Showing posts with label pollination. Show all posts

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, February 17, 2016

Sunrise, Sunset – Life In the Twilight

Biology concepts – activity patterns, crepuscular, co-evolution, active pollination

Nocturnal and diurnal activity patterns are like vanilla and chocolate cupcakes. But what if you like rose hip or green tea flavors – are there cupcakes out there for you?

Who knew lizards like cupcakes. I bet they just lick

off the icing.

In a word – yes. In fact, there are so many organisms that are neither nocturnal nor diurnal that I hesitate to call them exceptions – like how everyone has mocha cupcakes now. And I bet you know some of animals with extraordinary activity…… ever heard of sweat bees or deer or infants?

Diurnal animals have developed color vision and ways to deal with the heat and the sun. Nocturnal animals have sensitive vision and other adaptations to make use of the dark. But there are some animals that are active on the edges of both situations; dawn and dusk. What adaptations would help an animal succeed in this niche?

In general, crepuscular (latin for twilight) animals have vision most like nocturnal animals. A tapetum lucidum (Form Follows Function) is present behind the retinas of many crepuscular mammals. Your cat is crepuscular, although she will adapt to a diurnal pattern as a pet…..if she feels like it, you know how cats are.

Those great light shows put on by nature in the evenings

have a name – crepuscular rays. Impressive trivia

for your next party. Photo by van049.

There are advantages to crepuscularness (I just now invented that word). By curtailing activity during the heat of the day, less energy is spent conserving water. Not surprisingly, many desert species are crepuscular. Heat, on the other hand, doesn’t seem to be as much of an issue, since there are endothermic as well as ectothermic species that are active in these time frames, for instance desert lizards like the gila monster.

The dim light available at dawn and dusk is also an advantage for crepuscular animals. There may be enough light to see, but not enough to make these animals stick out like a sore thumb. This works for deer; along with their coloring, the dim light helps them blend into the background. Deer caught out during the day become very stressed and confused. They may end up playing in traffic; just a case of clouded judgment due to sunshine.

The aim of the crepuscular pattern is often to reduce the chance of being eaten. Most terrestrial predators are diurnal or nocturnal (except for several cat species), so crepuscular animals are active after diurnal predators have had their warm milk, and before nocturnal animals drink their coffee.

Chimney swifts perch on vertical surfaces and

have saliva that dries like glue and is practically

insoluble. They use it to build nests on chimney

walls. They have weak claws and can’t perch on

branches, they can perch on vertical surfaces

using their stiff tail feathers, but mostly they just

fly 16-18 hours each day.

Slightly more common are the crepuscular birds, including the American woodcock, which is a ground bird that eats worms and nests in brushy young forests. The chimney swift is also crepuscular, but it nests in chimneys and other vertical surfaces, eats insects out of the air, and can maintain flight for an entire year. These are birds with very different behaviors, diets, and ranges, but are both crepuscular. As is the rule in nature - maybe the only rule without an exception - it is impossible to predict the behavior of one species based on characteristics similar to other species.

In the plant/pollinator part of the community, some crepuscular pollinators have developed special relationships with plants that flower in the evening only. This represents a special form of crepuscularness (there’s that new word again, I think it will catch on) called vespertinal (vesper = evening in latin) activity. These plants and insects are active only in the evening, and often co-evolve mutualistic relationships.

In the desert where the Joshua tree lives, water is at a premium, and the heat doesn’t help the water situation. Remember in our discussion of nastic movements (Plants that Don’t Sleep) we saw that turgor pressure of water is responsible for the opening of the flowers. But open flowers promote water evaporation! Therefore, the best strategy for the Joshua tree is to have its flowers open outside the heat of the day. Et voila - it is vespertinal.

Joshua trees are native to the Mojave desert. They

were named by the Mormon settlers who were

reminded of Joshua raising his arms in prayer.

Predictions are that 90% of the trees could be wiped

out by global warming by the year 2100.

