Wednesday, July 13, 2016

The Perils of Plant Monogamy

Biology concepts – pollination, single pollinator, co-evolution, co-divergence

What’s bugging her, it’s supposed to be a party!

Imagine the best party of the year – it’s cold outside, but hot inside. The food is great, all your friends are there, everyone is receptive to flirtations by the opposite sex, it lasts for two nights in a row where it is, and then picks up in a new location again and again. Where is it and how do I get invited?

Last week we learned that the Philodendron selloum flower becomes endothermic for a 2 day period each year in order to facilitate its pollination. In this state, it attracts a single species of beetle, which parties down inside the flower and then takes the pollen to the next party - sort of BYOP.

The heating of the P. selloum spadix evaporates and spreads a pheromone that attracts male Cyclocephala beetles. Pollination by beetles (cantharophily) is not one of the most common mechanisms for the spread of pollen to ovules; pollination by bees (melittophily), butterflies (psychophily), or even the wind (anemophily) is more common. In most cases of cantharophily, the flowers are big, white, strong-smelling, and the male flowers are usually eaten but the ovaries are protected. This is exactly the scenario in P. selloum.

When the male beetles enter the flower, the angle of the spadix makes the male flowers available to be eaten, while the covering of the spathe discourages the beetles from leaving the pit. The nectar, pheromone, and flowers help draw the beetles in, but it is really the atmosphere and the company that keep them there.

Female beetles are drawn to the warm temperatures as well, as this affords the beetles the ability to feed and mate through the night, when the ambient temperature outside the flower would require them to slow down their activity (being ectotherms).

The Cyclocephala beetles, but there are other examples.

Orchids are famous for finicky polli beetles follow the

pheromone back to the flower, and after partying, the

flower closes down and kicks them out.

As the party winds down in this first P. selloum, the temperature is reduced and the spathe starts to close down on the spadix, forcing the beetles out – it’s closing time, you don’t have to go home, but you can’t stay here (lyrics by Third Eye Blind). As the beetles leave the flower two things happen, first they pass the viscidium, which coats the beetles with a sticky substance, and then they pass by the pollinia, which covers them with pollen grains.

The next night, a new P. selloum is ready to open its doors for the party. When the previous night’s revelers show up with their coating of pollen, they head directly to the nectar bar at the bottom of the pit, right where the female flowers are located. While getting their first drink of the night, they deposit the pollen on female flowers that reside there and pollinate the plant. The beetles do the work, but their rewards (increased mating time, increased food) are as important to them as the pollen is to P. selloum.

Pollinators can various animals.Non-animal

pollinators work in some cases too,

such as wind and water.

This is one type of pollination party, but not the only one. Most plants invite a variety of different pollinators, or a plant might self-pollinate. In some orchids of colder regions where pollinators are particularly rare, self-pollination can be a last resort. If the flower is not visited by any pollinator, the caudicle (the stalk on which the pollen resides) will shrivel up in a particular shape, dropping the pollen directly on the stigma (containing the eggs).

Most plants invite pollinators of different species. One bee may be a particularly effective pollinator of a particular plant, but that plant is probably also visited by a fly, a butterfly, a bird, a beetle, etc. Few plants have a single pollinator, but P. selloum is one exception. While Cyclocephala beetle may pollinate other plants as well, it is the only species that pollinates P. selloum.

Single pollinators provide advantages to the plant. The need to attract only one species reduces the energy a plant must expend to attract multiple pollinators. Some pollinators are attracted to color, some to different scents, some to different UV patterns, some to different nectars. To draw different pollinators the plant will have to have many attractants, and this costs energy.

The more pollinators a flower depends on, the more energy the plant must spend on attractants. For instance, some flowers use color and UV patterns, as on the left. Some flowers use nectar and visible light patterns, as with the pitcher plant. The red flower is rafflesia, the largest flower in the world. It smells like rotting flesh to attract flies. Some flowers use mimicry, like the bee orchid on the right, it looks like a female bee and attracts males that will try to mate with it.

Another advantage is seen after the pollen is gathered on the pollinator. In order for a pollination to be successful, the pollen must be delivered to the female organs of a plant of the same species. If a pollinator species has developed a relationship with a certain plant (attracted by a specific odor or color, etc.) then the chances are higher that it will visit another plant of the same species after gathering pollen. This increases the chances of cross-pollination.

The dependence of a plant on a specific pollinator amplifies the plant’s vulnerability if there is a decrease in the pollinator numbers. It has no second option for pollination. This is one reason why cross-pollination is preferred to self-pollination. Evolution does not anticipate the future; it proceeds as if the pollinators are present in good numbers. The plant needs the greatest diversity of gene mutations and rearrangements in order to adapt to unanticipated changes in pollinator number, behavior, or preference. This diversity is provided by cross-pollination with another plant, not by reiteration of existing gene patterns by pollination with the plant’s own genome.

The disadvantage of employing a single pollinator is becoming more obvious. Recent years have seen large decreases in wild pollinator populations. Honeybees have experienced colony collapse disorder, and 2011 figures indicate that 10% of American bumblebee species are near extinction. More than 40 species of pollinating insects in the US are endangered, and even more shocking, 1200 vertebrate species of pollinators are termed “at risk.” If this many pollinator species are in decline, even plants pollinated by multiple species might feel the pinch. Can you imagine how many plants that rely on a single species of pollinator might be in danger of extinction?

