual photosynthetic mechanisms, leading to perhaps alternating plant colors, depending on which sun is shining.

Chlorophylls provide energy through photosynthesis, but they also have a cost. The old saying, “It takes money to make money” applies to plants as well. It takes energy to make chlorophyll, so it only pays to make chlorophyll when there is ample sunlight to put through photosynthesis. When the daylight get shorter on Earth, the profit margin for producing chlorophyll goes down, so the plant just stops making it.

This is when we start to see the other pigments, those that might play a role on other planets. Other major pigments are the yellow, orange or red carotenoids and the flavonoids. When the plant reduces chlorophyll production, the green color is then a lower percentage of the total pigment in the leaf and the other colors can show through. This gives the bright colors of fall foliage.

But these same pigments can make it seem that a green plant is a non-green plant. Plants that produce large amounts of purple, brown, or maroon pigments have leaves that are so dark that they appear black. Purple, black, and red plants have chlorophyll aplenty, it’s just that the color is masked by other pigments.

Carotenoids are a diverse group of pigments, but yellows and oranges seem to predominate. Carrots get their color from carotene, one type of carotenoid. Xanthophyll is another, which reflects yellow light wavelengths. While chlorophylls absorb red and blue light, carotenoids absorb the blue wavelengths, as well as green light, reflecting only the lower energy yellow, orange, and red light.

Retinal is the major pigment used in our vision. Transduction

of light energy into chemical energy and a nerve impulse is

powered by a cis- to trans- conversion of part of the molecule.

Is it any wonder that this ability to capture light energy can

also be applied to photosynthesis.

By absorbing the green light that would usually be bounced back from chlorophyll, they can prevent us from seeing them as green. Additionally, non-green plant pigments can contribute to photosynthesis, serving as accessory pigments to chlorophyll.

Carotenoids absorb light energy, and while they can’t convert this directly to chemical energy through photosynthesis on Earth, they can transfer this energy to chlorophyll, which then carries it through photosytems I and II of photosynthesis.

In addition, some archaea use retinal (another pigment) to extract energy from the green wavelengths of light. So, why aren’t plants truly black? Wouldn’t it be most efficient to absorb all wavelengths of light for photosynthesis and reflect nothing, thereby appear black to us. Wouldn’t this be the most efficient use of the sun’s energy?

The answer is easy – evolution doesn’t work to maximum efficiency. Natural selection is random and works with what it is given – nothing in nature is engineered by decision to maximize efficiency. But that doesn’t mean there can’t be black plants around other stars, having undergone completely different evolutionary paths.

Even if something used carotenoids, retinal, xanthins and chlorophylls, could it extract energy by absorbing all light waves that strike the plant? Um, no. No plant comes close to absorbing all the light that it can use, and no plant is made of only pigment molecules. There will always be reflections from other molecules.

Plus, if all light was absorbed, can you imagine how hot the plant would get? Imagine a blacktop parking lot being alive; you can fry an egg on an asphalt surface during the summer!

Purple heart (left) and black pepper pearl (right) have lots of pigments

that make them colored purple and almost black. However, they have

chlorophyll too, it is just masked by the other colors. They do

photosynthesis just like other plants, but they certainly look

interesting.

Carotenoids are longer lived than chlorophyll. When autumn comes around, the plant breaks down chlorophyll so that the components can be reused, but the carotenoids stick around much longer. Therefore, the yellows and oranges are not masked by the greens, and the leaves change colors.

Anthocyanins of the flavonoid class are another set of plant pigments. These colors are also more stable than chlorophylls. Our King Crimson Maple makes a lot of red anthocyanin pigments that absorb the green light coming in to the leaf and perhaps a lot of the green light reflected by the chlorophyll. Therefore, as the amount of the anthocyanins in a leaf increases, the green color is masked by the red.  

Plants can use anthocyanins as “sunscreen” because in addition to absorbing green light, they also absorb ultraviolet light. Even though plants and animals need oxygen, they can also be damaged by the production of oxygen radicals (highly reactive compounds) produced by ultraviolet light energy striking oxygen-containing molecules and breaking them apart. Ultraviolet light can especially damage DNA, so anthocyanins can protect cells from mutations that might lead to inefficient activity or even cancer. It might be that on other planets, anthocyanins could be photosynthetic and plants live on UV light.

Sunscreen protects our skin from damage, just as red pigments protect the plant leaves. Even more, eating plants high in anthocyanins, like red grapes, blackberries, and blueberries, can transfer those antioxidant molecules to us for protection of our tissues and blood…. but don’t eat your Norway Maple.

