Floral Invasion

Onam_Flower_Arrangement            Throughout evolution, there have been, time and time again, key biological innovations that have utterly changed history thereafter. Perhaps the most obvious is the one you’re using to read this; the human brain. The development of the anatomically modern human brain has profoundly changed the face of the planet and allowed humans to colonize nearly every part of the globe. But an equally revolutionary innovation from an earlier time stares us in the face each day and goes largely unremarked upon. Flowers. (Stay with me here, guys… ) We think of them as mere window dressing in our lives. Decorations for the kitchen table. But the advent of the flowering plants, or “angiosperms”, has changed the world profoundly, including allowing those magnificent human brains to evolve in the first place.

 

Angiosperm percentage
From: Crepet & Niklas (2009) Am. J. Bot. 96(1):366-381

Having arisen sometime around the late Jurassic to early Cretaceous era (150-190 million years ago), angiosperms come in every form from delicate little herbs to vines and shrubs, to towering rainforest canopy trees. They exist on every continent, including Antarctica, which even humans have failed to develop permanent homes on, and in every type of climate and habitat. They exploded from obscurity to the dominant form of plant life on Earth so fast that Darwin himself called their evolution an “abominable mystery”, and biologists to this day are unable to nail down exactly why they’ve been so incredibly successful. Nearly 90% of all terrestrial plant species alive today are angiosperms. If we measure success by the number of species that exist in a given group, there are two routes by which it can be improved- by increasing the number of distinct species (“speciation”), or by decreasing the rate at which those species go extinct. Let’s take a look at a couple of the features of flowers that have likely made the biggest difference to those metrics.

Picture a world without flowers. The early forests are a sea of green, dominated by ferns, seed ferns, and especially, gymnosperms (that is, conifers and other related groups). Before the angiosperms, reproduction in plants was a game of chance. Accomplished almost exclusively by wind or water, fertilization was haphazard and required large energy inputs to produce huge amounts of spores or pollen grains in order that relatively few would make their way to the desired destination. It was both slow and inefficient.

1280px-Europasaurus_holgeri_Scene_2
The world before flowers. By Gerhard Boeggemann on Wikimedia Commons

The appearance of flowers drew animals into the plant reproduction game as carriers for pollen – not for the first time, as a small number of gymnosperms are known to be insect pollinated – but at a level of control and specificity never before seen. Angiosperms have recruited ants, bees, wasps, butterflies, moths, flies, beetles, birds, and even small mammals such as bats and lemurs to do their business for them. The stunning variety of shapes, sizes, colours, and odours of flowers in the world today have arisen to seduce and retain this range of pollinators. Some plant species are generalists, while others have evolved to attract a single pollinator species, as in the case of bee orchids, or plants using buzz pollination, in which a bumblebee must vibrate the pollen loose with its flight muscles. In return, of course, the pollinators are rewarded with nectar or nutritious excess pollen. Or are at least tricked into thinking they will be. Angiosperms are paying animals to do their reproductive work for them, and thanks to incentivisation, the animals are doing so with gusto. Having a corps of workers whose survival is linked to their successful pollination has allowed the flowering plants to breed and expand their populations and territory quickly, like the invading force they are, and has lowered extinction rates in this group well below that of their competitors. But what happens when you expand into new territory to find that your pollinators don’t exist there? Or members of your own species are simply too few and far between for effective breeding?

Selfing morphology
On the left, a typical outbreeding flower. On the right, a selfing flower of a closely related species. From: Sicard & Lenhard (2011) Annals of Botany 107:1433-1443

