An Orchid of Hybrid Origin

Hybridization is an often overlooked mechanism for evolution. We are taught in high school that hybrids such as mules and ligers are one-off's, evolutionary dead ends doomed to a life of sterility. Certainly this holds true in many instances. Species separated by great lengths of time and space are simply incompatible. However, there are instances throughout the various kingdoms of life in which hybrids do turn out viable.

If they are different enough from either parent, their creation may lead to speciation down the line. Such events have been found in ferns, butterflies, and even birds. One particular example of a hybrid species only recently came to my attention. While touring the Atlanta Botanical Garden I came across a fenced off bed of plants. Inside the fence were orchids standing about knee height. At the top of each plant was a brilliant spike of orange flowers. "Ah," I exclaimed, "the orange fringed orchid!" The reply I got was unexpected - "Sort of."

What I had stumbled across was neither the orange fringed orchid (Platanthera ciliaris) nor the crested yellow orchid (Platanthera cristata). What I was looking at were a small handful of the globally imperiled Chapman's fringed orchid (Platanthera chapmanii). Though there is some debate about the origins of this species, many believe it to be a naturally occurring hybrid of the other two. In many ways it is a perfect intermediate. Despite its possible hybrid origins, it nonetheless produces viable seed. What's more, it readily hybridizes with both parental species as well as a handful of other Platanthera with which it sometimes shares habitat.

Despite occasionally being found along wet roadside ditches, this species is rapidly losing ground. The wet meadows and pine savannas it prefers are all too quickly being leveled for housing and other forms of development. Although it once ranged from southeast Texas to northern Florida, and southeast Georgia, it has since been reduced to less than 1000 individuals scattered among these three states.

There is a light at the end of the tunnel though. Many efforts are being put forth to protect and conserve this lovely orchid. Greenhouse propagation in places like the Atlanta Botanical Garden are helping supplement wild populations while at the same time, maintaining genetic diversity. New populations have been located in Georgia and are now under protection. Though not out of the woods yet, this species serves as a reminder that a little bit of effort can go a long way.

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

Plant Architecture and Its Evolutionary Implications

I make it a point that during my field season I enjoy my breakfast out on the deck. It is situated about halfway up the canopy of the surrounding forest and offers a unique perspective that is hard to come by elsewhere. Instead of looking up at the trees, I am situated in a way that allows for a better understanding of the overall structure of the forest. Its this perspective that generates a lot of different questions about what it takes to survive in a forested ecosystem, especially as it relates to sessile organisms like plants.

Quite possibly my favorite plants to observe from the deck are the pagoda dogwoods (Cornus alternifolia). Hinted at by its common name, this wonderful small tree takes on a pagoda-like growth form with its stacked, horizontal branching pattern. It is unmistakable against the backdrop of other small trees and shrubs in the mid canopy. The fact that it, as well as many other plant species, can be readily recognized and identified on shape alone will not be lost on most plant enthusiasts.

The fact that diagrams like these exist in tree guides is proof of the utility of this concept.

The fact that diagrams like these exist in tree guides is proof of the utility of this concept.

Even without the proper vocabulary to describe their forms, anyone with a keen search image understands there is a gestalt to most species and that there is more to this than simply fodder for dichotomous keys. The overall form of plants has garnered attention from a variety of disciplines. Such investigations involve fields of study like theoretical and quantitative biology to engineering and biomechanics. It has even been used to understand how life may evolve on other planets. It is a fascinating field of investigation and one worth spending time in the literature. 

Some of the pioneering work on this subject started with two European botanists: Dr. Francis Hallé and Dr. Roelof Oldeman. Together they worked on conceptual models of tree architecture. Using a plethora of empirical studies on whether a tree branches or doesn't, where branches occurs, how shoots extend, how branches differentiate, and whether reproductive structures are terminal or lateral, they were able to reduce the total number of tree forms down to 23 basic architectural models (pictured above). Each model describes the overall pattern with which plants grow, branch, and produce reproductive structures. At the core of these models is the concept of reiteration or the repitition of form in repeatable sub-units. The models themselves were given neutral names that reflect the botanists that provided the groundwork necessary to understand them.  

Despite the fact that these models are based on investigations of tropical tree species, they are largely applicable to all plant types whether they are woody or herbaceous and whether they occur in the temperate zone or the tropics. The models themselves do not represent precise categories but rather points on a spectrum of architectural possibilities. Some plants may be intermediate between two forms or share features of more than one model. It should also be noted that most trees conform to a specific model for only a limited time period during their early years of development. Random or stochastic events throughout a trees life greatly influence its overall structure as it continues to grow. The authors are careful to point out that a trees crown is the result of all the deterministic, opportunistic, and chance events in its lifetime.  

Despite these exceptions, the adherence of most plants to these 23 basic models is quite astounding. Although many of the 23 models are only found in the tropics (likely an artifact of the higher number of species in the tropics than in the temperate zones), they provide accurate reference points for further study. For instance, the restriction of some growth forms to the tropics raises intriguing questions. What is it about tropical habitats that restricts models such as Nozeran's (represented by chocolate - Theobroma cacao) and Aubréville's (represented by the sea almond - Terminalia catappa) to these tropical environments? It likely has to do with the way in which lateral buds develop. In these models, buds develop without a dormancy stage, a characteristic that is not possible in the seasonal climates of the temperate zones. 

Reiteration is an important process in plant architectural development in which plants repeat their basic model. This is especially important in repairing damage. 

Another interesting finding borne from these models is that there doesn't seem to be strong correlations between architecture and phylogeny. Although species within a specific genus often share similar architecture, there are plenty of exceptions. What's more, the same form can occur in unrelated species. For instance, Aubréville's model occurs in at least 19 different families. Similarly, the family Icacinaceae, which contains somewhere between 300 and 400 species, exhibits at least 7 of the different models. Alternatively, some families are architecturally quite simple. For instance the gymnosperms are considered architecturally poor, exhibiting only 4 of the different models. Even large families of flowering plants can be architecturally simplistic. Take the Fabaceae, which is largely comprised of plants exhibiting Troll's model. 