Many orchids use a single pollinator. Orchids are the most

diverse flower group, as show by the Dracula orchid,

the spectacle orchid, and the small tongue orchid, left to right.

P. selloum is an interesting exception to the rule of multiple pollinators, but there are other examples. For instance, orchids are famous for finicky pollination. There are >25,000 species of orchids, the largest group of plants (in contrast, all birds represent only ~10,000 species), and single pollinators are responsible for the propagation of thousands of them. In South America and South Africa, the number of single pollinator species is quite high, including many orchid species. (Why? I have no idea.)

The specific interactions between the single pollinator and the plant it pollinates are often a result of co-evolution. In technical terms, this means describes the reciprocal natural selection and evolutionary change that occurs between two species by exertion of selective pressure on each other. The two species could be trying to outfox one another, like a parasite and its host, or could be working together, like the pollinator and pollinated.

As the two interacting species interact, they may evolve so that they rely solely on each other for that particular interaction. This could also lead to each species diverging from its closest relatives. This particular type of co-evolution is called co-divergence.

Co-divergent speciation can be seen in the host parasite relationship between the malaria parasite, Plasmodium falciparum, and humans. When humans and chimps diverged (about 4-7 million years ago), some P. falciparum evolved to infect only chimps, while others followed human evolution and became specific for humans.

The Darwin hawk moth wasn’t known when the star orchid

was first described. Charles Darwin just predicted it must exist.

Predicting the existence of a moth with 35 cm tongue didn’t win
Darwin many fans, but he was right.

In similar fashion, there are numerous plants that have co-evolved with a pollinator. The Angraecum sesquipedale orchid (star orchid) is a classic example. Charles Darwin was sent several examples of this flower and described them in an 1862 publication. Darwin noted that the nectar of this flower was located deep within a hollow spur. To reach the nectar, a pollinator would bump into the pollen and it would stick.

However, the tube was so narrow, that no known insect could have been considered a pollinator of this plant. Darwin predicted that an insect with a 30-35 cm proboscis (tongue-like appendage) would be found pollinating A. sesquipedale. He was ridiculed for such a bold proposition, but 40 years later, just such an insect was discovered, the Xanthopan morganii praedicta moth (named for Darwin’s prediction).

Nature is full of exceptions to the rule of multiple pollinators, including snapdragons that need a bee of specific weight to trip the opening mechanism of the flower. Several orchids that use the same single pollinator place the pollen on different parts of the pollinator’s body, so that female flowers of the same species will come into contact with the correct pollen. These are still exceptions, as the vast majority of plants use multiple pollinators – they just aren’t as interesting.

We have seen a plant that can become endothermic in order to pollinate. Next time we will look at a mammal that has gone the other direction, but for the same reason - survival.

Chupp AD, Battaglia LL, Schauber EM, & Sipes SD (2015). Orchid-pollinator interactions and potential vulnerability to biological invasion. AoB PLANTS, 7 PMID: 26286221 Whitehead MR, & Peakall R (2014).

Pollinator specificity drives strong prepollination reproductive isolation in sympatric sexually deceptive orchids. Evolution; international journal of organic evolution, 68 (6), 1561-75 PMID: 24527666

For more information, classroom activities and laboratories on P. selloum, pollinators and co-evolution, see:

P. selloum –

http://www.floridata.com/ref/p/phil_bip.cfm

http://faculty.ucc.edu/biology-ombrello/pow/Philodendron.htm

http://www.exoticrainforest.com/Philodendron%20bipinnatifidum%20pc.html

http://www.pburch.net/plants/C3397731/E20060506081651/

pollinators –

http://www.pbs.org/wgbh/nova/nature/pollination-game.html

http://pollinatorlive.pwnet.org/teacher/lessons.php

http://www.smithsonianeducation.org/educators/lesson_plans/partners_in_pollination/lesson1_main.html

http://www.nebraskawildlife.org/ww08-PollinatorCurriculum.htm

www.mbgnet.net/bioplants/downloads/pollination.pdf

www.nybg.org/files/pollination.pdf

www.ableweb.org/volumes/vol-22/12-puterbaugh.pdf

www.ppdl.purdue.edu/ppdl/expert/Pollinate_Tomato_Pumpkin.html

www.infocusmagazine.org/6.3/env_pollinators.htm

www.pollinator.org/Resources/facts.Gardeners.pdf

co-evolution –

www.pbs.org/americanfieldguide/teachers/insects/insects.pdf

http://www.teachersdomain.org/resource/tdc02.sci.life.evo.lp_speciation/

www.stanford.edu/group/stanfordbirds/text/essays/Coevolution.html

http://www.ucl.ac.uk/~ucbhdjm/courses/b242/Coevol/Coevol.html

http://evolution.berkeley.edu/evosite/evo101/IIIFCoevolution.shtml

http://bioweb.cs.earlham.edu/9-12/evolution/HTML/converge.html

http://www.universityofcalifornia.edu/news/article/4463

http://www.teachersdomain.org/resource/tdc02.sci.life.evo.lp_speciation/

Wednesday, July 6, 2016

Is It Hot In Here Or Is It Just My Philodendron?