Another unique feature that came with flowers is the ability to self-fertilise. “Selfing”, as it’s called, is a boon to the survival of plants in areas where pollinators can be hard to come by, such as very high latitudes or elevations; pollen simply fertilises its own flower or another flower on the same plant. Selfing can also aid sparse populations of plants that are moving into new territories, since another of its species doesn’t need to be nearby for reproductive success. It even saves on energy, since the flower doesn’t have to produce pleasant odours or nectar rewards to attract pollinators. Around half of all angiosperms can self-fertilise, although only 10-15% do so as their primary means of reproduction. Why, you may ask, since it’s such an effective strategy? Well, it’s an effective short term strategy. Because the same genetic material keeps getting reused, essentially, in each successive generation (it is inbreeding, after all), over time the diversity in a population goes down, and harmful mutations creep in that can’t be purged via the genetic mix-and-match that goes on in normal sexual reproduction. Selfing as a sole means of procreation is a slow ticket to extinction, which is why most plants that do it use a dual strategy of outbreeding when possible and inbreeding when necessary. As a short term strategy, however, it can allow a group of new colonists to an area to survive long enough to build up a breeding population and, in cases where that population stays isolated from the original group, eventually develop into a new species of its own. This is how angiosperms got to be practically everywhere… they move into new areas and use special means to survive there until they can turn into something new. I’m greatly simplifying here, of course, and there are additional mechanisms at play, but this starts to give an idea of what an unstoppable force our pretty dinnertable centrepieces really are.

Angiosperms are, above all, adaptable. Their history of utilising all possible avenues to ensure reproductive success is unparalleled. As I mentioned, we have the humble flower to thank for our own existence. Angiosperms are the foundation of the human – and most mammal – diets. Both humans and their livestock are nourished primarily on grasses (wheat, rice, corn, etc.), one of the latest-evolving groups of angiosperms (with tiny, plain flowers that you barely notice and which, just to complicate the point I’m trying to make here, are wind-pollinated). Not to mention that every fruit, and nearly every other type of plant matter you’ve ever eaten also come from angiosperms. They are everywhere. So the next time you buy flowers for that special someone, spare a moment to appreciate this world-changing sexual revolution in the palm of your hand.

Sources

  • Armbruster (2014) AoB Plants 6: plu003
  • Chanderbali et al. (2016) Genetics 202: 1255-1265
  • Crepet & Niklas (2009) American Journal of Botany 96(1): 366-381
  • Endress (2011) Annals of Botany 107: 1465-1489
  • Sicard & Lenhard (2011) Annals of Botany 107: 1433-1443
  • Wright et al. (2013) Proc. Biol. Sci. 280(1760): 20130133

**Top image by Madhutvin on Wikimedia Commons **

Bee_Orchid_(Ophrys_apifera)_(14374841786)_-_cropped
Photo by Bernard Dupont on Wikipedia

Pitcher Plants: Sweet Temptation and the Slippery Slope

(Via: Wikimedia Commons)

Common Name: The Asian Pitcher Plant

A.K.A.: Genus Nepenthes

Vital Stats:

  • Over 130 species in the genus
  • The vast majority of species have extremely narrow ranges of only a single island or small island group, and are considered threatened
  • Most recently discovered (2007) was Nepenthes attenboroughii, named for Sir David Attenborough, who is fond of pitcher plants

Found: Mountainous regions of Southeast Asia, Oceania, and Madagascar

It Does What?!

Plants have evolved a variety of different ways to deal with growing in nutrient-poor soils. Some become parasitic, some develop close symbiotic relationships with bacteria or fungi, and some of them… well, some of them just start eating animals.

Lizard: makes a nice, light snack.
(Via: Wikimedia Commons)

One group of plants that went this route are the Asian pitcher plants (not to be confused with the not-closely-related New World pitcher plants, which tend to have tall, flute-like pitchers). These smallish, climbing plants use highly modified leaves to form what are essentially external stomachs, complete with the plant’s own digestive fluid. These pitchers, which vary in size from one species to the next, have extremely slick, waxy inner walls. When visitors come to eat the nectar produced on the lid (or “operculum”) of the trap, they lose their footing and fall into the liquid below.

That liquid is actually a pretty complex mixture; it’s divided into two phases, like oil and water. The upper portion is mostly rainwater, but has been laced with a compound that makes it more viscous, preventing winged insects from just flying away, as they could from pure water. The trap’s lid actually functions to prevent too much rainwater from getting inside and diluting the fluid too much. The lower portion of the liquid is a digestive acid capable of breaking down flesh into useable molecules (particularly nitrogen and phosphorous), much like our own stomach acid. Analogous to our intestines, the lower inside surface of the pitcher is covered with special glands that absorb suspended nutrients.