So, at this point the question of what is governing these models becomes apparent. If most plants can be reduced to these growth forms at some point in their life then there must be some aspect of the physical world that has shaped their evolution through time. Additionally, how does plant architecture at the physical level scale up to the level of a forest? Questions such as this are fundamental to our understanding of not only plants as organisms, but the role they play in shaping the world around us. 

Although many scientists have attempted to tackle these sorts of questions, I want to highlight the work of one individual in particular - Dr. Karl Niklas. His work utilizes mathematics to explain plant growth and form in relation to four basic physical constraints:

1) Plants have to capture sunlight and avoid shading their own leaves.

2) Plants have to support themselves structurally.

3) Plants have to conduct water to their various tissues.

4) Plants must be able to reproduce effectively.

Using these basic constraints, Dr. Niklas built a mathematical simulation of plant evolution. His model starts out as a "universe" containing billions of possible plant architectures. The model then assesses each of these forms on how well they are able to grow, survive, and reproduce through time. The model is then allowed to change environmental conditions to assess how these various forms perform and how they evolve. 

An example of Niklas' model showing how simple branching pattern (bottom) can evolve over time into more complex, yet familiar, forms (top).

An example of Niklas' model showing how simple branching pattern (bottom) can evolve over time into more complex, yet familiar, forms (top).

The most remarkable part of this model is that it inevitably produces all sorts of familiar plant forms, such as those we see in lycophytes, ferns, as well as many of the tree architectural models mentioned above. What's more, later iterations of the model do an amazingly accurate job at predicting forest structure dynamics such as self-thinning, mortality, and realistic size/frequency distributions of various species. 

It would appear that the rules governing what we know as a plant are to some degree universal. Because constraints such as light capture and the passive movement of water are firmly grounded in the laws of physics, it makes sense that the successful plant architectures we know and love today (as well as those present through the long history of plant evolution on this planet) are in large part a result of these physical constraints. It also begs the question of what photosynthetic life would look like on other planets. It is likely that if life arose and made its living in a similar way, familiar "plant" architecture could very well exist on other planets.

Listen to my interview with Dr. Karl Niklas here.


Photo Credits: [1] [2] [3]

Further Reading: [1] [2] [3] [4] [5]

A Unique Case of Floral Mimicry

Pollination is one of the major advantages flowering plants have over the rest of the botanical tree. With a few exceptions, flowers have cornered this market. It no doubt has played a significant role in their rise to dominance on the landscape. The importance of flowers is highlighted by the fact that they are costly structures. Because they don't photosynthesize, all plants take a hit on energy reserves when it comes time to flower. Sepals, petals, pollen, nectar, all of these take a lot of energy to produce which is why some plants cheat the system a bit. 

Sexual mimicry is one form of ruse that has evolved repeatedly. The flowers of such tricksters mimic receptive female insects waiting for a mate. The evolution of such a strategy taps into something far deeper in the mind of animals than food. It taps into the need to reproduce and that is one need animals don't readily forego. As such, sexually deceptive flowers usually do away with the production of costly substances such as nectar. They simply don't need it to attract their pollinators. 

By and large, the world of sexual mimicry in plants is one played out mainly by orchids. However, there exists an interesting exception to this rule. A daisy that goes by the scientific name Gorteria diffusa has evolved a sexually deceptive floral strategy of its own. Native to South Africa, this daisy is at home in its Mediterranean climate. It produces stunning orange flowers that very much look like those of a daisy. On certain petals of the ray florets, one will notice peculiar black spots. From region to region there seems to be a lot of variation in the expression of these spots but all are textured thanks to a complex of different cell types. 

The spots may seem like random patterns until the flowers are visited by their pollinator - a tiny bee-fly known scientifically as Megapalpus nitidus. With flies present, one can sort of see a resemblance. This would not be a mistake on the observers part. Indeed, when researchers removed or altered these spots, bee-fly visitation significantly decreased. Although this didn't seem to influence seed production, it nonetheless suggests that those spots are there for the flies. 

When researchers painted spots on to non-textured petals, the bee-flies ignored those as well. It appears that the texture of the spots makes a big difference to visiting flies. What's more, although female flies visited the flowers, a majority of the visits were by males. It appears that the presence of these spots is keying in on the mate-seeking and aggregation behavior of their bee-fly pollinators. Further investigation has revealed that the spots even reflect the same kind of UV light as the flies themselves, making the ruse all the more accurate. This case of sexual mimicry is unique among this family. No other member of the family Asteraceae exhibits such reproductive traits (that we know of). Although it doesn't seem like seed production is pollinator limited, it certainly increases the chance of cross pollination with unrelated individuals.

Photo Credits: [1] [2] [3]

Further Reading: [1] [2]

The Mighty 'Ama'u

We tend to think of ferns as fragile plants, existing in the shaded, humid understories of forests. This could not be farther from the truth. Their lineage arose on this planet some 360 million years ago and has survived countless extinctions. In truth, they exhibit a staggering array of lifestyles, each with its own degree of adaptability. Take the Hawaiian tree fern, Sadleria cyatheoides for example.

Known in Hawai'i as the 'Ama'u, this tree fern is one of the first species to colonize the barren lava flows that make the Big Island so famous. This is an incredibly harsh landscape and many challenges must be overcome in order to persist. This does not seem to be an issue for the 'Ama'u. It is just as much at home in these water-starved habitats as it is in wetter forests. It is easily the most successful species in this genus, having colonized every island in the archipelago.

Much of its success has to due with a part of its life cycle that is much less obvious to us - the gametophyte stage. The tree fern we see is only half of the story. It is the spore-producing phase conveniently referred to as the sporophyte. When a spore finds a suitable site for germination, it grows into the other half of the life cycle, the gametophyte. This minute structure looks like a tiny green heart and it houses the reproductive organs of the plant. When water is present, male gametophytes release their flagellated sperm, which swim around until they find a female gametophyte to fertilize. Once fertilized, the resulting embryos will then grow into a new tree fern and start the cycle anew.