On the left is our King Crimson. The yellow arrow shows the darker leaves

that get more sunshine. The green arrow shows the shaded leaves that

make much less red pigment because they don’t need the protection. On

the right is the burning bush in the open, so it has more carotenoids that

show up in the Fall.

When fall comes, or it is time for the fruits to ripen, plants start to produce even more anthocyanins (as in green apples turning red), because as other compounds in the plant breakdown more oxygen radicals will be produced. Therefore, the plant needs more protection.

Returning to our maple and our fire bush, it would seem that the maple leaves are dark red, almost purple, because of the high anthocyanin pigment concentration relative to the chlorophyll concentration (red + green = almost purple). But not all of them are purple (see picture). Other examples of this, the purple heart plant and the oxalis regnelli, remain purple all through their growing cycle. 

Our burning bush is deep red in autumn because it is not shaded at all, so it produces more anthocyanin to protect its leaves in the summer. If it were shaded part of the time, it might be more pink. If the leaves need protection, they make more anthocyanin, and if not, they don’t.  Don't ask me about shade on other planets.

Next week, your Fourth of July ice cream may have a side effect - ever wonder how "brain freeze" works?



Behrendt, L., Schrameyer, V., Qvortrup, K., Lundin, L., Sorensen, S., Larkum, A., & Kuhl, M. (2012). Biofilm Growth and Near-Infrared Radiation-Driven Photosynthesis of the Chlorophyll d-Containing Cyanobacterium Acaryochloris marina Applied and Environmental Microbiology, 78 (11), 3896-3904 DOI: 10.1128/AEM.00397-12

O'Malley-James, J., Raven, J., Cockell, C., & Greaves, J. (2012). Life and Light: Exotic Photosynthesis in Binary and Multiple-Star Systems Astrobiology, 12 (2), 115-124 DOI: 10.1089/ast.2011.0678  

Kiang, N., Segura, A., Tinetti, G., Govindjee, ., Blankenship, R., Cohen, M., Siefert, J., Crisp, D., & Meadows, V. (2007). Spectral Signatures of Photosynthesis. II. Coevolution with Other Stars And The Atmosphere on Extrasolar Worlds Astrobiology, 7 (1), 252-274 DOI: 10.1089/ast.2006.0108

When Amazing isn’t Enough- Sea Slug Hybrids, part 2

As you undoubtedly remember, last time we talked about a fascinating exception in biology, an animal that can perform photosynthesis. The sea slug, Eylsia chlorotica, eats algae and places the intact, functional chloroplasts in its tissues by a process called kleptoplasty. From that point on, the animal can turn light and CO₂ into carbohydrates – it no longer needs to eat. You might also recall that I hinted that the mere ability to perform photosynthesis isn’t the most amazing thing about this animal. So let us jump in right there.

The average life span of our sea slug of interest is ten months. Not enough time to read War and Peace, but forever compared to the mere 24 hours allotted to the mayfly. So E. chlorotica has roughly a year to make hay while the sun shines. However, the life span of the proteins that are needed for photosynthesis is much shorter.

RuBisCO, a complex protein of photosynthesis

The single most abundant protein on earth is called RuBisCO (Ribulose-1,5-bisphosphate carboxylase oxygenase). This protein adds carbon from CO₂ to the growing carbohydrate during photosynthesis, and has a turnover rate of about 5 days. This is an abnormally long life time for a protein. Chlorophyll can have a turnover rate of a mere 10 hours in some plants. Proteins get old fast, they start to work poorly or just stop working altogether. This is especially true for proteins that work in photosynthesis, since light can damage the very proteins that harness its energy.

Scientists, under laboratory conditions, have kept E. chlorotica alive for 14 months using just water and sunlight. The take home message is that there is active photosynthesis in sea slugs for months and months, when the proteins that make photosynthesis work may need replacing in just a few hours. It makes one wonder how E. chlorotica maintains active chloroplasts for so long.

Science has considered three main possibilities, but there might be more. First, there is something unique about the V. litorea (the algae E. chlorotica eats) photosynthetic proteins that makes them extremely long-lived. This is a tenable possibility, as a few plants have chlorophyll that might never be replaced. But even with immortal chlorophyll, these plants have hundreds of other photosynthetic proteins that must be constantly replaced. So this idea must take a back seat.

Second, there might be something unique about E. chlorotica that keeps the proteins from degrading. This would be amazing, since the sea slug’s own proteins degrade just as in other animals and are replaced regularly. Again, not the strongest hypothesis. Third, E. chlorotica has managed to find a way to make photosynthetic proteins. Intriguing possibility, isn’t it?