Most of what gets caught in pitcher plants is about what you’d expect- winged insects, spiders, beetles, small scorpions. But occasionally, some larger animals find their way in. Things that should have known better, like frogs, lizards, and even birds. Arguably, these plants are doing evolution a favour by taking out any bird dumb enough to fly into its own watery grave. And yes, to answer your next question- they can eat rats, but only a single species has been documented to do this. Nepenthes rajah, the largest of all pitcher plants, has pitchers which grow to a height of nearly half a metre (1.6’) and hold up to three and a half litres (1gal.) of fluid, most of which is digestive juice.

Interestingly, pitcher plants have formed symbiotic relationships with several of the same types of creatures that it otherwise preys on. Nepenthes lowii, for example, provides nectar to a tree shrew. Instead of falling in and being digested, the shrew treats the pitcher as its personal toilet, thereby providing the plant with most of the nutrition it requires.

In one end and out the other.
(Via: Wikimedia Commons)

Other species form alliances with groups of carpenter ants. In exchange for a steady supply of nectar and a place to live- in this case a hollow tendril- the ants basically act as the plant’s evil henchmen (apparently a specialty of ants). When prey that is too large to be easily digested falls into the trap, the ants remove it, rip it to shreds, and then throw the bits back in again.

How’s that for a brilliant piece of evolution? Not only did these plants grow an external stomach… they get ants to chew their food for them.

[Fun Fact: Some pitcher plants primarily survive by digesting leaves that fall from trees into their traps – the ‘vegetarians’ of the carnivorous plant world.]

Says Who?

  • Bonhomme et al. (2011) Journal of Tropical Ecology 27: 15-24
  • Clarke et al. (2009) Biology Letters 5: 632-635
  • Krol et al. (2012) Annals of Botany 109: 47-64
  • Robinson et al. (2009) Botanical Journal of the Linnean Society 159: 195-202
  • Wells et al. (2011) Journal of Tropical Ecology 27(4): 347-353
So big it makes them vaguely uncomfortable.
(Via: Wikimedia Commons)

The Plant That Time Forgot (Welwitschia mirabilis)

(Via: Wikimedia Commons)

Common Name: Welwitschia mirabilis

A.K.A.: Welwitschia

Vital Stats:

  • Welwitschia is a gymnosperm, like pines or firs, and thus reproduces via male and female cones
  • Considered a “living fossil”
  • Named after one of its discoverers, Austrian botanist Friedrich Welwitsch
  • In mature specimens, the woody stem can grow up to one metre (3.3’) across

Found: In the Namib desert, along the west coast of Namibia and Angola

It Does What?!

Restricted to a tiny, arid swath of African desert, Welwitschia mirabilis represents the last remaining species of a very unusual lineage of plants. Close relatives met with extinction over the aeons, while welwitschia, tucked away in its remote and harsh desert range with little competition, just kept going. The fact that the species is alone, not just in its genus, but also in its family and order (the two ranks above genus in plant systematics), speaks to just how distantly related to any other living plant it is. For the sake of comparison, the Rosales, the order to which roses, apples, and pears belong, contains around 7700 species in 9 families and 260 genera. So original and captivating is welwitschia among plants that it has been the subject of more than 250 scientific articles since it was first described in 1863.

A mere infant. But probably still older than you are.
(Via: Lizworld.com)

So what makes this thing so weird? Well, plants typically have what’s called an apical meristem at the tips of their stems and/or branches. You can think of this as a clump of stem cells that keeps dividing, throwing off new leaves and buds in its wake. If you cut off the apical meristem, the plant must either develop a new one elsewhere, or stop producing new tissue.