What sets the 'Ama'u apart from its rarer cousins is the fact that its gametophyte appears to be quite capable of both outcrossing and self-fertilization. Outcrossing, of course, promotes genetic diversity, however, the ability to self-fertilize means that a new plant can grow from only a single spore. This is super advantageous when it comes to colonizing new habitats. Its cousins seem to lack this ability to self-fertilize successfully, restricting them to more localized areas. Taken together, I think it's safe to say that the 'Ama'u is one tough cookie. 

Photo Credits: [1] [2]

Further Reading: [1] [2]

 

A Fern Unchanged

Ferns are old. Arising during the late Devonian period, some 360 million years ago, ferns once dominated the land. These ancient ferns were a bit different than the ferns we know today. It wasn't until roughly 145 million years ago, during the late Cretaceous period, that many extant fern families started to appear. However, a recent fossil discovery shows that at least one familiar fern was hanging out with dinosaurs as far back as 180 million years ago!

A team of scientists in Sweden recently unearthed an exquisitely preserve fossil of a fern from some early Jurassic deposits. Usually the fossilization process does not preserve very fine details, especially not at the cellular level, but that is not the case for this fossil. Falling into volcanic hydrothermal brine, the fern quickly mineralized. The speed at which the tissues of the fern were replaced by minerals preserved details that scientists usually only dream about. Clearly visible in the fossilized stem are subcellular structures like nuclei and even chromosomes in various stages of cell division!

 

source

source

Using sophisticated microscopy techniques, the team was able to analyze the properties of the nuclei undergoing division. What they discovered is simply amazing. The number of chromosomes as well as other properties of the DNA matched a fern that is quite common in eastern North America and Asia today. This fossilized fern, as far as the team can tell, is a cinnamon fern (Osmundastrum cinnamomeum). Based on the fossil evidence, cinnamon ferns were not only around during the early Jurassic, they have remained virtually unchanged for 180 million years. Talk about a living fossil!

Further Reading: [1]

Not All Pitchers Are Equal: How Prey Capture Has Driven Speciation in the genus Nepenthes

Species of the genus Nepenthes are as bizarre as they are beautiful. Known the world around for their carnivorous lifestyle, these plants looks like something out of a macabre art exhibit. It is easy to get caught up in this beauty. I often find myself lost in thought while staring at full grown specimen. How did this genus come to be? Why are they so diverse? What is going on with the morphology of these plants?

Nepenthes hail from nutrient poor habitats, which has driven them to supplement their growth with nutrients gained via the breakdown of a variety of organisms. The business ends of a Nepenthes are their pitchers. We get so caught up in the bewildering diversity of shapes, colors, and sizes that we often overlook them as the anatomical marvels of evolution that they truly are. Whereas the main body of these plants often look quite similar among different species, it's the pitchers that really allow us to separate them out as distinct species. Pitcher morphology not only gives us a convenient means to identify these plants, research is now showing that the structure of these pitchers is likely to be the driving force in their evolution. 

Let's back up for a second. Before we get to the subject of adaptive radiation, we should take a closer look at the anatomy of these plants. To put it simply, the pitchers of Nepenthes are actually leaves, albeit highly modified versions. What we readily recognize as the photosynthetic leaves of a Nepenthes plant are actually modified leaf bases or petioles. Over evolutionary time, these bases have flattened to increase the amount of surface area available for photosynthesis.

From the tip of each of these "leaves" is produced a tendril. Gradually this tendril will elongate and the tip starts to swell. This tip will eventually become the pitcher. The pitchers themselves are highly modified leaves. They are some of the most specialized leaves in all of the plant kingdom. As the tip grows larger, it becomes clear that there is a distinctive lid apparatus. Once the pitcher is fully mature, this lid pops open revealing the death trap filled with digestive fluids.

As if producing pitchers wasn't cool enough, each species of Nepenthes produces two distinct forms - lower pitchers, which are produced by young plants as well as on mature plants near the ground, and upper pitchers, which are produced up on the climbing stems as they vine through the canopy. The upper and lower pitchers look radically different from one another to the point that one may easily confuse them for different species. The reason for such stark differences has to do with the type of prey captured. Lower pitchers are generally larger and can capture prey that crawls along the forest floor. Upper pitchers tend to be more slender and most often capture flying insects as well as other creepy crawlies hanging out in the forest canopy.

The key to the success of these traps seems pretty straight forward - insects attracted by bright colors and sweet nectar land on the traps and fall to their death. Certainly this holds true throughout the genus, however, there are at least two major variations on this theme and a handful of bizarre mishmashes. As the lid of a Nepenthes pitcher starts to open, a ring of tissue called the peristome unfurls. The shape and color varies wildly between species and this has to do with the methods in which they capture their prey. These variations are the key to the amazing diversity of Nepenthes we see throughout the range of this genus.

Nepenthes vogelii

Nepenthes vogelii

The first of the three strategies is referred to as the 'insect aquaplaning' strategy. Insects walking around on the peristome of the pitcher find it hard to get a foothold. These are species such as N. raja, N. ampullaria, and N. bicalcarata (just to name a few). The slipperiness of the peristome of these species is further enhanced when humidity is high. Considering how much it rains in these habitats, it is no wonder why capture efficiency is often as high as 80%. Although there is some variation on this theme, pitchers that utilize the insect aquaplaning strategy often lack waxy cells on the interior of the pitcher walls.

Slippery pitcher walls are the second strategy that Nepenthes have converged upon. These are species such as N. diatas, N. mirabilis, and N. alata (again, just to name a few) Insects attracted to the pitchers are often lured in by sweet nectar. Once they cross the lip of the pitcher, prey find it hard to hang on and inevitably fall inside. Once this happens, waxy cells lining the interior walls make it impossible for anything to climb back out. It should be mentioned that a slippery peristome and waxy pitcher walls are not mutually exclusive. That being said, there are clear trends among species that show a reduction in waxy cells as peristome size and slope increases.