The price of water also has also driven the Joshua tree to produce no nectar – it must have some other way to attract the yucca moth. It is the yucca moth who really taken this upon its (her) shoulders. She has found a way to make pollinating the Joshua tree flowers pay off for her species. But only the yucca moth has made this connection, and this makes their relationship an exception to a biological rule.

Since they are available to one another in the same part of the day (evening) it is more likely that vespertine plants might have a single pollinator, which we learned a few weeks ago (The Perils Of Plant Monogamy) is the exception to the rule of multiple pollinators.

The female yucca moth is not drawn to the flowers by nectar, but by the need to propagate her species. At one flower, the female moth gathers pollen and balls it up into a large mass. Palps (appendages like arms but located near the mouth) hold the pollen ball as she travels to another Joshua tree; almost always to another tree. We know that cross-pollination is better than self-pollination (Is It Hot In Here), but the question remains, how does the moth know that?

At a second tree, the yucca moth lays an egg inside the carpal (which houses the ovule and is where the seeds will form once the flower is pollinated), but only in one or two of the many caprals. Then the moth swipes the pollen ball over the stigma (the top of the carpals), ensuring that the seeds will develop.

Most pollinators are passive, they transfer pollen as a result being drawn by some attractor (nectar, odor, color, etc.). Pollen transfer is not the reason for their visits. But yucca moths are an exception to this rule; they are active pollinators. They visit the flowers with the express intent of collecting and transferring pollen. But why spend energy to purposefully pollinate?

In the left image, you can see the palps that the yucca moth uses to gather up a pollen ball. These are modified mouth parts, and mouth parts are modified ancient legs. The middle image shows the yucca moth actively pollinating the flower after it laid its egg inside the carpal. The right image shows the larvae growing inside one ovule tube (the top-left cavity), eating the seeds as food.

The seeds are the payoff. The moth larva eats the seeds of that one carpal while developing. This is symbiotic mutualism, both species benefit from their relationship – food for the moth larva, and sure pollination for the Joshua tree.

But certain precautions must be taken. Production of seeds (and fruit) takes energy. If the flower won’t produce enough seeds to make it worth the energy expenditure, the tree will abort the flower. So, if moths deposit eggs in too many carpals of the same flower, the larvae will eat too many seeds, and the flower will commit suicide. This will kill the larvae as well. To prevent this, the moths emit a chemical scent to indicate that a flower has been visited and pollinated; other moths will move on to flowers that have not been marked as occupied.

The yucca plants and yucca moth are an example of the vespertine lifestyle, but are there organisms that live exclusively on the other edge of the night? Yep.

Several types of bees are active only in the early morning hours, just after sunrise. This type of activity pattern is called matutinal (Matuta, the Roman goddess of dawn). Some flowers open up very early in the morning, and these are the targets of matutinal bees. The morning glory is a good example, although the flowers remain open long after the early bird bees have gone to bed.

This is the false dandelion. Sweat bees and

schinia moths appreciate for giving them

food and helping in reproduction. I appreciate

it for not being a true dandelion– the lawn

care expert’s mortal enemy!

Other matutinal flowers include the plants of the pyrrhopappus family. These are perennial herbs of the American southwest, south central, and southeast grass lands, and include the carolinus species that is called a false dandelion. They flower for two-four days a year, opening at sunrise and closing by 10:00 am on a hot, sunny day.

The matutinal flower moth (schinia mitis) and the sweat bee (Hemihalictus lustrans) have relationships with the pyrrhopappus plants. The bees use them as their exclusive source of pollen, although they must visit other flowers, like the morning glory, for nectar.

The Schinia mitis moth is more dependent on pyrrhopappus than are the sweat bees. Food, shelter, mating, and a place to lay eggs are all supplied by these specific herbs, as well as shelter and a food source for the larvae. The moths mate on the open flowers between 7:00 am and 9:45 am (rain or shine), and the female then lays the eggs deep within flower.

The mitis moth egg is laid deep within

the false dandelion flower for protection.