In welwitschia, this isn’t the case. At the beginning of the plant’s life, the apical meristem produces just two leaves, and then dies. The plant will never grow another leaf, which is much more surprising when you consider that it may well live for more than a thousand years. How do you get through a millennium with only two leaves?! The answer is, these aren’t ordinary leaves. Uniquely, welwitschia’s two strap-like leaves have a band of meristematic tissue built into their base, which means they can continue to elongate outward indefinitely. The leaves will continue to grow at a rate of around half a millimetre (0.02”) per day for as long as the plant lives. If you’re thinking that this must mean leaves that are several hundred metres long, unfortunately, no, they aren’t. The leaves are abraded away by sand storms and eaten by passing animals. Even in the best case scenario, the cells at the leaf tips have a maximum lifetime of about ten years (still pretty good for a leaf…). What’s more, the leaves tend to get frayed and split over time, and end up looking like a lot more than just two leaves. Despite all the punishment, though, each leaf can reach a length of up to four metres (13’), giving a mature welwitschia a width of up to eight metres (26’) across.

Welwitschia’s answer to the pinecone.
(Image by Friedrich A. Lohmuller)

As you might expect from a long-lived relic of the past, there aren’t a lot of these plants around. For once, this has less to do with human disturbance than natural circumstances. Over millions of years, the range where welwitschia grows has dried out considerably, and in fact continues to get drier even now. Today, the plant relies largely on fog to meet its water needs, restricting its range to a thin strip of desert coastline where fogs occur regularly. Unlike cactuses or succulents, welwitschia has never evolved the ability to store water. Also problematic is a fungus, Aspergillus niger, which frequently infects and destroys germinating seeds. These factors together can mean that a welwitschia colony can sometimes go many years without successfully reproducing.

And of course, no threatened species would be complete without some human interference. In recent decades, unscrupulous collectors have removed plants from already small breeding populations, making it even more difficult to sustain their numbers. Interestingly, it’s noted in Wikipedia that plants in Angola are actually better protected from collecting than those in Namibia due to the higher concentration of landmines there.

So… landmines: bad for humans, good for endangered plants.

You think you have problems with split ends?
(Via: Natural History Museum)

Says Who?

  • The Gymnosperm Database
  • Dilcher et al. (2005) American Journal of Botany 92(8):1294-1310
  • Henschel & Seely (2000) Plant Ecology 150:7-26
  • Jacobson & Lester (2003) Journal of Heredity 94(3):212-217
  • Rodin (1958) American Journal of Botany 45(2):96-103

Killing Me Softly, or, The Fatal Embrace of the Strangler Fig

(Via: Wikimedia Commons)

Common Name: Strangler Figs

A.K.A.: Ficus species

Vital Stats:

  • There are around 800 sp. of figs, over half of which are hemi-epiphytes, like stranglers
  • Around 10% of all vascular plants are epiphytes (about 25,000 species)
  • The trees which produce the figs we eat are terrestrial, and do not grow in other trees

Found: Tropical forests of Latin America, Southeast Asia, and Australia

It Does What?!

What does it take to squeeze the life out of a full-grown tree? A lot of time and some very long roots, apparently. Many parasites eventually bring about the untimely death of their hosts, but few do it as slowly and as insidiously as the strangler fig.

Stranglers begin life as a tiny seed that leaves the back end of a bird and happens to land on a tree branch high in the rainforest canopy. The seed germinates, and the young fig begins to grow as an aerial plant, or epiphyte, taking its moisture from the air and its nutrients from the leaf litter on its branch. Thousands of plant species, including most orchids, grow in this manner. But then an odd thing begins to happen. The seedling produces a single long root. Very long. From tens of metres up in the canopy, this root grows all the way down to the ground. Many young stranglers will die before their questing root reaches the earth, but for those that make it, a connection is formed with the soil through which water and nutrients can be extracted. From this point on the great, towering giant which holds this tiny little interloper is in mortal danger.