Biology concepts – thermoregulation, pollination, tropisms, flower structure, plant communication

In many ways, plants are “smarter” than people (forgive the anthropomorphism). We can change our environment to suit our needs or move to a better environment. But plants can’t flip the light switch, can’t buy a bottle of water to quench their thirst, can’t turn on the air conditioner, and can’t hire a truck and move all their stuff to a better place.

Plants can react to many physical signals. We can sense gravity, but they can

differentiate parts of themselves with gravity – roots grow towards gravity

(positive geotropism) and stems grow away from gravity (negative geotropism).

So what can plants do given these limitations? They can make their own food (photosynthesis) – they’ve got us beat right there. They can turn to face the light (phototropism) or the sun (heliotropism). These abilities were explained by none other than Charles Darwin and his son in an elegant series of experiments in 1880.

Plant stems can grow away from gravity (negative geotropism or gravitism), while their roots grow toward gravity (positive geotropism) or water (hydrotropism). Finally, plants can twist around a wire and hold on (thigmotropism). Pretty talented, wouldn't you say?

But wait, there's more. Plants can also communicate with one another. They alter their biochemistry to become less appealing to predatory insects or microorganisms, and their responses become better with each attack. After they develop a good defense for a particular predator, they will warn nearby members of the same species via dispersed chemicals. The warned plants then generate the best defense the first time they are attacked.

Plants can also recognize kin – and be nice to them. Research shows that plants grow less aggressively when surrounded by seedlings from the same mother plant compared to when surrounded by non-kin competitors. I wish I could get my kids to act that nicely towards one another.

Plants also commune with animals. The acacia tree has an arrangement with the ants that live on it. The tree produces hollow thorns for the ants to live in, and produces food for them to eat. In exchange, the ants protect the plant from predators such as caterpillars by attacking them. The ants will also prune away dead leaves and destroy nearby plants that might compete with their tree for light.

The acacia tree provides hollow thorns for ants to live in; the tree’s wood is so hard

that the ants can’t hollow it out on their own. The acacia wood was once used as nails.

The acacia is related to the mimosa (sensitive plant) we discussed previously.

This is a great arrangement for both ant and tree (symbiotic mutualism), but becomes tricky during pollination. The ants will attack any insect that touches their tree; even potential pollinators. So the acacia produces a chemical at the flower when an insect lands to feed on the nectar; it says, “this guy is O.K., don’t kill him.” Amazing - I can’t get the cats to come when I call them - and I feed them! Maybe saying someone is as dumb as a potted plant isn’t much of an insult.

Plants may be “smart” about temperature as well. They don’t regulate their own heat, and are usually the same temperature as the surrounding environment. Remember from the last post that it takes lots of energy to be an endotherm, so ectothermic plants enjoy great energy savings by adopting room temperature as their own.

A few plants can spike their temperature for a short time, usually to attract pollinators, but they can’t regulate the temperature. It is like setting a fire; it burns at as high a temperature as the fuel will allow, and then goes out.

P. selloum grows in tropical environments, but can

be found as a landscape planting in Georgia, the

Carolinas, and the gulf coast. It can grow 8 meters

(26 ft) tall and the leaves can be 1 meter (3 ft) in width.

Our exception to the rule of plant ectothermy is the philodendron. Many species of this genus can raise their temperature during the period when they produce pollen, and can regulate that temperature over a short period of time (2 days). The species Philodendron selloum (P. selloum, also called Philodendron bipinnatifidum, split leaf philodendron, tree philodendron) has been the most studied and will serve as our model.

P. selloum flowers in a structure called an inflorescence. This consists of a covering spathe and a spadix in the center. The flowers are located on the spadix, with a specific arrangement of male and female flowers, making the philodendron a monoecious plant (male and female on same plant). However, the flowers are incomplete, since each individual flower has only the male (pollen producing stamen) or female (ovule containing pistil) organs.

The flower of P. selloum is about 25 cm (10 in)

tall and the flowers are plain white, as it does

not use color to attract pollinators.

The male flowers are located on the top half of the spadix, while the middle region contains sterile male flowers, and the female flowers are located at the base. This arrangement, with the sterile gap in the middle, decreases the chances that the pollinators will pollinate a female flower on the same plant (self-pollination).

Self-pollination reduces genetic diversity as the offspring are clones of the parent (we will talk more about this next time). Also to help prevent self-pollination, the male flowers produce pollen in the first evening of the anthesis; the time period when the flower is open and fully functional. The female flowers can receive pollen on the second evening.

The spadix can reach and hold temperatures of 45 ˚C (113˚F)

and is most concentrated in the sterile male flowers. The female

flowers don’t produce heat, as this would damage the ovules.

P. selloum raises the temperature of the spadix, specifically the male flowers. The attractant is a female beetle sex pheromone that makes male beetles of a specific species think that potential mates are on this particular flower. To maximize the effect of the pheromone, the increased temperature of the spadix volatilizes the chemical (evaporates it into the air) so it can spread a greater distance. The beetles just follow their nose back to the correct plant!

The heat comes from a special reaction within the plant. Photosynthesis is actually an endergonic (energy consuming) reaction, it eats up heat, leaving the plant cooler. But respiration (creating ATP from the carbohydrates of photosynthesis) is exorgonic (heat releasing). These two processes are basically a wash, so P. selloum needs another way to generate the heat for the spadix.

