One Orchid Two Colors

Bumblebees are no dummies. Far from being mindless drones whose sole purpose it to benefit the colony, these industrious insects are quite capable of learning and memory. They are constantly evaluating their foraging strategies and are quick to abandon a food source that doesn't deliver. For plants that rely on bumblebees, this presents a particular challenge. 

Of course, plants want to maximize their reproductive effort while at the same time minimizing their energy investments. For this reason, some plant species have foregone any sort of reward. Nectar is costly to produce after all. This non-rewarding strategy is particularly widespread among the orchids. Take for instance the case of the elder-flowered orchid (Dactylorhiza sambucina) of Europe. A species of meadows and alpine grasslands, it prefers calcarious conditions. What is most stunning about this species are its floral displays. 

Its inflorescence is made up of a dense cluster of flowers. Unlike what we are used to with most flowering plants, the flowers of the elder-flowered orchid come in two distinct color morphs - purple and yellow. They are so drastically different that one could be excused for thinking they were two different species. What's more, the different color morphs cooccur throughout the species' range. What could be causing this dimorphism? The answer lies in the flowers themselves. 

The edler-flowered orchid is one of those non-rewarding species. It has no nectar and its pollen is bunched up in sacs called pollinia that bees can't really harvest. The main pollinators of this species are bumblebees. As I have hinted, bumblebees are all about optimizing their foraging efforts. They quickly learn which plants are worth visiting and which plants are not. They do this via a highly tuned search image. Any plant that doesn't give them what they want will soon be shunned. 

This is where having different colored flowers comes in handy. Researchers have discovered that the color ratios of any given orchid population are under what is referred to as "negative frequency-dependent selection." Here's how it works: naive bumblebees that visit a non-rewarding flower of one color (purple in this example) are then much more likely to visit a flower of a different color (yellow). It just so happens that the plant with a different flower color (yellow) often turns out to be the same species of orchid. 

The result of this behavior is that in any given population, the plants with the rarer flower color (yellow) get visited more often. Because flower color is under genetic control, that particular morph (yellow) will gradually rise in frequency. Once it becomes the dominant flower color, the reverse happens and the first color (purple) is then visited more often. 

Over time this causes back and forth shifts in flower color that eventually settles on some sort of stable ratio of purple to yellow flowers. Thus anyone botanizing a high-elevation meadow in Europe can find purple and yellow flowered orchids in the same population. By tapping into the bees' natural foraging tendencies, this non-rewarding orchid species is able to maintain its presence in the landscape without having to invest valuable energy into floral rewards. 

Photo Credit: Emilio (http://bit.ly/22CHigV)

Further Reading:
http://www.pnas.org/content/98/11/6253.full.pdf

The Sexual Ruse of the Bee Orchids

There are flowers out there that offer rewards far more enticing than any amount of pollen or nectar - they offer sex. Sexual deception is a rather tricky way of achieving pollination. By duping sex-crazed male insects into thinking they have found a female rather than a flower, such plants have tapped into an irresistible force of nature. Nowhere is this more beautifully illustrated than the orchids belonging to the genus Ophrys

Collectively referred to as the bee orchids, Ophrys grow native throughout much of Europe, North Africa, the Canary Islands, and parts of the Middle East. Phylogenetically speaking, they are a bit of a mess. Estimates of the number of species range from as few as 20 to as many as 130. The range of variation in floral color is staggering and has everything to do with the evolution of this genus. 

The reason they are called bee orchids is because that is exactly what they have entered into an evolutionary syndrome with. And what an evolutionary relationship it is! The bee orchids have evolved to trick male bees into thinking their flowers are receptive females. 

The most obvious aspect of this ruse is their appearance. Though there is quite a lot of variation, the overall theme is that the labellum acts as a female dummy complete with hairy abdomens and, in some species, iridescent wing marks. The ruse does not end there. Far more convincing than their appearance is the odor released by each flower. 

Ophrys produce chemical compounds called "allomones." These allomones closely mimic the pheromones released by female bees. What's more, each species of bee orchid produces allomones specific to the species of bee they are trying to attract. For some this can be very specific, attracting males of only a single species. For others it would seem that a small handful of different species have fallen for the orchid's trick. 