The strangler fig, playing “harmless epiphyte.”
(Screenshot from The Private Life of Plants, BBC)

A secure connection to the soil allows the fig to speed up its growth and to begin sending more and more roots earthward. Rather than dropping straight down, like the initial root, these later organs will twine around the bark of the host tree. At first, the roots are tiny, like mere vines crawling over the host trunk. Over time, however, they thicken, covering more and more of the trunk’s surface. Where they touch or overlap, the roots actually fuse together, forming a mesh over the surface of the bark. Up above, the stem of the strangler is growing as well. It rises through and above the host branches, soaking up the light and leaving the other tree shaded and starved for energy.

In fact, this is a war fought on two fronts. As the starving host tree struggles to gather light energy to send downward from the leaves, it is also increasingly unable to bring water up from its roots. This is because the tree’s trunk continues to expand even as the strangler’s grip grows tighter around it. These opposing forces effectively girdle the tree, crushing the vascular tissues that carry moisture from the soil. Eventually, the battle is lost and the tree dies. Fortunately for the fig, its major investments in root growth have paid off – the dead host tree does not fall, taking the strangler with it. Instead, it simply rots where it stands. Finally, many years after its arrival on the scene, the strangler fig has achieved independence. It is now a free-standing tree, completely hollow and supported by its interwoven lattice of aerial roots.

The first root finds the ground.
(Screenshot from The Private Life of Plants, BBC)

So what happens when more than one strangler fig seed lands on a particular tree? Something quite unique… the roots of the different individuals fuse and form an organism which is indistinguishable from a single tree, except by molecular testing. These are what biologists refer to as ‘genetic mosaics.’ What’s more, the individuals actually begin to act like a single tree. You see, figs typically have staggered flowering times, such that it is unlikely for numerous trees in a small area to be in bloom at the same time. This helps in keeping their wasp symbionts well nourished. Once trees fuse, however, they seem to become physiologically linked as well, with researchers reporting that they bloom as a single individual.

The most hurricane-proof tree ever.
(Screenshot from The Private Life of Plants, BBC)

[Fun Fact: Some strangler fig species have very high growth rates, and huge individuals have actually been found engulfing abandoned buildings in the tropics.]

Says Who?

  • Harrison (2006) Journal of Tropical Ecology 22(4): 477-480
  • Perry & Merschel (1987) Smithsonian 17: 72-79
  • Schmidt & Tracey (2006) Functional Plant Biology 33: 465-475
  • Thomson et al. (1991) Science 254: 1214-1216
Don’t meditate under strangler figs.
(Via: Flickr, by vincenzooli)

The Bloodhounds of the Plant World (Cuscuta sp.)

(Via: Marine Science)

Common Names: Dodder, Goldthread, Witch’s Shoelaces

A.K.A.: Genus Cuscuta

Vital Stats:

  • Approximately 200 species
  • Part of the Convolvulaceae family, which includes morning glory and sweet potato
  • Only 15-20 species are considered to be problematic crop parasites

Found: Throughout temperate and tropical parts of the world

It Does What?!

We’ve discussed a few parasites on this blog already, and they’ve all been pretty typical of what comes to mind when we think of parasitic organisms- tiny, malignant little creatures that invade the host’s body, steal its resources, and, in some cases, eat its tongue. But when we think ‘parasite,’ we don’t usually think ‘plant.’ As it turns out, there are an estimated 4500 parasitic species just among the angiosperms, or flowering plants. Among them, dodders have to be one of the strangest.

Found nearly throughout the world, these vine-like plants begin as tiny seeds that germinate late in the spring or summer, after their potential host plants have established themselves. The young seedling has no functional roots and little or no ability to photosynthesize, so initially, it must make do with what little nutrition was stored in its seed. This isn’t much, so the plant has only a few days to a week to reach a host before it dies. To better its chances, the dodder stem swings around in a helicopter-like fashion as it grows, trying to hit something useful.

Much more impressive is the plant’s other method of finding suitable hosts- a sense of smell. Recent research has found that, uniquely among plants, the dodder can actually detect odours given off by surrounding plants and grow towards them. In experiments, the seedlings were found to grow toward the scent of a tomato, even if no actual plant was present. What’s more, they are capable of showing a preference among hosts. Presented with both tomato plants, which make excellent hosts, and wheat plants, which make poor hosts, seedlings were found to grow toward the aroma of tomatoes much more often. Like herbivores, they can use scent to forage amongst a variety of species for their preferred prey.