This brings us to the oddballs. There are species like N. lowii, whose pitchers function as a toilet bowl for shrews, and N. aristolochioides, whose pitchers seemed to have abandonded both strategies and now function as light traps similar to what we see in Darlingtonia. Regardless of their strategy, the diversity in trapping mechanisms appear to be the driving force behind the bewildering diversity of Nepenthes

Nepenthes   aristolochioides

Nepenthes aristolochioides

All of the evidence taken together shows that prey capture is at the core of this radiation. There seems to be incredibly strong selective pressures that result in strong divergence in pitcher morphology. The disruptive selection that seems to be driving a wedge between the insect aquaplaning strategy and the waxy wall strategy may have its roots in reducing competition. Nutrients are low and competition for food is high. Different Nepenthes species could be evolving to capture different kinds of prey. Even closely related species such as N. ampullaria, N. rafflesiana, N. mirabilis, N. albomarginata, and N. gracilis all seem to occupy their own unique spot on the spectrum of prey capture strategy.

It could also be that Nepenthes are responding to the specific characteristics of the habitats in which they are found. Those inhabiting drier sites may favor the waxy wall strategy whereas those living in wetter habitats tend to favor the slippery peristome. More work needs to be done to investigate where and how these different strategies are maximized. Until then, I think it is safe to say that the diversity of this incredible genus has a lot to do with obtaining food. 

Photo Credits: [1] 

Further Reading:

[1] [2] [3]

 

On Soil and Speciation

Lord Howe Island

Many of you will undoubtedly be familiar with some variation of this evolutionary story: A population of one species becomes geographically isolated from another population of the same species. Over time, these two separate populations gradually evolve in response to environmental pressures in their respective habitats. After enough time has elapsed, gradual genetic changes result in reproductive isolation and eventually the formation of two new species. This is called allopatric speciation and countless examples of this exist in the real world.

At the opposite end of this speciation spectrum is sympatric speciation. Under this scenario, physical isolation does not occur. Instead, through some other form of isolation, perhaps reproductive or phenological, a species gives rise to two new species despite still having contact. Examples of this in nature are far less common but various investigations have shown it is indeed possible. Despite its rarity, examples of sympatric speciation have nonetheless been found and one incredible example has occurred on a small oceanic island off the coast of Australia called Lord Howe Island.

Howea    belmoreana  and  Howea   forsteriana

Lord Howe Island is relatively small, volcanic island that formed approximately 6.4–6.9 million years ago. It is home to four distinct species of palm trees from three different genera, all of which are endemic. Of these four different palms, two species, Howea belmoreana and Howea forsteriana, are quite common. Interestingly enough, H. forsteriana, commonly known as the kentia palm, is one of the most commonly grown houseplants in the entire world. However, their horticultural value is not the most interesting thing about these palms. What is most remarkable is how these two species arose. 

Multiple genetic analyses have reveled that both species originated on Lord Howe Island. This is kind of odd considering how small the island actually is. Both palms can regularly be found growing in the vicinity of one another so the big question here is what exactly drove the evolution of their common ancestor? How does a single species growing on a small, isolated island become two? The answer is quite surprising.

Howea    belmoreana

When researchers took a closer look at the natural histories of these two species, they found that they were in a sense isolated from one another. The isolation is due to major phenological or timing differences in their reproductive efforts. H. forsteriana flowers roughly six weeks before H. belmoreana. Flowering time is certainly enough to drive a wedge between populations but the question that still needed answering was how do such phenological asynchronies occur, especially on an island with a land area less than 12 square kilometers? 

As it turns out, the answer all comes down to soil. Individuals of H. belmoreana are restricted to growing in neutral to acidic soils whereas H. forsteriana seems to prefer to grow in soils rich in calcarenite. These soils have a more basic pH and dominate the low lying areas of the island. Growing in calcarenite soils is stressful as they are poor in nutrients. This physiological stress has caused a shift in the way in which the flowers of H. forsteriana mature. When found growing on richer volcanic soils, the researchers noted that the flowers mature in a way that is more synchronous, not unlike the flowers of H. belmoreana.

Thanks to their attention to detailed life history events and conditions, researchers were able to show that soil preferences caused a phenological shift in the flowering of these two related species. Because they flower at completely different times when growing on their respective soil types, enough reproductive isolation was introduced to disrupt the random mating process of these wind pollinated palms. As soon as such reproductive biases are introduced, speciation can and will occur.

Photo Credits: [1] [2] [3]

Further Reading: [1]

The Power of Leaves

When we think of the dominance of flowering plants on the landscape, we usually invoke the evolution of flowers and seed characteristics like endosperm and fruit. However, evolutionary adaptations in the structure of the angiosperm leaf may have been one of the most critical factors in the massive diversification that elevated them to their dominant position on the landscape today. 

Leaves are the primary organs used in water and gas exchange. They are the centers of photosynthesis, allowing plants to take energy from our closest star and turn it into food. To optimize this system, plants must balance water loss with transpiration in order to maximize their energy gain. This requires a complex plumbing system that can deliver water where it needs to be. It makes sense that plant physiology should maximize vein production, however, there are tradeoffs in doing so. Veins are not only costly to construct, they also displace valuable photosynthetic machinery. 

It appears that this is something that flowering plants do quite well. Because leaves fossilize with magnificent detail, researchers are able to look back in time through 400 million years of leaf evolution. What they found is quite incredible. There appears to be a consistent pattern in the vein densities between flowering and non-flowering plants. The densities found in angiosperm leaves both past and present are orders of magnitude higher than all non-flowering plants. These high densities are unique to flowering plants alone. 

This innovation in leaf physiology allowed flowering plants to maintain transpiration and carbon assimilation rates that are three and four times higher than those of non-flowering plants. This gives them a competitive edge across a multitude of different environments. The evolution of such dense vein structure also had major ramifications on the environment. 

The massive change in transpiration rates among the angiosperm lineage is likely to have completely changed the way water moved through the environment. These effects would be most extreme in tropical regions. Today, transpiration from tropical forests account for 30-50% of precipitation. A lot of this has to do with patterns in the intertropical convergence zone, which ensures that such humid conditions can be maintained. However, in areas outside of this zone such as in the Amazon, a high abundance of flowering plants with their increased rates of transpiration enhances the amount of rainfall and thus forms a sort of positive feedback.