Moreover, the P. selloum heat production must correlate to the time when the pollen is mature, must be localized to the spadix, and must be regulated. To do this, the philodendron has independently evolved the same trick that human babies use to stay warm!

Babies have a big surface area compared to their volume, so they tend to lose heat rapidly. This is why parents dress babies warmer than they dress themselves. To generate more heat, babies have brown fat (brown adipose tissue or BAT). BAT has more mitochondria than regular adipose (fat) tissue, and the iron in the mitochondria make this fat appear almost brown in color. The increased mitochondrial number helps to generate more heat as the fat is metabolized.

Fat is metabolized to generate heat instead of carbohydrates because it has more energy. Fat carries almost 9 kcal/gm, while carbohydrates contain only 4 kcal/gm. This is also why fat is used to store energy, it would take more than 2.5x the volume to store the same energy if it were all in the form of carbohydrate, especially since carbohydrates are connected with water when stored, while fats are not. Being fat is actually the most compact way to store energy.

Brown adipose tissue (BAT) has a centrally located nucleus and

several small lipid droplets in order to make room for the many

mitchondria. On the right, white fat cells have an offset nucleus

and are completely filled with a single lipid droplet.

To really up the heat ante, the mitochondria have an uncoupling protein (UCP) that disconnects the burning of fat from the generation of ATP. Instead of putting some of the energy into making ATP, all the energy is put toward giving off heat. Since babies aren’t coordinated enough to exercise to increase heat, and shivering isn’t that efficient, this non-shivering thermogenesis (NST) is their way to stay warm.

It was thought that adults didn’t have BAT, but recent studies indicate that most adults have some, and some people have a lot. BAT generation can actually help keep you thin, because the BAT is more readily metabolized –regular fat is a guard against bad times and the body holds on to it tightly, but BAT it is meant to be burned. New research suggests that chronic cold can stimulate BAT development, so forget your winter coat and just freeze your way into that size two.

P. selloum has developed BAT as well, an excellent example of convergent evolution (unrelated organisms develop similar characteristics). Plants use the alternative oxidase protein to uncouple fat metabolism from ATP generation instead of UCP, but the process is nearly the same. Using non-shivering thermogenesis, P. selloum can raise the temperature of the spadix to 104-113˚F and hold it there.

More amazing, P. selloum can somehow sense the ambient temperature and keep the spadix temperature 20-30˚F above that of the environment during that first evening. During the second day, the temperature is held around 80-95˚F, but is not controlled so stringently. The second evening sees a slow, regulated decrease in temperature to ambient by the third morning. It's a complex mechanism, but the payoff is survival of the species.

The whole thing is pretty smart for a plant, or for any organism. Next time, we will investigate the relationship between the pollinator beetle and P. selloum, and how limiting pollination to one species of beetle breaks a rule.

Ito K, & Seymour RS (2005). Expression of uncoupling protein and alternative oxidase depends on lipid or carbohydrate substrates in thermogenic plants. Biology letters, 1 (4), 427-30 PMID: 17148224

Dötterl, S., David, A., Boland, W., Silberbauer-Gottsberger, I., & Gottsberger, G. (2012). Evidence for Behavioral Attractiveness of Methoxylated Aromatics in a Dynastid Scarab Beetle-Pollinated Araceae Journal of Chemical Ecology, 38 (12), 1539-1543 DOI: 10.1007/s10886-012-0210-y

Technorati claim token TTVE88EU8BQT

For more information, classroom activities, or laboratories on tropisms, pollination, plant communication, or P. selloum:

Plant tropism –

http://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropism/tropisms.html

http://www.discoveryeducation.com/teachers/free-lesson-plans/the-importance-of-tropisms.cfm

http://leavingbio.net/Plant%20Responses.htm

http://www.botanical-online.com/lasplantasmovimientosvegetalesangles.htm

http://virtualastronaut.tietronix.com/textonly/act25/text-plants.html

www.the-aps.org/education/k12curric/activities/pdfs/watson.pdf

www.spacegarden.net/downloads/Gravitropism.pdf

www.sas.upenn.edu/~ltlick/Pedagogy.pdf

www.pjteaches.com/PDF/LifeScience/labs/lab13f.pd

http://www.sciencebuddies.org/science-fair-projects/project_ideas/PlantBio_p041.shtml

http://www.hcs.ohio-state.edu/hort/biology/Lab/tropismslab.html

pollination –

http://www.mbgnet.net/bioplants/pollination.html

http://www.fs.fed.us/wildflowers/pollinators/

http://www.brainpop.com/science/cellularlifeandgenetics/pollination/preview.weml

http://www.nbii.gov/portal/server.pt?open=512&objID=222&mode=2&in_hi_userid=2&cached=true

http://www.youtube.com/watch?v=ge3EM8AERV0

http://www.discoveryeducation.com/teachers/free-lesson-plans/plant-pollination.cfm

http://www.smithsonianeducation.org/educators/lesson_plans/partners_in_pollination/lesson1_main.html

http://ag.arizona.edu/pubs/insects/ahb/lsn43.html

www.ableweb.org/volumes/vol-22/12-puterbaugh.pdf

http://userwww.sfsu.edu/~biol240/labs/lab_13pollination/index.html

plant communication –

http://www.sciencedaily.com/releases/2009/06/090619171244.htm

http://www.animalintelligence.org/2007/10/15/does-plant-communication-imply-intelligence/