Regardless, male bees find these flowers irresistible at first, often preferring flowers to actual females. However, the males soon learn to avoid flowers, which results in consistently low pollination rates. This doesn't seem to be much of an issue for these orchids as a single plant can produce tens of thousands of seeds. 

This pollination syndrome has obviously worked for this genus. Slight mutations on the allomones produced have led to the massive radiation of Ophrys species we see today. Even more amazing is that research suggests that most of this radiation has occurred since the Pleistocene. 

Photo Credit: http://bit.ly/23pJWe0

Further Reading: [1] [2] [3] [4]

A Curious Case of Gerbil Pollination

The hedgehog lily a.k.a. Massonia depressa hails from arid regions of South Africa. As with most plants from this region, it is highly adapted to its semi-desert environment. Its bizarre yet beautiful appearance belies something peculiar - a pollination syndrome that may surprise you. The hedgehog lily is pollinated by desert rodents such as gerbils.

Let's back up a second though. The genus Massonia has a certain level of confusion hanging around. For starters, despite its common name, it is not a lily at all. It was originally placed in the family Hyacinthaceae but now resides in the family Asparagaceae.

During the hot summer months this plant goes dormant, retreating underground in the form of a bulb. Come winter, two broad leaves are produced that lay flat on the ground. The positioning of the leaves may serve a few different purposes for the hedgehog lily. For starters, leaves laying flat on the ground may help the plant avoid herbivory. It may also help reduce water loss from both the underside of the leaves as well as from the soil surrounding its roots. Finally, it may also play a role in temperature regulation. Many different plant families in this region seem to have converged on a similar strategy.

Now let's get back to the flowers. Winter is also the flowering season. A stunning inflorescence is borne between the leaves. The cream colored flowers lay flush with the ground and are quite stiff. What is most peculiar about these blooms is that they emit a yeasty odor. All of these are adaptations for attracting its pollinators - rodents.

A study published in 2001 showed that when rodents were excluded from the flowers, seed set was highly reduced. Throughout the study, the authors noted four different species of rodents visiting the flowers at night. Two of these rodents were gerbils. Another adaptation for rodent pollination, albeit a subtle one, is extremely viscous nectar. The nectar of the hedgehog lily is 400 times more viscous than other nectar solutions with a similar sugar content.

This allows the rodents to effectively lap up the nectar, thus enticing them to visit the flowers more often. The authors also found that these rodents are often covered in hedgehog lily pollen throughout the blooming season. It coats their fur and makes up a large portion of their feces. Though this is not the only plant species to utilize rodents as pollinators, this is nonetheless a rare pollination syndrome.

Photo Credits: Graham Duncan, Kirstenbosch Botanical Garden and Steven D. Johnson, Anton Pauw, and Jeremy Midgley

Further Reading: [1] [2]

An Orchid With Body Odor

Aside from ourselves, mosquitoes may be humanity's largest threat. For many species of mosquito, females require blood to produce eggs. As such, they voraciously seek out animals and in doing so can spread deadly diseases. They do this by homing in on the chemicals such as CO2 and other compounds given off by animals. What is less commonly known about mosquitoes is that blood isn't their only food source. Males and females alike seek out nectar as source of carbohydrates.

Though mosquitoes visit flowers on a regular basis, they are pretty poor pollinators. However, some plants have managed to hone in on the mosquito as a pollinator. It should be no surprise that some orchids utilize this strategy. Despite knowledge of this relationship, it has been largely unknown exactly how these plants lure mosquitoes to their flowers. Recent work on one orchid, Platanthera obtusata, has revealed a very intriguing strategy to attract their mosquito pollinators.

This orchid produces human body odor. Though it is undetectable to the human nose, it seems to work for mosquitoes. Researchers at the University of Washington were able to isolate the scent compounds and found that they elicited electrical activity in the mosquitoes antennae. Though more work needs to be done to verify that these compounds do indeed attract mosquitoes in the wild, it nonetheless hints at one of the most unique ruses in the floral world.

Photo Credit: Kiley Riffell and Jacob W. Frank

Further Reading:

http://bit.ly/1JXP2jk

Shhhh... Let Him Finish

Sexual deception is rampant in the orchid family. Orchid genera all over the world produce flowers that trick sexually charged male insects into failed mating attempts. The orchids go to great lengths to resemble females both in appearance and smell. Indeed, many sexually deceptive orchid species emit odors that precisely mimic the pheromones of specific insect species. 