Its going to cramped quarters for the larva.

The reason for flowers developing a matutinal lifestyle might be similar to those for vespertine or fully crepuscular species, ie, water and energy savings. But the pollinators, especially the bees, seem to have followed suit for other reasons. True sweat bees (many people misidentify them) take advantage of the early morning hours to avoid the lines at the flowers; it is a simple matter of reduced competition. On the other hand, the mitis moth has co-evolved with the flower and become completely dependent upon it. If the flower is open only in the morning, the moth better be ready on time.

Who knew that so many plants and animals had thrown off the yoke that tethered them to either day or night activity, and now work at the edges of both? Next time we will take it even further; some organisms have stopped working on any schedule at all.

Chen Y, & Seybold SJ (2014). Crepuscular flight activity of an invasive insect governed by interacting abiotic factors. PloS one, 9 (8) PMID: 25157977

Rockhill, A., DePerno, C., & Powell, R. (2013). The Effect of Illumination and Time of Day on Movements of Bobcats (Lynx rufus) PLoS ONE, 8 (7) DOI: 10.1371/journal.pone.0069213

For more information, classroom activities, or laboratories on crepuscular activity, yucca moth reproduction, or Schinia mitis moth reproduction, see:

Crepuscular activity –

www.emnrd.state.nm.us/prd/.../5thGradeAnimalAdaptations.ppt

http://www.bio.davidson.edu/people/midorcas/animalphysiology/websites/2010/Pacious/DiurnalNocturnalCrep.htm

http://www.circadian.org/dictionary.html

http://www.enotes.com/topic/Vespertine_%28biology%29

http://www.gleaming.org/crepuscular/

http://yomi.mobi/egate/Crepuscular/a

Yucca and yucca moths –

http://waynesword.palomar.edu/ww0902a.htm

http://indianapublicmedia.org/amomentofscience/yucca-flowers-and-yucca-moths/

http://www.emporia.edu/ksn/v41n2-june1995/KSNVOL41-2.htm

http://www.insectman.us/articles/butterflies-and-moths/yucca/yucca--plant.htm

http://www.desertusa.com/animals/yucca-moth.html

http://www.fs.fed.us/wildflowers/pollinators/pollinator-of-the-month/yucca_moths.shtml

http://bss.sfsu.edu/holzman/courses/Fall99Projects/yucca.htm

http://www.blueplanetbiomes.org/joshua_tree.htm

http://www.willamette.edu/~csmith/SCRP.htm

False dandelions and moths –

http://www.eattheweeds.com/pyrrhopappus-hypochoeris-dandelion-impostors-2/

http://www.cas.vanderbilt.edu/bioimages/species/pyca2.htm

http://www.worldfieldguide.com/wfg-species-detail.php?taxno=18360

http://www.butterfliesandmoths.org/species/Schinia-mitis

http://www.jstor.org/pss/25084173

Wednesday, April 15, 2015

Boy Plants Are From Mars …..

Biology concepts – sexual dimorphism, plants, monoecious, dioecious, pistil, stamen, floral scent, ecology, ecological selection

Charles Darwin missed the boat on linking his sexual

and natural selection in animals to plants as well.

This is odd because he was quite the botanist and

spent many years studying grasses and such with his

son. The parallels between selection in plants and

animals might have strengthened his initial argument,

but there would still be dissent about the descent of man.

Charles Darwin was a smart guy. He had a lot to do with recognizing sexual dimorphism and natural selection and sexual dimorphism to come up with the idea of sexual selection (natural selection based on preference of one sex for certain characteristics in the other) in evolution. But he blew it when it to recognizing sexual dimorphism in plants.

Does it seem weird to talk gender differences in plants? Yes, they do have genders and sex chromosomes, but it's way more complicated in plants than in animals. More complicated probably means more exceptions, but now let’s focus on the main types of reproductive systems in plants and the types of sexual dimorphism they create.