http://www.euromed.org.uk/plant-communication.html

http://www.csrees.usda.gov/newsroom/lgunews/plants/plant_stress_signal.html

http://www.nytimes.com/2009/12/22/science/22angi.html?em

http://www.plosone.org/article/info:doi/10.1371/journal.pone.0013324

http://indianapublicmedia.org/amomentofscience/plant-communication-allergies-related/

http://www.usask.ca/education/coursework/mcvittiej/resources/livingthings/plants.htm

http://www.ted.com/talks/stefano_mancuso_the_roots_of_plant_intelligence.html

P. selloum –

http://www.floridata.com/ref/p/phil_bip.cfm

http://faculty.ucc.edu/biology-ombrello/pow/Philodendron.htm

http://www.exoticrainforest.com/Philodendron%20bipinnatifidum%20pc.html

http://www.pburch.net/plants/C3397731/E20060506081651/

Wednesday, June 29, 2016

Why Can’t We Just Go With the Flow? – The High Cost of Heating

Biology concepts – endothermy, ectothermy, poikilothermy, thermoregulation

So you are lying in bed, cold and hungry, contemplating living like a monk just so you can have more time to live like a monk (see this post on increasing life span via reduced core temperature and caloric restriction). Your temperature is going down after bedtime and coming back up in the early morning. This implies that you can control and maintain a constant temperature - pretty impressive. But don’t get a big head, most mammals can do it, even your pet hamster.

The hypothalamus is located in the lower center of the

brain. Different parts are involved in sensing and

regulating temperature, but also for blood pressure,

circadian rhythms, and feeling full after eating.

To be able to control your temperature (thermoregulate), you must know what your temperature is in the first place. Mammals have sensors in their skin and organs which relay information about temperature to the hypothalamus of the brain. The neural sensors in skin (peripheral thermoreceptors) sense the temperature just under the surface. This can be quite different from the core temperature. Central thermoreceptors sense the temperature in the brain, bladder and muscles. Your hypothalamus sets your skin thermostat at about 72˚C, so you still feel hot when the ambient temperature is >75˚F even though your core temperature averages 98.6˚F (37˚C).

Heat is constantly being generated by your metabolism (The breaking down and building up of molecules in your cells). Burning ATP to produce work also produces heat as a byproduct, and this goes a long way to keeping our temperature around 98.6˚F. Generating internal heat to maintain a body temperature is called endothermy (endo = within, therm = heat). Mammals are endotherms, and we hold a constant temperature, so we are also homeotherms (homeo= same). However, we have seen that constant temperature is a relative term, since our circadian rhythm cycles our core temperature up and down as the day goes on.

This is an infrared image of the human body.

Temperatures increase from blue to green to

yellow to orange to red. The head is often one

of the warmer parts of the body since the brain

uses so much energy, while the testicles are

housed outside the body to keep them cooler.
image source: http://www.medicalir.com/

In addition, holding a constant temperature doesn’t mean that all parts of the organism are the same temperature. Just like your skin is cooler than your core body temperature, parts of your internal body can be warmer than your average core temperature. During intense exercise, your muscle temperature can go to 107˚F or higher! On the other hand, sperm is damaged by high temperature, so the testicles are usually housed in an external pouch in order to keep their temperature one or two degrees below body temperature.

The higher than average temperature is O.K. for a short while or in a small part of the body, but if it involves too much volume or stays high for too long, then your core temperature can rise to dangerous levels (104˚F). On the other hand, having too low a temperature in any part of the human body can be dangerous. If ice crystals form in the cell, the jagged edges will cut the cell to ribbons and kill it; this is frostbite.

Your body thermoregulates to maintain a healthy temperature range. It finds ways to dissipate heat when the core temperature rises, such as sweating in humans, panting in dogs, or pushing more blood through the large ears of rabbits. If your core temperature is too low, you can generate heat by shivering (small muscle spasms that mean more ATP burned and more heat). Chattering teeth is just a spasm in the buccinator muscles of your jaw. These are pretty big muscles (bigger on some people I know) and can produce enough heat to keep your head warm.

Some endotherms are exquisitely adept at regulating the temperature in different parts of their body, and can save lots of energy through this differential regulation. Ground squirrels in hibernation reduce their abdominal temperature to match ambient temperature down to 0˚C, and some birds can hold a body temperature just one degree above freezing all night. These types of animals are referred to as endothermic poikilotherms (poikilo = varied).

Lizards are ectotherms, so they have temperatures near

ambient. In this image, the ambient temperature was

76.1 ˚F, so the lizard remains near that temperature.

However, the human hand is much warmer, as it has an

internal source of heat.

We are biased toward believing that all animals control their body temperature just because we do, but the vast majority of animals are ectotherms (ecto = outside). They get most of their heat from the environment, and this works for them.

Ectotherms like reptiles and insects will have low activity when it is cool, but absorbing heat by sunning themselves will speed them on their daily errands. This is because the rates of most cellular activities increase with temperature right up to the point of boiling, but low temperatures slow them down greatly. So most animals need an external source of heat to allow them to hunt, protect themselves, or seek shelter.

