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

Aquatic Angiosperm: A Cretaceous Origin?

It would seem that yet another piece of the evolutionary puzzle that are flowering plants has been found. I have discussed the paleontological debate centered around the angiosperm lineage in the past (http://bit.ly/1S6WLkf), and I don't think the recent news will put any of it to rest. However, I do think it serves to expand our limited view into the history of flowering plant evolution.

Meet Montsechia vidalii, an extinct species that offers tantalizing evidence that flowering plants were kicking around some 130–125 million years ago, during the early days of the Cretaceous. It is by no means showy and I myself would have a hard time distinguishing its reproductive structures as flowers yet that is indeed what they are thought to be. Detailed (and I mean detailed) analyses of over 1,000 fossilized specimens reveals that the seeds are enclosed in tissue, a true hallmark of the angiosperm lineage.

On top of this feature, the fossils also offer clues to the kind of habitat Montsechia would have been found in. As it turns out, this was an aquatic species. The flowers, instead of poking above the water, would have remained submerged. An opening at the top of each flower would have allowed pollen to float inside for fertilization. Another interesting feature of Montsechia is that it had no roots. Instead, it likely floated around in shallow water.

This is all very similar to another group of extant aquatic flowering plants in the genus Ceratophyllum (often called hornworts or coon's tail). Based on such morphological evidence, it has been agreed that these two groups represent early stem lineages of the angiosperm tree. Coupled with what we now know about the habitat of Archaefructus (http://bit.ly/1S6WLkf), it is becoming evident that the evolution of flowers may have happened in and around water. This in turn brings up many more questions regarding the selective pressures that led to flowers.

What is even more amazing is that these fossils are by no means recent discoveries. They were part of a collection that was excavated in Spain over 100 years ago. Discoveries like this happen all the time. Someone finds a interesting set of fossils that are then stored away on a dark shelf in the bowels of a museum only to be rediscovered decades or even centuries later.

All in all I think this discovery lends credence to the idea that flowering plants are a bit older than we like to think. Also, one should be wary of anyone claiming to have found "the first flower." The idea that there could be a fossil out there that depicts the first anything is flawed a leads to a lot of confusion. Instead, fossils like these represent snapshots in the continuum that is evolution. Each new discovery reveals a little bit more about the evolution of that lineage. We will never find the first flower but we will continue to refine our understanding of life on this planet.

Photo Credits: Bernard Gomeza, Véronique Daviero-Gomeza, Clément Coiffardb, Carles Martín-Closasc, David L. Dilcherd, and O. Sanisidro,

Further Reading:
http://www.pnas.org/content/112/35/10985.abstract