Smells like lunch… even to other plants.
(Via: Wikimedia Commons)

Once a host plant is found, the dodder begins to twine itself around the stem and to form haustoria (singular: haustorium). These are like tiny tap roots that pierce the host’s stem and actually push between the living cells inside until they reach the vascular system. Once there, the haustoria enter both the xylem (where water and minerals move upward from the roots) and the phloem (where sugars from photosynthesis move around the plant). From these two sources, the dodder receives all its nutrients and water, freeing it from any need for a root system, or even a connection to the soil. And since it doesn’t need to capture solar energy, all green pigment fades from the parasite, and it turns a distinctive yellow or red colour. Leaves aren’t necessary either, which is why the plant is essentially nothing but stem, explaining its common name of “witch’s shoelaces.”

Not what you want to see when you head out to weed the garden.
(Via: County of Los Angeles)

Once it gets comfortable on its new host, the dodder can grow at a rate of several centimetres a day (impressive for a plant) and produce stems of a kilometre or more in length, quickly overrunning an area. It can also attach itself to additional hosts – hundreds, in fact – which is problematic, because at this point it becomes the plant equivalent of a dirty shared needle. Since the vasculature of the hosts is connected, any virus present in one host can be freely transferred to any other. This ability, coupled with its affinity for potatoes, tomatoes, tobacco, and several other important crops, makes dodder a major nuisance for many farmers. And since it’s able to regenerate from just a single, tiny haustorium left in a host plant, it’s really hard to get rid of. There’s always a flip side, though; in some ecosystems, dodder can actually maintain biodiversity by preferentially parasitising the more competitive plants, allowing the weaker ones to survive. It seems dodder may also be the Robin Hood of the plant world.

[Extra Credit: Here’s a video showing how dodder can completely take over a group of nettle plants, complete with ominous soundtrack. Narrated by the fantastic Sir David Attenborough.]

Says Who?

  • Costea (2007-2012) Digital Atlas of Cuscuta (Convolvulaceae). Wilfred Laurier University Herbarium, Ontario, Canada
  • Furuhashi et al. (2011) Journal of Plant Interactions 6(4): 207-219
  • Hosford (1967) Botanical Review 33(4): 387-406
  • Pennisi (2006) Science 313: 1867
  • Runyon et al. (2006) Science 313:1964-1967

    Cuscuta: 1, Acacia: 0
    (Via: Wikimedia Commons)

Missing Carpels & the Building Blocks of Science

(Photo by: Domingos Cardoso)

Common Name: Amarelão (Brazil), Grapia (Argentina), Khare Khara (Bolivia)

A.K.A.: Apuleia leiocarpa

Vital Stats:

  • Once considered a genus of three different species, now collapsed down to one by taxonomists
  • The only trimerous (three-parted) flower in the whole legume family
  • Male flowers grow an extra stamen in place of the missing carpel

Found: In the rainforests of central South America

It Does What?!

Nothing quite as bizarre as our usual subjects, actually, but stick with me here. This week, I’m attending the annual conference of the American Society of Plant Taxonomists in Columbus, Ohio. I’ll be giving a talk on some of my research dealing with Apuleia and the development of its flowers. I thought I’d take this week to share some of that research here, and to try to make it interesting for people who aren’t into obsessing over obscure plants. If you still find this entry painfully tedious, though, rest assured, we’ll be back to freaks and oddities next week.