Some ectotherms, like moths and bees, can have their wing muscles go into spasms in order to generate enough heat for them to take off. If they can raise their temperature in any way (sunning or spasming), they are called ectothermic poikilotherms, although some might call moths and bees partial endotherms, since the source of heat is internal. On the other hand, a few ectotherms like some fish, always have the same temperature as their environment no matter their activity or needs. These animals are referred to as ectothermic homeotherms.

There are many more ectotherms than endotherms in the world because it is a successful strategy for saving energy. It is extremely costly to maintain a high metabolic rate and a constant internal temperature, like running your furnace all winter to stay comfortable – we all know how expensive that can be. An adult human (endotherm) needs 1300-1800 kCal/day to maintain its temperature and activity, while a crocodile (ectotherm) of the same size requires only 60 kCal! Fewer calories needed means less energy expended hunting or foraging which makes surviving times easier when less food is available.

Even though they may be called cold-blooded, don’t assume that ectotherms are always cold. Rimicaris exoculata, an ectothermic shrimp that lives next to hydrothermal vents (undersea volcanoes that spew superheated water), is happy with an internal temperature of 350˚C (662˚F). The water doesn’t boil because it is under so much pressure (for every 33 feet of water, the pressure doubles); otherwise they would be shrimp toast.

On the left is a deep-sea hydrothermal chimney called a black smoker. The temperature in the hot water column is near 700˚F. The right side image is the vent shrimp, Rimicaris exoculata. The bright spots are the dorsal eyespots and are rich in rhodopsin. They glow like cat’s eyes when light is shone on them. In the deep ocean, there is no light, so the

shrimp don’t glow normally.

Also don’t assume that ectotherms are looking for a way to warm up. Some fish are perfectly comfortable in antarctic waters at (-2˚C to -4˚C; the ocean water doesn’t freeze because the salt disrupts crystal formation). For example, Dissostichus mawsoni fish have proteins that help important molecules resist cold damage (heat shock proteins) and to stay functional at low temperatures (chaperonins).

The left image shows Dissostichus mawsoni, the Antarctic toothfish,

swimming under an ice sheet. Up close, we can see the teeth, and

that he isn’t going to win any beauty contests.

D. mawsoni also has an antifreeze protein in its blood that binds to ice crystals and keeps the fish from freezing solid. Now that’s cold-blooded. These notothenioid (notothen = “from the south” in Greek) fish are successful enough in this environment to make up 90% of the fish biomass in the Antarctic.

Unfortunately, the terms warm-blooded and cold-blooded have become popular for all organisms. This is wrong on so many levels. We think of snakes as cold-blooded, but on a hot sunny day, the internal temperature of a snake will be much higher than that of a mammal. And we already talked about birds, endotherms of the highest order, that can allow the temperature of their feet to come within a degree of freezing. Now, which is warm-blooded and which is cold blooded?

And where do these terms leave plants? They don’t have blood – so they can’t be cold-blooded or warm-blooded – but they are ectotherms. Some plants are even poikilotherms- they can generate some heat at certain points in their life cycle. They can’t maintain or regulate it, so they are still ectotherms, but let’s not be prejudiced against them by calling them cold-blooded.

Low and behold, there are exceptions to the rules of body temperature – wouldn’t you know it. There is a plant that can maintain a constant temperature by producing heat – even if it is only for two days a year. And there is a mammal that seems to think ectothermy is the way to go. We’ll talk about these rule-breakers starting next time.

Shillito B, Le Bris N, Hourdez S, Ravaux J, Cottin D, Caprais JC, Jollivet D, & Gaill F (2006). Temperature resistance studies on the deep-sea vent shrimp Mirocaris fortunata. The Journal of experimental biology, 209 (Pt 5), 945-55 PMID: 16481583

Kiss AJ, Mirarefi AY, Ramakrishnan S, Zukoski CF, Devries AL, & Cheng CH (2004). Cold-stable eye lens crystallins of the Antarctic nototheniid toothfish Dissostichus mawsoni Norman. The Journal of experimental biology, 207 (Pt 26), 4633-49 PMID: 15579559

For more information, classroom activities, or laboratories on endothermy, ectothermy, or thermoregulation:

thermoregulation –

http://www.seas.
harvard.edu/courses/es96/spring1997/web_page/health/thermreg.htm

http://www.unm.edu/~lkravitz/Article%20folder/thermoregulation.html

http://www.biog1105-1106.org/demos/105/unit3/bodytempregulation.html

http://dwb.unl.edu/teacher/nsf/c01/c01links/www.science.mcmaster.ca/biology/4s03/thermoregulation.html

http://www.scienceforums.net/topic/37403-thermoregulation-activityexperiment/

http://www.teachersdomain.org/resource/lsps07.sci.life.reg.heatexchange/

endothermy –

http://planet.uwc.ac.za/nisl/biodiversity/loe/page_115.htm

http://www.ucmp.berkeley.edu/diapsids/endothermy.html

http://www.issuesinbiology.com/why-did-endothermy-evolve/

http://mysite.verizon.net/redslime/Lessons/endothermy.html

http://www.elp.manchester.ac.uk/pub_projects/2003/MNZO0MLK/lecture14.htm

http://amoebamike.wordpress.com/tag/endothermy/

http://edhelper.com/ReadingComprehension_54_27.html

ectothermy –

http://reptilis.net/cold-blood.html

http://hsc.csu.edu.au/biology/core/balance/9_2_1/921net.html

http://www.learner.org/jnorth/monarch/spring2011/journal022411.html

http://roberta-goli.suite101.com/what-is-an-ectotherm-a124054

www.ableweb.org/volumes/vol-24/13-hodson.pdf

Wednesday, June 22, 2016

Antibiotics Are Going Viral

Biology concepts – bacterial immunity, bacteriophage, antibiotics

There are recognized characteristics that all living organisms

share. However, they are not black and white; take a look at

these characteristics and think about fire. Fire grows,

consumes energy, gives off energy, metabolizes, reproduces.