Because precipitation is the single greatest factor in maintaining plant diversity in these regions, increases in rainfall due to angiosperm transpiration effectively helps to maintain such diversity. As angiosperms rose to dominance, this effect would have propagated throughout the ecosystems of the world. Plants really are the ultimate ecosystem engineers. 

Photo Credit: Bourassamr (Wikimedia Commons)

Further Reading: [1]

Mighty Magnolias

Magnolias are one of those trees that even the non-botanically minded among us will easily recognize. They are one of the more popular plant groups grown as ornamentals and their symbolism throughout human history is quite interesting. But, for all this attention, few may realize how special magnolias really are. Did you know they they are one of the most ancient flowering plant lineages in existence?

Magnolias first came on to the scene somewhere around 95 million years ago. Although they are not representative of what the earliest flowering plants may have looked like, they do offer us some interesting insights into the evolution of flowers. To start with, the flower bud is enclosed in bracts (modified leaves) instead of more differentiated sepals. The "petals" themselves are not actually petals but tepals, which are also undifferentiated. The most striking aspect of magnolia flower morphology is in the actual reproductive structures themselves.

Magnolias evolved before there were bees. Because of this, the basic structure that makes them unique was in place long before bees could work as a selective pressure in pollination. Beetles are the real pollinators of magnolia flowers. The flowers have a hardened carpel to avoid damage by their gnawing mandibles as the feed. The beetles are after the protein-rich pollen. Because the beetles are interesting in pollen and pollen alone, the flowers mature in a way that ensures cross pollination. The male parts mature first and offer said pollen. The female parts of the flower are second to mature. They produce no reward for the beetles but are instead believed to mimic the male parts, ensuring that the beetles will spend some time exploring and thus effectively pollinating the flowers.

It is pretty neat to think that you don't necessarily have to track down a dawn redwood or a gingko to see a plant that has survived major extinction events. You can find magnolias very close to home with a keen eye. Looking at one, knowing that this is a piece of biology that has worked for millennia, is quite astounding in my opinion.

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

The World's Only (Known) Photosynthetic Vertebrate

You may be asking yourself right now why I have posted a picture of a salamander this morning. This is a plant blog after all! Well, what I am about to tell you may seem a bit crazy, but I assure you this discovery has opened up some doors that science never really considered a possibility before. The yellow spotted salamander (Ambystoma maculatum) is the first and only (known) photosynthetic vertebrate ever discovered!

That's right. You heard me. A photosynthetic animal. More accurately speaking, it is the embryos of this species that undergo photosynthesis. To understand why this happens we must back up a little bit. Yellow spotted salamanders are a species of mole salamander that can be found in wet areas of eastern North America. They spend most of their adult lives underground, hiding beneath logs and rocks in the forest, feeding on any manner of invertebrates. Once a year (around this time) adult yellow spotted salamanders undertake a massive migration down to the pools where they mate. On the first few warm, rainy nights, thousands of salamanders can be seen trucking their way to vernal pools and ponds to breed. It is an amazing sight to behold.

The thing about yellow spotted salamanders is they will only breed in fishless ponds. Their larvae would be an easy meal for many predatory fish species. The problem that arises out of this breeding strategy is that fishless ponds tend to be very low in oxygen. It has long been known that the eggs of this species form a symbiotic relationship with an algae. The algae produce oxygen for the developing embryo and the embryo feeds the algae via its nitrogen rich waste and CO2. This relationship was always thought to be external, that is until Ryan Kerney of Dalhousie University in Halifax, Nova Scotia discovered that embryos of a certain age actually had algae living within their cells.

They algae don't seem to start off inside the cells though. This may be why this relationship wasn't discovered earlier. Roger Hangarter at Indiana University found that it isn't until parts of the salamander's nervous system begin to develop that the algae move into the embryo and set up shop. The algae then reside near the salamander's mitochondria, which are the powerhouses of the cell. So where are the algae coming from? While more research needs to be done, Karney also discovered the presence of algae in the oviducts of adult female spotted salamanders. It is looking like mother salamanders are actually passing the algae on to their offspring. 

Though this is the first and only instance we know of this sort of photosynthetic relationship in vertebrate animals, this discovery has opened the door for exploring the possibility of other photosynthetic symbionts. It has also allowed scientists a different avenue to explore just how cells recognize and deal with foreign bodies. We live in such an amazing world!

Further Reading: [1] [2] [3]

 

Seed Anchor

Epiphytic plants live out their entire lives on the trunks or branches of trees. Using their roots, they attach themselves tightly to the bark. Spend any amount of time in the tropics and it will become quite clear that such a lifestyle has been very successful for a plethora of different plant families. Still, living on a tree isn't easy. Epiphytic plants must overcome harsh conditions among or near the canopy.

One of the biggest challenges these plants face starts before they even germinate. This is especially true for orchids. Orchid seeds are more like spores than they are seeds. They are so small that thousands could fit inside of a thimble. Upon ripening, the dust-like seeds waft away on the slightest breeze. In order for epiphytic species to germinate and grow, their seeds must somehow anchor themselves in place on a trunk or branch. Inevitably most seeds are doomed to fail. They simply will not land in a suitable location. It stands to reason then that any adaptation that increases their chances of finding the right kind of habitat will be favored. That's where the strange coils on the tip of Chiloschista seeds, a genus of leafless orchids native to southeast Asia, New Guinea, and Australia, come in. For these orchids, this process is aided by some truly unique seed morphology.

Unlike most orchid seeds that are nothing more than a thin sheath surrounding a tiny embryo, the seeds of Chiloschista have additional parts. These "appendages," which are specialized seed coat cells, are tightly wound into coils. Upon contact with water, these coils shoot out like tiny grappling hooks that grab on to moss and bark alike. In doing so, they anchor the seed in place. By securing their hold on the trunk or branch of a tree, the seeds are much more likely to germinate and grow. This is one of the most extreme examples of seed specialization in the orchid family.