An animal that makes RuBisCO or chlorophyll takes the idea of a plant/animal hybrid to a whole new level. It isn’t just the ability to selectively save chloroplasts from digestion and then make use of them. It would be as if the sea slug bought an old motor (the chloroplasts) and but produces replacement parts by itself. But to make the replacement parts, the instructions must be there, and this means DNA.

For our sea slug to have the proper DNA, the plant genes must be consumed, avoid digestion, and be transported to the animal cell nucleus. What is more, the genes must be incorporated into the animal's chromosomes. This is a tall order.

Chloroplasts do have some of their own DNA, since they used to be their own organism (remember endosymbiosis?), but biologists know that many of the hundreds of photosynthesis genes have been transferred to the plant nucleus and are no longer housed in the chloroplast. Therefore, just maintaining functional chloroplasts is not sufficient to produce the proteins needed to keep them active.

Perhaps the slug retains the algae nucleus after feeding. This would provide all the genes needed to produce the proteins needed for photosynthesis, as long as the animal cell can reach and read the plant DNA. Since the chloroplast is not digested, perhaps neither is the nucleus. This would be a good idea, except that scientists have starved E. chlorotica for months, and then searched the slug for plant nuclei. They haven’t found any, so it is probable that the nuclei aren’t retained.

This leaves us with the possibility that the plant genes needed for photosynthesis have been donated by the algae and added to the animal’s cell chromosomes. Don’t laugh, this happens all the time in bacteria. It is called lateral (or horizontal) gene transfer, and it can account for things like antibiotic resistance and sex change in gut bacteria (yes, bacteria can change sex). Even viruses can help accomplish horizontal gene transfer. Viruses can insert their own DNA into the infected cell’s DNA and when they cut themselves back out, they may bring more than they put in. The next infected cell is then the recipient of DNA it may not have had previously.

In vertical transmission, all DNA in the offspring

comes from the parent. In horizontal gene transfer,

the movement is between two different organisms

of the same generation; the recipient cell now has

DNA it did not have before.

Lateral gene transfer can also occur in eukaryotes, but it is usually at the primitive end of the scale. The transfer of some chloroplast and mitochondrial genes to the nucleus millions of years ago is an example of horizontal gene transfer. Horizontal gene transfer with passage of the new genes to the next generation is easy in bacteria or lower eukaryotes because they don’t reproduce through sex. In fungi, even though some progeny are produced by mating, the DNA transferred to the progeny is still the same DNA that was laterally transferred.

Sex on the other hand, means sex cells. The DNA in sex cells (gametes) is the only DNA that gets passed on to the progeny (you get half your DNA from Mom’s egg and half from Dad’s sperm). For DNA to be passed on through horizontal gene transfer, the new DNA must be transferred into either an egg or sperm, and that has to be the particular egg or sperm that participates in fertilization. This is especially difficult to imagine for E. chlorotica, as the algae is eaten, and the chloroplasts are put into the gut cells. Nothing about this leads to algae nuclear DNA getting anywhere near the sex cells. It doesn’t seem very likely - but this is exactly what happens.

Pea aphids have incorporated fungal genes to

help them blend in to their surroundings.

Scientists have found several photosynthesis-specific genes in both mature E. chlorotica that have been starved for algae for months and in immature veligers that have never fed on algae. This can only mean that the genes have been passed vertically, from parent to child, and this means that the plant DNA has entered the gamete cells. The only similar instance I can think of is the transfer of a fungal carotenoid (pigment) gene to pea aphids (ant cows, a neat story on their own) that changed the aphid’s color to match their environment, giving them a camouflage advantage. This is itself a biological exception, the only instance of an animal that produces carotenoid pigment.

Lets summarize. We have an animal that can do photosynthesis – amazing. This same animal has taken up DNA from algae, and has incorporated the new genes into its germ line cells so that they are passed on to its offspring – more amazing. Next time, we’ll talk about how one of the greatest ideas of science might be run aground by a sea slug. Could it be that a discarded version of evolution might be correct?

For more information on horizontal gene transfer, as well as web-based activities and experiments, go to:


http://www.psrast.org/hrtrintr.htm
www.genomenewsnetwork.org/articles/05_01/Lateral_gene_lit.shtml
http://amrls.cvm.msu.edu/microbiology/molecular-basis-for-antimicrobial-resistance/acquired-resistance/acquisition-of-antimicrobial-resistance-via-horizontal-gene-transfer
http://mbio.asm.org/content/2/1/e00005-11.full