So if fire can be fit into some of these, maybe an argument

can be made for viruses?

Many scientists do not consider viruses a form of life, but that doesn’t mean the idea is universal. Even virologists can’t agree. Viruses do blur the lines between life and non-life, and it gives us something to debate when parties get quiet. It makes for a great debate in Biology classes too, if you don't have a party to go to.

For many, it comes down to this - viruses don’t react to changes in their environment, grow, or metabolize, so they can’t be alive. They lack all these characteristics because these processes take energy, and viruses themselves don’t make or consume energy. This is a big sticking point for anyone trying to make an argument for including viruses as life.

But they seem to do O.K. at making their way in the world, and are becoming quite the model for immune stimulation. A 2011 study at the Emory Vaccine Center used virus-sized nanoparticles to try to induce life-long immunity as natural viruses do. It is hypothesized that virus particles bind to several different types of innate immune receptors (called Toll-like receptors, TLRs) and this diverse stimulation by one antigen is responsible for longer immunity.

The nanoparticles were composed of a synthetic polymer particle complexed with two stimulators. One is similar to a part of the bacterial cell wall, and the other mimics viral mRNA. The particles also stimulated several different TLRs in mice, and it is hoped they will do similar in humans. Nice to see we can take advantage of viruses, since they take advantage of us so often. Important to our topic today, viruses can even take advantage of bacteria.

What a great image of a T2 bacteriophage. What

look like layers are …. layers. Each is a protein and

what is more, they self assemble! The head carries

the nucleic acid, the legs attach to the bacterium, and

the shaft creates the hole and injects the nucleic acid.

Since bacteria are prokaryotes, it would be right to assume that the viruses that infect them look and act differently than the viruses that infect eukaryotic cells. They even have a different name – bacteriophages (backtron = small rod, and phage = to feed on). Infection of a bacterium by a virus may seem a trivial event - we have our own problems to deal with. But there are several ways in which this infection affects animals.

Bacteriophages insert their nucleic acid into the bacteria from the outside; the virus doesn’t enter the cell. Similar to the bacteriocin delivery system recently discovered in bacteria, bacteriophage also use a spike system to punch a hole in the target cell. Scientists in Switzerland, Russia, and Indiana collaborated in 2011 to show that the bacteriophage spike has a single iron atom at the tip, and it punches, not drills, a hole in the target bacterium.

Once inside, the nucleic acid can have different fates. In many cases, the phage DNA is inserted into the bacterial chromosome and stays there for a while, not harming anyone, but also not making new virus particles. This is called lysogeny. Lysogens (cells infected with lysogenic phage) will then pass on the prophage (the phage nucleic acid that is integrated) on to their daughter cells.

Other bacteriophages don’t have the patience to just hang out in the bacterial genome; they take over the cell, make many copies of themselves and then destroy the bacterium by lysing it (breaking it open). These are the lytic bacteriophages.

You might recognize that lysogenic phage DNA, just sitting there in the chromosome, would die out with the cell (or daughter), so they must have another side to themselves. These phages can be lysogenic if the environment suits them, or lytic if they have the right signals, and they can switch from lysogenic to lytic if the environment changes, so they are called temperate bacteriophage. Do I have to point out that they can’t go the other direction (lytic to lysogenic); how could you insert yourself into the bacterial genome if you have already caused bacterial destruction?!

There are 19 recognized bacteriophage types (probably

more now). They have different kinds of head proteins, and

some are filamentous. Cystovirus (cytovirus) is the only

virus with RNA for a nucleic acid instead of DNA. Tectivirus

is the only phage that infects both archaea and bacteria.

There are currently 19 different classes of bacteriophage that infect bacteria and archaea. That’s a bunch of different ways that a bacterium would have to defend itself, but it can. Bacteria have several different ways to prevent bacteriophage infection. In some cases, the bacteria will produce cell wall molecules to prevent phage binding or nucleic acid injection.

In other cases, the bacteria will identify its own nucleic acid, usually by adding methyl groups to DNA. In some cases, the bacteria will methylate its own DNA, and then cut up (called restriction, this is where the restriction enzymes used in molecular biology come from) any DNA that isn't methylated. In other cases, the bacteria will methylate the incoming viral DNA and target all methylated DNA for restriction.

Recent evidence show that bacteria even have a version of adaptive immunity. The CRISPR system takes spacer DNA (short repeats outside genes) from the bacteriophage and places them in specific CRISPR spots in its own chromosome. These serve as a memory in case that bacteriophage is encountered again. If it is, the appropriate spacer can be turned in to a piece of RNA that will target the phage DNA for destruction (called RNAi, the “i” stands for interfering, the process for another discussion).