Photo Credit: [1] [2]

Further Reading: [1]

Pollination with a Twist

Ensuring that pollen from one flower makes it to another flower of that species is paramount to sexual reproduction in plants. It's one of the main drivers of the diversity in shapes, sizes, and colors we see in flowers across the globe. Sometimes the mechanism isn't so obvious. Take, for instance, the flowers of Impatiens frithii.

The flowers of this Cameroonian endemic have been a bit of a puzzle since its discovery. Like all Impatiens, they have a long nectar spur. However, the spur on I. frithii is uniquely curved. This puzzled botanists because most of the Impatiens in this region are pollinated by sunbirds. The curved spur would appear to make accessing the nectar within quite difficult for a bird. Still, just because we can't imagine it, doesn't mean that it's impossible. Something must pollinate this lovely little epiphyte in one way or another. This is where close observation comes in handy.

Thanks to remote cameras and lots of patience, botanists were able to record pollination events. They quickly realized that sunbirds are indeed the primary pollinator of this species. This was a bit of a surprise given the shape of the flower. However, the way in which the flowers deposit pollen on this birds is what is most remarkable. As it turns out, successful reproduction in I. frithii all comes down to that curved nectar spur. 

When a sunbird probes the flower for nectar, its beak follows the contour of the spur and this causes the entire flower to twist. As it twists, the anthers and stigma make contact with the chin of the bird. This is unlike other Impatiens which deposit the pollen on top of the heads of visiting birds.

Such an adaptation is quite remarkable in many ways. For one, it is elegantly simple. Such a small alteration of floral architecture is all that is required. Second, by placing pollen on the underside of the head, the plant guarantees that only pollen from its species will ever come into contact with the stigma. This is what we call reproductive isolation, which is an important driver in speciation.

Photo Credit: [1]

Further Reading: [1]

Convergent Carnivores

A carnivorous lifestyle has evolved independently in numerous plant lineages. Despite the similarities between genera like Nepenthes, Sarracenia, and Cepholotus they are not closely related. Researchers have wondered how the highly modified leaves of various carnivorous plant species evolved into the insect trapping and digesting organs that we see today. Thanks to a recent article published in Nature, it has been revealed that the mechanisms responsible for carnivory in plants are a case of convergent evolution.

This research all started with the Australian pitcher plant Cepholotus follicularis. More closely related to wood sorrels (Oxalis spp.) than either of the other two pitcher plant families, this species offers a unique window into the genetic controls on pitcher development. Cepholotus produces two different kinds of leaves - normal, photosynthetic leaves and the deadly pitcher leaves that have made it famous the world over.

By observing which genes are activated during the development of these different types of leaves, the research team was able to identify which alleles have been modified. In doing so, they were able to identify genes involved in producing the nectar that attracts their insect prey as well as the genes involved in producing the slippery waxy coating that keeps trapped insects from escaping. But they also found something even more interesting.

Next, the team took a closer look at the digestive fluids produced by Cepholotus as well as many other unrelated carnivorous plant species from around the world. In doing so, the team made a startling discovery. They found that the genes involved in synthesizing the deadly digestive cocktails among these disparate lineages have a similar evolutionary origin.

Although they are unrelated, the ability to digest insects seems to have its origins in defending plants against fungi. You have probably heard someone say that fungi are more similar to animals than they are plants. Well, the polymer that makes up the cell walls of fungi is the same polymer that makes up the exoskeleton of insects - chitin. By comparing the carnivorous plant genes to those of the model plant Arabidopsis, the team found that similar genes became active when plants were exposed to fungal pathogens.

It appears that carnivorous plants around the world have all converged on a system in which genes used to defend themselves against fungal infection have been co-opted to digest insect bodies. Taken together, these results show that the path to carnivory in plants is surprisingly narrow. Evolution doesn't always require the appearance of new alleles but rather a retooling of genes that are already in place. 

Photo Credits: [1] [2]

Further Reading: [1]

 

 

Captive Pollinators

In order to ensure pollination, leafflower trees in the genus Glochidion have entered into an intimate relationship with a small family of moths. Their flowers have become so specialized that no other insect is capable of pollinating them. In return, female moths are provided with an edible place to lay their eggs - the fruit of the tree. One species of leafflower has taken this relationship to the extreme. It holds its pollinators captive. In order to understand this bizarre relationship, we must first take a closer look at this interesting pollination syndrome.

Ecologists refer to this type of pollination syndrome as "brood pollination." In the case of the leafflower trees, pollination is achieved thanks to female moths known commonly as leafflower moths. Gravid female leafflower moths locate the blooms thanks to a special perfume tailored specifically for each species. The females first visit the male flowers where they pick up some pollen. Next they visit the female flowers where they will then deposit the pollen into a special chamber that can only be accessed by the female moths' proboscis.

After pollination, the female leafflower moth will then locate the ovaries of the flower and using a needle-like ovipositor, will deposit eggs within the undeveloped fruits. The larvae within eventually hatch right next to their food source - leafflower seeds. The larvae aren't gluttons. They will only eat one or two of the dozens of seeds developing within the fruit.

Although this may seem wasteful on the part of the plant, it makes a lot of sense from an evolutionary perspective. Essentially it reduces the likelihood that the moths will try to cheat the system. Gluttonous larvae that eat more than one or two seeds will be penalized and, in the long run, fewer seeds could equate to fewer host plants. By tying the reproductive abilities of the moth to the production of fruit, the tree ensures regular pollination.

For most of the leafflower/moth pairs, once the seed meal is over, the larvae chew out of the fruit and fall to the ground to pupate. However, this is not the case for a leafflower known scientifically as Glochidion lanceolarium. A native of southeastern China, it takes this relationship a step further by holding the larvae captive for nearly a year.

Cut open an fruit of this leafflower and there is a chance you might find a fully formed moth waiting patiently inside one of the swollen chambers. Instead of chewing out before it pupates, the moth is held captive within. Only when the fruits mature and split open will the moths be released. This happens just as the new crop of flowers is opening. The tree is literally controlling when its obligate pollinator is available to do its reproductive bidding.

The uniquely intimate nature of this relationship goes beyond simply being interesting. By studying how these two partners interact in relation to the other leafflower/moth partners around the Old World tropics researchers are gaining a better understanding of how such mutualisms evolve.