We have talked about how plants come in many varieties-angiosperms, gymnosperms, bryophytes, etc. in previous posts. Many can reproduce both sexually and asexually, but for today, let’s limit ourselves to angiosperms (flowering plants) and sexual reproduction.

More than 90% of flowering plants have flowers with male reproductive organs (stamen) and female reproductive organs (pistil) on the same individual plant. The term in botany for this is monoecy – the plant is monoecious (mono = one and ecious = house).

The anther produces pollen (the male microgametophyte, see this post) and the carpel produces the ovule with the egg cells (female microgametophyte). How could a plant have differences based on sex if both sexes are on the same flower? Can a flower be dimorphic with itself? Well…. no, but it isn’t always that simple.

Carnovali’s painting of Hermaphroditus and Salmacis

from 1856. The nymph became so smitten with him on

first sight that she prayed they would never be parted.

The wish was granted. Note the soft outlines, this was a

radical move in Italy in the 1800’s and got some of his

paintings rejected by the church.

If one flower has both the male and female reproductive structures on it, it is called a perfect flower - I wonder if they have inflated egos. This makes them true hermaphrodites. The term comes from the Greek mythology; Hermaphroditus was the son of the Greek gods Hermes (dad) and Aphrodite (mom). Hermaphroditus (male) was fused with the nymph Salmacis (female) so that the result was a demigod with male and female characteristics.

In science we use the term more strictly; it means one individual that has both and female reproductive organs. This happens to rarely in animals. But in flowering plants it’s the rule rather than the exception.

But in some monoecious plants, the flowers aren’t perfect. Individual flowers will have either male or female reproductive parts. These are called…. you guessed it, imperfect flowers.

So, can an imperfect monoecious plant be sexually dimorphic? On one hand, it is a hermaphroditic individual plant, just like a plant with perfect flowers; both male and female reproductive organs are found in one individual. This would argue that it can’t be sexually dimorphic, just like the perfect plant.

But on the other hand, it has two different types of flowers, and they look different because one has staminate structures (anther + filament) and the other has pistillate structures (pistil = ovary + stigma + style). Because of this, the two types of flowers are different morphs (different shaped versions), and that makes them sexually dimorphic. Does it matter that they’re on the same individual plant? I leave that argument to you.

The top line drawing is a perfect flower, both male and

female. The bottom drawings are of a male (left) and

female flower. These can exist on the same plant –

monoecious imperfect, or on dioecious plants. It’s not

quite this simple, but wait until next week for that.

That takes care of the hermaphrodite majority, but about 6% of angiosperms are what is called dioecious (two houses). They have individuals with male flowers and individuals with female flowers. Now we’re talking sexually dimorphic for sure; different individuals of a species with different characteristics based on sex. Sounds just like the animals we talked about last week.

Sexual dimorphism in plants comes in two main flavors, just like in animals. One is obvious; differences in reproductive organs (primary sex characteristics) will make the flowers look different. The second type is more interesting. You can have difference in characteristics not directly related to reproductive organs (secondary sex characteristics). Dimorphisms could include the shape, color, number, or smell of the flowers, or even differences in the vegetation of the plants. Who knew that plant sexual characteristics could be so complicated?

In general, male flowers are smaller and more numerous than female flowers. Think about it. Males need to spread as much pollen as possible, whereas females spend much more energy to make fruits and seeds. Just like in animals, males make lots of reproductive cells, and females make fewer – so more, smaller flowers in males makes sense.

The male soapwort flowers are on the left. They have

smaller flowers and more variability in coloring. The

female flower is on the right. The tall structures are the

pistil. Compare them to the anthers in the male flowers.

But there can be differences in individual flowers too. A 2014 study showed that in Saponaria officinalis (soapwort), an insect pollinator can discriminate between male flowers and female flowers based on shape and color (males are a little pinker). The most successful male flowers (whose pollen got to female flowers and fertilized them) were just a little different from females, so the insect works to keep the dimorphism low.