Finally, bacteria can oppose phage by giving up. Like the apoptosis in our cells or the plant hypersensitive reaction we have discussed, bacteria can kill themselves in order to prevent themselves from becoming virus factories. In the case of bacteriophage-infected bacteria, the process is called high frequency of lysogeny. This system prevents the bacterium from carrying the prophage and passing it on to daughter cells by having the cell die before it replicates.

So bacteria infected by phage can defend themselves, but in some cases, they don’t need to. In fact, it may help them out. Consider a lysogenic phage of one type and lytic phage of another type. Which would a bacterium consider living with – certainly not the lytic phage. But many viruses, including phage have mechanisms to prevent superinfection (infection with a second virus); phage of one type cannot survive in a bacterium infected with a phage of another type. If the lysogenic phage got there first, it could actually protect the bacterium from a death by a lytic phage.

Cholera toxin is carried by the CTX bacteriophage.

The phage needs TCP, a type IV pillus to infect the

V. cholerae. Once the bacterium is growing on the

intestinal surface, the phage is activated, reproduces,

infects other bacteria, and the cholera toxin is

produced. So to cause disease, the bacteria must

undergo horizontal gene transfer to gain the pillus,

and be infected by the CTX phage.

We may chuckle at the idea of bacteria getting infected – in many cases it serves them right – but it can also affect us. Certain bacteriophages possess DNA that can make an infected bacterium even better at causing humans distress. The cholera toxin of Vibrio cholerae is carried by the CTX bacteriophage, and the diphtheria toxin gene of Cornybacterium diphtheriae is also transferred from bacterium to bacterium by a phage.

But phage may also be turned from the dark side and used to help mankind. In the spirit of our recent discussions on when it is beneficial to be infected, how about letting your doctor infect you with bacteriophage to kill off your bacterial infection?

It is no secret that antibiotic resistance is becoming a large problem in medicine. If we know that viruses can infect bacteria, why don’t we use them as a type of antibiotic? This may very well be a good idea, but it isn’t a new one.

Before the advent of penicillin and other traditional antibiotics, bacteriophages were used to treat bacterial infections in the Soviet Union and Eastern Europe. However, the 1920-30's trials were not without their flaws, mostly because scientists didn’t have a good idea of how phages worked. For many years the West remained behind, because Soviet research was not widely distributed.

To kill bacteria, lytic phages would be the tools of choice. But there is a downside, we use bacteria to stay alive. You wouldn’t want to kill of your gut flora, you need them to digest food and absorb vitamins. So, bacteriophage must be delivered to the site of the infection only, replicate there but not travel, and kill only the target bacteria. This is a tall order, but trials are in progress for bacteriophage as antibiotics against drug resistant Staphylococcus and others. Bacteriophages are even being tested in bacterially-infected plants.

The SOS system is one way a bacterium can repair

DNA damage. The damage stimulates RecA protein

function. This is an important protein. It works in

many forms of DNA repair, as well as being responsible

for homologous recombination. The SOS repair genes

are controlled by RecA degrading the protein that

represses their production. They go on to fix the DNA

problem.

On another front, research at MIT and Boston University from 2010 suggests that it may be possible to inhibit bacterial antibiotic resistance mechanisms, and once again making the resistant bacteria susceptible to conventional antibiotics. In this case, bacteriophage were engineered to target the bacterial DNA repair system in the target cells. The SOS system (see picture to right) is induced when bacteria are treated with antibiotics, but the bacteriophage-treated cells were more susceptible to the antibiotic. This could prevent resistance from developing, but may also be useful in strains that have developed some other antibiotic resistance mechanism.

Another potential bacteriophage aid to humanity has nothing to do with disease. May 2012 work from the University of California has made use of the mechanical energy of the bacteriophage inside bacteria, turning it into electrical energy (piezoelectricity, piezo = to press or squeeze). While this is a very small amount of power per cell, it is hoped that this may soon be harnessed to run your smart phone and iPad.

Next week we will start a series on heat in Biology.

Browning, C., Shneider, M., Bowman, V., Schwarzer, D., & Leiman, P. (2012). Phage Pierces the Host Cell Membrane with the Iron-Loaded Spike Structure, 20 (2), 326-339 DOI: 10.1016/j.str.2011.12.009

Lu, T., & Collins, J. (2009). Engineered bacteriophage targeting gene networks as adjuvants for antibiotic therapy Proceedings of the National Academy of Sciences, 106 (12), 4629-4634 DOI: 10.1073/pnas.0800442106

For more information or classroom activities, see:

Bacteriophage –

http://jmbe.asm.org/index.php/jmbe/article/view/63/html_33

https://sites.google.com/a/tjc.sg/wow-attachment/home/bacteriophage-class

http://jc-schools.net/ce/virus.html

http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_p029.shtml

http://www.cellsalive.com/phage.htm

http://www.k8science.org/slides/slide01.cfm?tk=42&pg=2

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=16&ved=0CDoQFjAFOAo&url=http%3A%2F%2Fwonderstruck.co.uk%2Ffiles%2FStudent-Resource-Sheet-2.2-Building-Viruses.pdf&ei=yNJlUNGiHMepyAGW4oFw&usg=AFQjCNGbRS00EnRS7kafEbcmHli_2hAabw

www.wsfcs.k12.nc.us/cms/lib/.../3021/Attack_of_the_Viruses.pdf

https://sites.google.com/site/coralandphage/phage-outreach

http://vimeo.com/8313889