Photo Credits: [1] [2]

Further Reading: [1]

A Unique Passionflower Endemic to Costa Rica

I love small flowers, especially if they pack in a lot of detail. That's is why this passion flower caught my eye. Meet Passiflora boenderi, a charismatic vine endemic to a small region of Costa Rica. Apparently this species had been sitting around in herbaria for years under a different name. It wasn't until living specimens were observed that botanists realized it is a distinct species.

There is a lot to look at on this species. The flowers themselves are some of the smallest in the genus. They pack in all of the detail of a larger passion flower, just in miniature. The leaves are quite stunning as well. They're bilobed with a tinge of purple and covered in bright, orange-yellow spots. The spots themselves serve an important role in protecting this plant from herbivores.

The genus Passiflora is part of an intense evolutionary arms race with a genus of butterfly known as Heliconius. Their caterpillars feed on the foliage of passion flowers. As such, Passiflora have evolved a variety of means that help them to avoid the attention of gravid female butterflies. The orange spots on the leaves of P. boenderi are one such adaptation and they serve a dual function.

The first is a visual deterrent. Female Heliconius prefer to lay their eggs on caterpillar-free leaves. This makes sense. Why bother laying eggs where there will be ample competition for food. The spots mimic, both in size and shape, the appearance of Heliconius eggs. A female looking for a spot to lay will see these spots and move on to another plant. In addition to the visual mimicry, these spots also secrete nectar. The energy-rich nectar inevitably attracts ants, which viciously defend them as a food source. If a caterpillar (or any other herbivore fore that matter) were to start munching on the leaves, the ants quickly drive them off.

Because of its limited range, P. boebderi is under threat of extinction. Habitat destruction of its lowland habitat for palm oil, pineapples, and vacation resorts is an ongoing threat to the long term survival of this species and many others. I was fortunate enough to have encountered this plant growing in the Cliamtron at the Missouri Botanical Garden but I fear that if we keep on doing what we humans are so good at, botanical gardens may be the only place this species will be found growing in the not too distant future.

Further Reading: [1] [2]

A Shrub and Its Buffer Zone

Confession: I can be really bad at recognizing patterns. It's not my strong point. As such, I tend to remain skeptical of what I think I am seeing. Sometimes I am right, though. A recent stroke of luck came while I was hiking through some scrubby habitat in northern Florida. I walked out into a clearing to get a better view of a pond when I saw some interesting shrubs up on the banks. The area was largely covered in tufts of warm season grasses but the shrubs seemed to be ringed by barren, white sand. My botanist friend was kind enough to point out that the shrubs I was looking at were none other than sandhill rosemary (Ceratiola ericoides). With a name attached to these species, I could dive into the literature on this plant to see if I was actually seeing a true pattern or not.

Before we get to the meat of the article, it would be nice to first introduce you to the sandhill rosemary. C. ericoides is not a true rosemary at all. Its resemblance to the culinary mint is purely superficial. C. ericoides is a heath (family Ericaceae) and is the only member of its genus. It can be found growing in dry, scrubby habitats throughout southeastern North America. It is dioecious meaning each shrub is either male or female. Unlike its showier cousins, this heath is not pollinated by insects. Instead, it relies on wind. Because of this, C. ericoides flowers are highly reduced structures produced in the axils of leaves near the tips of its branches.

The scrubby habitats it calls home are challenging places for plants to live. The sandy soils drain water rapidly and are prone to shifting with the winds. The stout, needle like leaves are a fantastic adaptation for minimizing water loss during the hottest, driest months of the year. Also, regular fires are the norm. Burning is vital to the health of this region, however, C. ericoides does not seem to be very well adapted to cope with it. Instead, these shrubs are killed by fire, relying on the seed bank for regeneration. But that isn't the only thing this species has evolved in order to cope with a regular fire regime. As it turns out, C. ericoides may be utilizing chemical warfare to increase its chances of survival.

C. ericoides is what we call "allelopathic." Allelopathy can be defined as "the direct or indirect harmful or stimulatory effect of one plant on another through the production and release of chemical compounds" and is "an important form of plant-to-plant interference in natural and agricultural settings" (Rice 1984). In other words, allelopathic plants utilize chemicals that are toxic to other plants in order to gain an upper hand when it comes to acquiring space to grow. For C. ericoides, this may also mean keeping itself safe from fire.

As anyone who has tried to light a fire knows, a little fuel goes a long way. In the wild, plant materials make up the fuel load. The more plant material lying around, the more fuel the fire has to burn through. Much of the understory of these habitats are filled with fire adapted plant species. Grasses are possibly the most fire tolerant of them all. What's more, grasses often release specialized compounds when they burn that actually increase the temperature of the fire. This is often bad news for less fire adapted plants in the vicinity. If grasses and other fire adapted species were to be growing near C. ericoides, they would not only increase the chances of fire reaching the shrub but also increase the intensity of the flames. This is where the allelopathy comes in.

A stem full of female flowers  [SOURCE]

A stem full of female flowers [SOURCE]

Evidence from greenhouse experiments has shown that the allelopathic compounds from C. ericoides inhibit the germination and growth of other plant species. This is especially true for fire adapted grasses. Although these chemicals are abundant in the leaves of C. ericoides, research also shows that they are produced in the roots as well. As the leaves fall off, they decompose and release their chemical cocktails into the soil immediately surrounding the shrub. This keeps plants at bay in the immediate vicinity while the roots go a bit further. Since the roots of C. ericoides branch outwards from the plant, the effectiveness of its chemical warfare is increased by a few meters radius around the shrub. Indeed, it is believed that C. ericoides is actually keeping the surrounding area clear of most fire adapted vegetation. In doing so, the shrubs are creating a fire-free buffer zone.

Although more work needs to be done in order to understand the degree to which these effects occur in the wild, this is nonetheless tantalizing evidence that such plant interactions are shaping the landscape in ways we don't fully understand. On a personal note, it was exciting to know that there really was something to the barren ring patterns I observed around each shrub.