Scent can be another dimorphism. Most often, the scent of male and female flowers is very similar; this is so they can attract the same pollinators. But an exception is Phyllanthaceae plants as shown in a 2013 paper. A parasitic moth is the pollinator. It gathers pollen from males, offers it to the female flowers where it lays its eggs. The larvae then eat the seeds - so the females need to be fertilized for this system to work. This means that the beetle really needs to find male flowers first.

Mated female beetles prefer the smell of the male flower, so female beetles being mated drives them to collect pollen by attracting them to the male flowers more than the female ones. Then, when it visits the female flower to lay its eggs, it brings along the pollen to ensure a food source (seeds) for the larvae.

The differences between the sexes can be seen in the plants as a whole too. Longer living dioecious plants often have males that are larger. Male seeds will be heavier and germinate earlier than females. The extra endosperm gives them a chance at establishing themselves and growing larger, and the early germination also gives them a head start on the females. So no wonder they are often bigger.

The quaking Aspen is an exception. It is long lived, but

the females are usually bigger and have more clonal

propagation. And the exception to the exception – Pando,

one of the oldest and largest organisms on Earth is a

clonal population of quaking aspens in Utah –

but they’re male!

The investment females make in fruits and seeds also keeps them smaller; they have to conserve energy for building expensive reproductive structures. A 2010 study showed that in many serotinous species (plants that release seeds after a fire), the females branch less than the males and have fewer but bigger leaves. In studying which individuals release the most seeds after a fire, those that looked least like males did the best. Saving energy by making fewer branches and leaves really does pay off, especially because the females have to invest so much in keeping the cones alive until there’s a fire.

Another example of plant size dimorphism is the Rumex hastatulus from a paper from 2012. This is a wind pollinated and wind dispersed (for seeds) plant. The male grows taller than the females early, so it's taller when the pollen is dispersed (better distance). Then the female grows more and is taller than the males when the wind disperses the seeds (better distance again). Neat how that works out.

On the other hand, plants that live shorter lives usually have females that are bigger. In the perennial plant Silene latifolia, growth and survival are same in male and female until reproduction begins, the females grow bigger and live longer. There is a live fast and die young strategy for the males – their job is done first, a lot like female spiders that eat the males after mating. By providing their mate with a meal (himself), the male spider improves the chance she will lay healthy eggs.

On the left is the male L. xanthoconus plant with flowers.

These provide the Pria beetle in the middle with nectar.

The female plant and flowers on the right are shaped

very differently and provide the beetle with shelter. It

doesn’t mean to pollinate the female, it’s just trying to

get out of the rain.

Ecology plays a role in plant sexual dimorphism as well. The environment and the pollinators can bend plants to their will. The Leucadendron xanthoconus of South Africa is pollinated by a single beetle species (Pria cinerascens) according to a 2005 study. It gets nectar and lays it’s eggs in the male, but seeks shelter from the rain in the differently shaped female flower. It only gets food from the male, and only receives shelter from the female. Even though the female doesn’t offer any nectar – the system works and therefore evolution keeps the males and females from being similar.

The same plant demonstrates another ecologically driven dimorphism. Males that maximize their number of flowers get visited by more beetles, but the investment makes them die sooner. Bond and Maze in 1999 showed that the males spend more on non-photosynthetic flowers (because they aren't green) and that these flowers cover up more of the photosynthetic leaves. The dimorphism is that the female plants live longer.

In many dioecious plants, the sex ratio is nearly 1:1 male:female. That makes sense, unless there are longevity issues induced by one sex or the other (like females dying younger because they put more into making fruits or the example immediately above). However, in some plants the ratio may be way off. Males may be bigger, and they may make less defense toxins, therefore, they may get eaten by herbivores more. This example not withstanding, male bias is more common than female bias.

Biologically, an interesting picture. Stephen Colbert is

about six feet tall, sexually dimorphic than most females

based on height and haircut. The bald eagle has reverse

sexual size dimorphism, the females are usually larger.