In many instances, the orchids ruse is so powerful that male insects will often preferentially visit the flower over an actual female. For many of the sexually deceptive orchids, all that is required is the male to pay a visit. No attempt at copulation is necessary, though that doesn't stop vigorous attempts. Because of this, it is easy to see how the minute cost incurred to the insects is not enough to drive evolution away from deception. However, there is a group of tongue orchids (genus Cryptostylis) from Australia that seem to throw a wrench into this finely tuned system.... or do they?

The tongue orchids rely on deceiving male wasps in the genus Lissopimpla into mating with their flowers. As mentioned above, the males simply cannot resist the attempt. However, unlike many other reported cases, the male wasps actually mate to completion, depositing their sperm onto the flower. This should be disastrous for the wasps since males not only prefer flowers to wasp females, but they also waste their precious few mating attempts. How could this have evolved?

Most sexually deceptive orchids rely on bees and wasps (family Hymenoptera) for their pollination. Thus, the answer to this evolutionary conundrum lies in the mating system of these insects. Queens are genetically haplodiploid. I will spare you the details on that but basically what it means for Hymenoptera is that female offspring are produced via fertilized eggs whereas male offspring are produced via unfertilized eggs. 

The orchids have (unknowingly of course) tapped into this system to their benefit. If by mating with the flower and not a female wasp meant that no offspring were produced, this system surely would not have evolved to the level that it has. Instead, female wasps that have not been mated with or received less sperm than usual end up producing a higher amount of male offspring.

The orchids are effectively skewing the sex ratio of their pollinators! "How is this a sustainable system?" you may be asking. Well, by causing female wasps to produce more males, the orchids are ensuring that there will be more naive males in the population the next time they are in bloom. Also, by skewing the sex ratio towards males, there are now fewer females to mate with so that males become less choosy and more readily mate with orchids. Finally, with more sexually charged males flying around, each female has a greater chance of being fertilized. Because of the unique mating system that has evolved in Hymenoptera, the orchids have thus been able to evolve this pollination strategy with little harm to the pollinators.

Photo Credit: photobitz

Further Reading:
http://instructional1.calstatela.edu/kfisher2/BIOL360/classroom.activities/species_interact._casestudies/orchid.sex.pseudo.II.pdf

American Witch Hazel

With October nearly over, temperatures are starting to dip. The asters and goldenrods have traded their floral displays for their wind-dispersed seeds that take advantage of the fall breeze. Alas, floral displays in the northern hemisphere are nearly over. There is one major show left for those living in eastern North America. From October through November (and even into December in some regions) one species of understory shrub puts forth a display reminiscent of a firework extravaganza if the fireworks only came in yellow.

I am, of course, talking about American witch hazel (Hamamelis virginiana). This wonderful shade-loving shrub goes largely unnoticed throughout the summer. Come fall, however, it makes up for its subtle appearance by offering up some of the last flowers of the season. Seemingly overnight their branches become adorned with unique little flowers whose petals shoot out like four little party streamers. They somehow manage to look both modest and showy all at once.

It may seem strange for any plant to be flowering so late. What possible advantage could this entail? Some experts believe that late flowering evolved as a way for American witch hazel to avoid competition with other flowering plants. Indeed, it certainly attracts its fair share of pollinators in desperate search of a late season meal. Flies and bees make up a majority of pollinator visits. It could also be possible that American witch hazel flowers so late to avoid hybridizing with its spring-flowering cousin, the Ozark witch hazel (Hamamelis vernalis). Regardless of its "intentions," this fall flowering strategy comes at a cost.

Despite garnishing a fair amount of pollinator attention, American witch hazel doesn't have enough time following pollination to produce fruit before winter hits. As such, fertilization of the ovaries is delayed until May the following year. The fruits, which are contained in woody capsules, spend the entire growing season maturing into viable propagules. Once mature, the seed capsules begin to dry until they become so taught that the capsule bursts. If you are lucky and attentive enough, you may be able to hear a small snap as the seeds are forcibly ejected from the capsule.

What's more, fruit set in this species is rather low. Analyses of over 40,000 witch hazel flowers showed that less than 1% produced viable seeds. Despite all of this, American witch hazel is nonetheless a successful species in eastern North American forests. It is proof that evolution need not be all or nothing. Any slight advantage is still an advantage. This hardy shrub is, at the end of the day, a survivor.