Further Reading: [1] [2] [3]

Floral Deterrent

There is no mistaking the allure of certain floral scents. Entire industries have developed around capturing the essence of plant reproduction. Millions of years before we humans adopted floral scents for our own bidding, plants were producing them to attract pollinators. Even plants that produce sickening smells meant to mimic rotting flesh are trying to get the attention of something. It seems so obvious that these perfumes evolved as attractants but there are some plant species whose floral scents do the opposite. 

When I first caught a whiff of the blooms of the fragrant olive (Osmanthus fragrans) it didn't take long to track down its source. In my opinion it is one of the most enticing fragrances I have ever encountered. Native throughout much of Asia, this evergreen shrub produces sprays of tiny, pale, 4 lobed blossoms. Its attractive form and heavenly scent have led to its popularity in horticultural circles the world over. Surely its intense fragrance also makes it quite popular with pollinators. The truth is actually quite surprising.

In its native range, few insects seem to show it any attention. Aside from some hover flies, the list of floral visitors is surprisingly quite depauperate. What is going on? Why would this shrub go through the trouble of producing such volatile compounds? Has its intended pollinator gone extinct? Though it can be difficult to answer such inquiries, research done in Japan suggests that these scents may actually function as a deterrent rather than a lure. 

Researchers looked at a common butterfly species known to pollinate other flowering species in the area. The results of their experiments were quite shocking to say the least. Even when food was withheld for a period of time, the butterflies never visited the flowers of the fragrant olive. Only when the team replaced the flowers with replicas infused with a neutral scent did the butterflies pay them any attention. 

The fact that hover flies do in fact visit the flowers suggests that the scent compounds serve a dual function - attracting the intended pollinators while at the same time deterring potential nectar thieves. More work is needed before we can be completely confident in these conclusions but the idea nonetheless highlights the fact that more is going on with flowers than we realize. 

Photo Credit: hto2008 (http://bit.ly/2hPOhnO

Further Reading: [1]

A Primer on Trigger Plants

I would like to introduce you to another group of plants capable of abrupt movements. Whereas many species have evolved moving parts as a means of capturing prey or deterring herbivores, the following genus moves as means of achieving pollination. Meet the genus Stylidium a.k.a. the trigger plants.

Native to parts of Asia and Australia, these beautiful little herbs are quite diverse, making generalizations difficult. Still, there is one thing they all share, a fused set of reproductive organs that lash out at unsuspecting pollinators. When a visiting insects of sufficient size lands on a flower, its weight causes a rapid change in turgor pressure within the column's tissues.

Stylidium debile     [SOURCE]

Stylidium debile  [SOURCE]

The rapid change in pressure sends the column flying. The position of this reproductive hammer varies from species to species. Some bash their pollinators on the back whereas others strike them under the abdomen. When the flowers first mature, only the male portions are ripe. Thus, the initial visit dusts the insect with pollen. Once the pollen is gone, the column resets itself and the female portions start to mature. The next time an insect visits the bloom, the stigma will do the bashing. With any luck, the visiting insect will have already been dusted with pollen from a previous plant. In this way, the plant avoids self pollination.

Another morphological aspect shared among member of this genus are the production of glandular trichomes. These minute hairs cover the body of the plant and produce sticky mucilage that ensnares tiny insects. It was originally thought that this was a merely a defense mechanism that may represent a form of proto-carnivory.

However, analysis of the mucilage revealed that plant is also producing digestive enzymes capable of breaking down insects unfortunate enough to have been caught. It remains to see whether or not the plants absorb nutrients in the same way as sundews but the fact that these plants share the same nutrient-poor habitats as many other Australian carnivores lends some credibility to asking that question.

Photo Credit: http://bit.ly/2hJjMyc and Francis Nge

Further Reading: [1] [2]

The Ant-Farming Tillandsias

Tillandsias are all the rage. Their relative ease of care has found them included in seemingly every terrarium sold these days; often in very inappropriate circumstances that result in their death. There is no denying that these epiphytic relatives of the pineapple are unique and beautiful plants but I would argue that their ecology is probably the coolest aspect about them. I am particularly fond of the bulbous species because of their relationship with ants.

That's right, there are upwards of 13 species of bulbous Tillandsia that offer up housing for ants. If you look closely at the leaves of these species, you will notice that they roll up to form tubes that lead down into the bulb at the base. The space between the leaves forms a hollow chamber, functioning as a perfect microclimate for ants to nest. In many habitats, these Tillandsia offer better housing than the surrounding environment. One would be surprised at how many ants can fit in there too. Colonies containing anywhere between 100 - 300 ants are not unheard of.

The rewards for the plant are obvious. Ants provide nutrients as well as protection. In return the ants get a relatively safe and dry place to live. Ant domatia have been recorded in roughly 13 different species, many of which are some of the most commonly sold Tillandsias on the market such as T. baileyi, T. balbisiana, T. bulbosa, and T. caput-medusae. If this doesn't make your hanging glass Tillandsia orb even cooler then I don't know what will.

Photo Credits: scott.zona (http://bit.ly/16kZ1RR) and Alex Popovkin (http://bit.ly/1BXMEUH)

Further Reading: [1] [2]

A Cave Dwelling Nettle From China

Caves and plants do not seem like a good combo. Plants need sunlight and caves offer very little to none of it. However, plants in general never seem to read the literature we write about them. As such, they are constantly surprising botanists all over the world. 

A recent example of this was published back in September of 2012. A team of botanists exploring limestone gorges in southwestern China stumbled upon three new members of the nettle family. One of these nettles seemed to be right at home growing well within two limestone caves. 

Needless to say this was quite a shock to the botanists. The regions in which these plants were growing were quite dim, with light levels ranging from a mere 0.04% to a measly 2.78 % of full daylight! Although this is by no means complete darkness, it is an incredibly low amount of sunlight for a plant that still relies on photosynthesis to get by. 

They named the nettle Pilea cavernicola in reference to its cave-dwelling habit. While it has only just been discovered, the IUCN considers this species vulnerable. Only two populations are known and their proximity to expanding human activity puts them in danger of rapid extinction. 

Photo Credit: Monro & Wei

Further Reading: [1]