Apuleia leiocarpa is part of the legume family, which, if you’re from a temperate part of the world, brings to mind little annuals like beans and peas and clover. In the tropics, though, legumes are just as often towering trees of the rainforest canopy (like Apuleia) or scraggy shrubs of arid grasslands (such as Acacia). Most of the nearly 20,000 species of legumes have flowers with the same basic groundplan: 5 sepals, 5 petals, 10 stamens (the male organs), and a single carpel (where the fruit and seeds form). There are closely related chunks of the family, though, in which some of these floral organs have been lost over the course of evolution. (Now, ‘lost’ can mean two things; either the organ starts to grow and is suppressed before it finishes developing, or it just never forms at all. To the naked eye, these two kinds of loss look exactly the same. I’ll come back to this later.) Apuleia is one such legume- it has entirely lost two of its sepals, two of its petals, and most of its stamens, making for a very simplified flower.

The Hermaphodite (‘Normal’) Flower
S=sepal, P=petal, A=anther/stamen, C=carpel, St=stigma

What’s more, it now forms two different types of flower (called ‘morphs’). If you were to look closely at a flowering branch on one of these trees, you would see that the vast majority of the flowers were male-only, having no carpel with which to form fruit. Only every fourth or fifth flower would be the hermaphrodite type that we think of as a ‘normal’ flower. Botanists refer to this type of plant as being andromonoecious (pronounced “an-dro-mon-ee-shus”). So why would a tree evolve to become andromonoecious? There are a couple of different theories, based on two different ways that the male-only flowers can be produced.

In the first and most common type of andromonoecy, all the flowers on the plant begin as normal hermaphrodites. There are flowers of all different ages, so while some are beginning to open, others haven’t finished forming yet. Pollination starts on the earlier flowers, and the plant detects that it has far more ovaries (future seeds) than it’s going to need. Maybe the soil isn’t providing enough nutrition to produce all those potential fruits, or maybe there’s a drought in progress. So, according to its needs, the tree simply suppresses the development of the carpels in the younger flowers before they have time to mature, leaving parts of each branch with hermaphrodite flowers and parts with male flowers.

The Male-Only Flower
S=sepal, P=petal (both removed)
Arrow= where the carpel would have been

In scenario two, some flowers never develop carpels; they are male-only from the time they are first formed. This type of andromonoecy is thought to occur because the tree requires large amounts of pollen to reproduce successfully (perhaps the species is wind pollinated and individuals tend to be far apart, for example), and it’s “cheaper” to produce male flowers than hermaphrodites. In this situation, we don’t see the pattern of younger versus older flowers with respect to which ones are male.

That white asterisk in the very middle shows the hole through which the carpel would have emerged. It’s just a small, empty cavity in the male flower.

So which type of andromonoecy does Apuleia have? In order to find out, a colleague and I studied pressed herbarium specimens as well as flowers preserved in alcohol. The flowers, we dissected and viewed under an incredibly powerful microscope called a scanning electron microscope, which allowed us to see minute details, such as where a suppressed carpel might have been. In the end, we found that male Apuleia flowers show no sight of having ever developed a carpel. We also noticed that the hermaphrodite flowers always occurred symmetrically, right in the centre of a group of male flowers, a pattern that we wouldn’t see if the andromonoecy was environmentally influenced.

So in the end, we’re able to say that in this species, the different floral morphs probably arose in evolution due to an increased need for pollen, rather than as a control on fruit production. Groundbreaking… right? Well, maybe not, but obscure little discoveries like this are the building blocks for the big important breakthroughs we read about in the news. If you want to make something huge, you need a good foundation to start from.

Now imagine spending three hours of your life staring at this.
Science is so glamourous.

Says Who?

  • Beavon & Chapman (2011) Plant Systematics and Evolution 296: 217-224
  • de Sousa et al. (2010) Kew Bulletin 65: 225-232
  • Gibbs et al. (1999) Plant Biology 1: 665-669
  • Spalik (1991) Biological Journal of the Linnean Society 42: 325-336
  • Zimmerman et al. (In Press) International Journal of Plant Sciences

EVOLUTION TAG TEAM, Part 2: Sex & the Synconium

The second in an ongoing series of biology’s greatest duos. (Check out Parts One and Three)

(Via: Mastering Horticulture)

Common Name (Plants): Fig Trees

  • A.K.A.: Genus Ficus

Common Name (Wasps): Fig Wasps

  • A.K.A.: Family Agaonidae

Vital Stats:

  • Approximately 800 species of figs
  • Most are trees, but some are shrubs and vines
  • Approximately 640 species (20 genera) of fig wasps
  • All are obligate pollinators of figs

Found: Throughout the Tropics

It Does What?!