Further Reading:
http://www.amjbot.org/content/89/1/67.abstract

Is it a Fungus? Is it a Forb? No, it's a Tree!

Botanical gardens are winter sanctuaries for a northerner like myself. Winter tree ID can only do so much for me during these times. As such, I try my best to make regular trips to tropical houses wherever and whenever I can. On a recent excursion to the Missouri Botanical Garden, I came across something completely unexpected.

I was perusing their tropical house aptly named "The Climatron." As I rounded a corner I happened to look down and saw what looked like something only a member of the birthwort family (Aristolochiaceae) could produce. There, lying near the ground were a cluster of some of the coolest flowers I have personally laid eyes on.

I began searching for the plant that produced them. Up until this point, I have only encountered members of this family in the form of low-lying understory herbs and scrambling vines dangling from the canopy. There were no apparent leaves associated with these flowers and the part of my brain responsible for search images became confused. I traced the flower stems to their place of origin and realized they were attached to the nearest trunk. I followed the trunk upwards and realized that what I had found was in fact a small tree!

The species I was looking at was none other than Aristolochia arborea, a small tree native to the tropical forests of Central America. Needless to say I was floored. There is something to be said about any plant family than can vary this much in size and habit. The coolest aspect about this tree is that, similar to the more herbaceous members of this family, the flowers are produced close to or directly on the forest floor.

A closer inspection of these strange blooms reveals an interesting morphology. It would appear that they are mimicking fungi in the genus Marasimus. Now this could simply be a manifestation of apophenia. Was I seeing patterns where there are none? Of course, this was a job for scientific literature.

It seems I may have been on to something. Botanists agree that in the wild this plant is pollinated by fungus gnats and flies. However, no direct observations of this have ever been made. That being said, the flowers do emit a rather musty smell that could very well be described as "fungal." Regardless, this is an excellent choice of tree to showcase in a botanical garden because stumbling into it like I did led me down an curious path of discovery.

Tree photo credit: Cymothoa exigua (Wikimedia Commons)

Further Reading: [1] [2]

Mighty Mighty Squash Bees

It's decorative gourd season, ladies and gentlemen. If you are anything like me then you should be reveling in the tastes, smells, and overall pleasing aesthetics of the fruit of the family Cucurbitaceae. If so, then you must pay your respects to a hard working bee that is responsible for the sexual efforts of these vining plants. I'm not talking about the honeybee, no no. I am talking about the squash bees. 

If we're being technical, the squash bees are comprised of two genera, Peponapis and Xenoglossa. They are not the hive forming bees we generally think of. Instead, these bees are solitary in nature. After mating (which usually occurs inside squash flowers) the females will dig a tunnel into the ground. Inside that tunnel she places balls of squash pollen upon which she will lay an egg. The larvae consume the protein-rich pollen as they develop. 

The story of squash bees and Cucurbitaceae is a North American story. Long before squash was domesticated, these bees were busy pollinating their wild relatives. As a result, this bee/plant relationship is quite strong. Female squash bees absolutely rely on squash flowers for the pollen and nectar needs of their offspring. In fact, they often dig their brood tunnels directly beneath the plants. 

Because of this long standing evolutionary relationship, squash bees are the best pollinators of this plant family. The flowers open in the morning just as the squash bees are at their most active. Also, because they are so specific to squash, the squash bees ensure that pollen from one squash flower will make it to another squash flower instead of an unrelated plant species. Honeybees can't hold a candle to these native bees. What's more, crowds of eager honeybees may even chase off the solitary squash bees. For these reasons, it is often recommended that squash farmers forgo purchasing honeybee hives for their crops. If left up to nature, the squash bees will do what they are evolutionarily made to do. 

Photo Credit: MJI Photos (https://www.flickr.com/photos/capturingwonder/4962652272/)

Further Reading:
http://www.researchgate.net/profile/Victor_Parra-Tabla2/publication/226134213_Importance_of_Conserving_Alternative_Pollinators_Assessing_the_Pollination_Efficiency_of_the_Squash_Bee_Peponapis_limitaris_in_Cucurbita_moschata_(Cucurbitaceae)/links/549471010cf20f487d2a95b8.pdf

http://www.jstor.org/stable/25084168?seq=1#page_scan_tab_contents

http://extension.psu.edu/plants/sustainable/news/2011/jan-2011/1-squash-bees

Blue

Blue is a strange color. This may seem like an odd statement yet, when you think about it, so few things in nature are truly blue. It is estimated that, of all the colors plants utilize to attract pollinators, blue only occurs in less than 10% of species. This isn't a pattern restricted to plants either. Blue is an infrequent occurrence throughout the biological world.