And the pistachio tree is dioecious and has a sexually

dimorphic ratio; there is about one male for every

10 females.

On the other end of the scale, dimorphism can end up affecting the environment as well. Spatial segregation of the sexes (SSS) can occur if the resources are spaced differently in the environment and the different sexes need different resources (it happens). This could lead to areas that are mostly male and areas that are mostly female. It’s not a problem unless they get so far apart that the wind or the animal pollinators don’t make it from the males to the females.

A report in 2010 stated that SSS usually works out so the males end up in areas of less resources and females in areas of more. Females need the resources more, and while the less ideal areas mean fewer competitors for the males. That’s the general rule – males are limited by competition, and females are limited by resources – think about that.

Next week, the sexual dimorphism of plants seems strange to us because we don’t really see the difference between males and females (maybe in holly plants). But it does get weirder. Animals have males, females, and hermaphrodites. But plants take it to a whole other level.

Davis, S., Dudle, D., Nawrocki, J., Freestone, L., Konieczny, P., Tobin, M., & Britton, M. (2014). Sexual Dimorphism of Staminate- and Pistillate-Phase Flowers of Saponaria officinalis (Bouncing Bet) Affects Pollinator Behavior and Seed Set PLoS ONE, 9 (4) DOI: 10.1371/journal.pone.0093615

HEMBORG,., & BOND, W. (2005). Different rewards in female and male flowers can explain the evolution of sexual dimorphism in plants Biological Journal of the Linnean Society, 85 (1), 97-109 DOI: 10.1111/j.1095-8312.2005.00477.x

Bonduriansky, R., Maklakov, A., Zajitschek, F., & Brooks, R. (2008). Sexual selection, sexual conflict and the evolution of ageing and life span Functional Ecology, 22 (3), 443-453 DOI: 10.1111/j.1365-2435.2008.01417.x

Pickup, M., & Barrett, S. (2011). Reversal of height dimorphism promotes pollen and seed dispersal in a wind-pollinated dioecious plant Biology Letters, 8 (2), 245-248 DOI: 10.1098/rsbl.2011.0950

Okamoto, T., Kawakita, A., Goto, R., Svensson, G., & Kato, M. (2013). Active pollination favours sexual dimorphism in floral scent Proceedings of the Royal Society B: Biological Sciences, 280 (1772), 20132280-20132280 DOI: 10.1098/rspb.2013.2280

For more information or classroom activities, see:

Monoecious/dioecious –

http://passel.unl.edu/pages/printinformationmodule.php?idinformationmodule=1087230040

http://www.saylorplants.com/SaylorPlants/Ref_Info/Dioecious2w.htm

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CEkQFjAA&url=http%3A%2F%2Foklahoma4h.okstate.edu%2Flitol%2Ffile%2Fothers%2Fenrichment%2F4h500.pdf&ei=aXvKT4TuMIeC2wW1q4HaCw&usg=AFQjCNGyz1D_o5uVX0nMrNa2VpjchuJHgw

http://botany.csdl.tamu.edu/FLORA/Wilson/pp/s99/flowers.htm

http://www.oakleafgardening.com/glossary-terms/hermaphrodite-monoecious-dioecious/

http://www.bushmansfriend.co.nz/dioecious-plants-xidc18308.html

Spatial segregation of sexes –

http://www.jstor.org/discover/10.2307/2461752?sid=21105872536271&uid=2&uid=3739256&uid=70&uid=4&uid=2129&uid=3739664

https://stat.ethz.ch/pipermail/r-sig-geo/2012-May/015047.html

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&ved=0CFUQFjAH&url=http%3A%2F%2Fwww.simonqueenborough.com%2Fassets%2Fpubs%2F2012-Gao-JTFS.pdf&ei=qMseVeWdDYewogSJi4HwDg&usg=AFQjCNEtvuIR1yAJLZBXaKeCAkaPHlOBuw&sig2=EfJdbpE9l_lakRzE4aAosw