Snacked on any Fig Newtons lately? Tasty, right? Like the ad says, “A cookie is just a cookie, but a Newton is fruit and cake.”  …And wasps.

They must have run out of space on the package for that last part.

Before you toss out your favourite teatime treat, I should point out that without those wasps, the figs themselves wouldn’t exist. [Personally, I love Fig Newtons and will eat them regardless of any insects present.] This plant-insect pairing actually represents one of the most stable symbioses out there, with evidence suggesting it has existed for over 65 million years.

Now with 10% more Wings
(Via: Wikipedia)

While it’s not entirely clear how this arrangement evolved in the first place, fig trees produce a unique structure called a synconium, in which the flowers are actually inside the part we think of as the fruit. This synconium, which can contain up to 7000 flowers, depending on the fig species, has a tiny hole at the tip called an ostiole. In order for the flowers to be pollinated and the fruit to grow, a female wasp must squeeze through that hole, often losing her wings and antennae in the process, and distribute pollen that she carries in a sac on her abdomen. As she does so, she also uses her ovipositor to reach down into some of the female flowers and lay her eggs in their ovaries, where a gall is formed and the larvae can develop. Then she dies and ends up in a cookie. The End.

But hold on, let’s remove humans from the equation for a moment. She dies, but her eggs hatch into little moth larvae which use the growing fig for nutrition. Once they’re old enough, the young wasps mate with one another inside the fig (another nice mental image for snacktime), and the females gather pollen from the male flowers and store it inside their abdominal pollen baskets (yes, that’s actually what they’re called). The wingless male wasps have a simple, three step life: 1) mate with females, 2) chew a hole through the fig so they can leave, 3) die. That’s pretty much it for them. They may escape the nursery with the females, but they’ll die shortly thereafter, regardless. In fact, even the females have a pretty rough deal; from the time they’re old enough to mate, they have about forty-eight hours to get their eggs fertilized, gather pollen, find a new synconium, distribute the pollen, and lay their eggs. Two days, and their life is over. No pursuit of happiness for the fig wasp, I’m afraid.

“What does it all mean?”
(Via: BugGuide.net)

As with any long-standing mutualism, there are, of course, parasites ready and waiting to take advantage of it. These parasites are wasps which are able to enter the synconium and lay their eggs, but which do not pollinate the fig. Although their eggs will crowd out those of the fig wasps, decreasing the number of fig wasp larvae born, they are kept in check by the fact that any unpollinated synconium will be aborted by the tree and drop to the ground, taking the parasite eggs with it. The nonpollinating wasps are therefore kept from being a serious threat to the tree’s pollinators.

So there you have it, another of evolution’s great matches. The wasps get an edible nursery, the trees get pollinated, and we get tasty fruits with suspicious crunchy bits that probably aren’t dead wasp bodies, so just try not to think about it too much…

Seeds, or wasp eggs? You be the judge!
(Via: This Site)

[Fun Fact: The symbiosis between fig species and their corresponding wasp partners is so specific (often 1:1), that the shape of the ostiole actually matches the shape of the head of the wasp species which will pollinate it.]

[For those who would like to read about figs and fig wasps in much greater detail (such as how this works when the male and female flowers are in different figs), check out this excellent site for all you could ever want to know.]

Says Who?

  • Compton et al. (2010) Biology Letters 6: 838-842
  • Cook et al. (2004) Journal of Evolutionary Biology 17: 238-246
  • Kjellberg et al. (2001)Proceedings of the Royal Society of London, Biology 268: 1113-1121
  • Proffit et al. (2009) Entomologia Experimentalis et Applicata 131: 46-57
  • Zhang et al. (2009) Naturwissenschaften 96: 543-549