When it does appear, the color blue is usually the result of structure rather than pigment. The feathers of a bluejay, the wings of a morpho butterfly, and the sheen of a beetle's elytra - these blues owe their vibrancy to refracted light, not pigment. Without light, the crystalline cells responsible for the blue hue would appear dull brown. As light enters their structure, it is bent in a way that gives off blue wavelengths.

Plants have adopted this strategy as well. The berries of Pollia condensata use a similar crystalline structure that results in blue. However, there are true blue flowers out there. How have species with blue flowers managed to overcome the rarity of blue pigments?

The simple answer is that they haven't. There are no blue pigments in the floral world. Instead, plants utilize what can only be described as an evolutionary hack. Blue flowers obtain their color by doing something we all did in art class, blending pigments (similar to the one true black flower http://bit.ly/1FDmU8l). By producing varying amounts of anthocyanins (the pigments responsible for reds) floral cells are able to make blue flowers.

The anthocyanins can also be tweaked to appear blue. One way of doing this is through changes in pH. Blue petunias, for example, have a defect in the proton pumps found inside their flower cells. This causes the cells to become more basic than acidic, which manifests in blue, rather than purple, flowers.

Despite the lack of blue in the floral world, it nonetheless seems to work well when it comes to pollinators. I watched multiple different species of bee visit the flowers of this downy gentian (Gentiana puberulenta). Hummingbirds often visit the amazing floral display produced by the great blue lobelias (Lobelia siphilitica) in my garden. Anyone that has looked over a patch of blue lupine or delphiniums can attest to the success of this color.

Further Reading:
http://www.sciencedirect.com/…/article/pii/S2211124713007547

The Forgotten Zingirberales

Let's dedicate this morning to remembering the forgotten Zingiberales in the family Lowiaceae. This largely overlooked family contains one genus - Orchidantha. Their namesake comes from the uncanny resemblance their flowers have to those of orchids. One species is so easily mistaken that taxonomists named it Orchidantha maxillarioides, which means "Orchid flower that looks like a Maxillaria."

Not much is known about this group. They hail from southern China to Borneo and some species have evolved a pollination syndrome with dung beetles. Aside from that, this family is wide open for investigation as well as more respect!

Photo Credits: Tom Ballinger (https://www.flickr.com/photos/polylepis/) and Scholtz, C. H., Davis, A. L. V. and Kryger, U.

Further Reading:
http://www.amjbot.org/content/86/1/56.full

True Black

Black seems to go with everything. It is a sleek and powerful color. This is a fact not lost on many plant breeders. Much work has been done to produce flower varieties that exhibit pure black coloration. This is not an easy task as true black is not a pigment that plants have readily available to utilize. By breeding plants with increased amounts of anthocyanin pigments in their petals, breeders have been able to produce some varieties that are either so red or so violet that, to us, they appear jet black. It was long thought that all black flowers simply did not exist in the wild but a gentian from Central America blows that assumption out of the water.

Meet Lisianthius nigrescens. Often referred to as the “Flor de Muerto,” this striking gentian produces the most extraordinarily jet black flowers known in the plant kingdom. Indeed, researchers have looked at the pigments responsible for the black coloration and found that they do in fact absorb all wavelengths in the visible spectrum of light. Thus these flowers are truly black.

It was suspected that, with their long, tubular shape, they must attract animals like hummingbirds for pollination and therefore must emit light high in UV wavelengths. Nope. Other pigments in the flowers also absorb all UV light. These flowers are about as black as it gets! Field observations actually found that bees are the primary pollinators of this species. Whats more, the flowers are virtually scentless. How exactly this plant attracts pollinators remains a mystery, a fact I much enjoy about the natural world.

Photo Credit: Lauren Zarate and the Biodiversity Heritage Library http://www.projectnoah.org/spottings/37532042

Further Reading: http://www.znaturforsch.com/ac/v59c/s59c0625.pdf