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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
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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
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How to Stay Cool the Lungfish Way

Via: Science News for Kids

Common Name: The Lungfish

A.K.A.: Subclass Dipnoi

Vital Stats:

  • 6 species; 4 in Africa, 1 in South America, 1 in Australia
  • Some species can reach up to 2m (6.6’) long and weigh 43kg (95lbs.)
  • Omnivorous, eating plants, insects, crustaceans, worms, fish, and frogs
  • Largest genome of all terrestrial vertebrates at ~133 billion base pairs

Found: Slow-moving freshwater bodies in South America, Africa, and Australia

It Does What?!

Well, they’re not much to look at, but in the “quietly carrying on while everything drops dead around you” department, the lungfishes are tops. These large, eel-looking creatures are what biologists refer to as “living fossils”, species which have existed in more or less their present form for a very, very long time. In the case of the lungfishes, around 400 million years. For the sake of comparison, this was around the same period that plants developed roots and leaves. That long ago. In fact, researchers believe that the lungfishes are the closest living relatives of the terrestrial vertebrates (that is, anything with a spinal column that lives on land).

These will probably outlast humanity.
Via: One More Generation

So what makes these things so interesting, besides being old? First off, they breathe air, as you might have guessed from their name. Australian lungfishes have a single lung, and, while they normally breathe through their gills, are able to supplement their oxygen intake with air during times of high exertion or when their water gets stale (Fun side note: During mating, Australian lungfishes make loud burping noises at the surface of the water which are thought to be part of the courtship ritual. I’ll refrain from making any Aussie jokes here… ). African and South American lungfishes, on the other hand, have two lungs and breathe nothing but air. Their gills are completely atrophied, such that they could actually drown if kept under for much longer than their usual 5-8 minutes between breaths.

“Hey! I’m trying to aestivate in here!”
Photo by: Tobias Musschoot

This ability to breathe without water results in the other fantastic ability of subclass Dipnoi. South American and African lungfish live in habitats which often dry up completely during the hottest part of the year. The fishes’ gross but brilliant answer to this is to burrow up to half a metre down into the soft mud and excrete a huge amount of mucous. As the surrounding mud dries up, the mucous forms a hard shell which keeps the curled up lungfish moist and cool. A small hole at the top of this snot-cocoon allows the fish to breathe. It’s metabolism slowed to only a small fraction of the normal rate, the creature will aestivate (like ‘hibernate’, but without the cold) like this for several months until the rains return. Laboratory experiments have shown that an African lungfish can remain alive under these conditions for as long as six years.

“Granddad”: probably older than your Granddad
Via: Shedd Aquarium

Aside from their amazing survival abilities, these fish have unusual lives, as fish go. They are extraordinarily long-lived. The Shedd Aquarium in Chicago holds an Australian lungfish known as “Granddad” which arrived there as an adult in 1933, making him at least 80 years old. Females of this species don’t even mate until they’re at least 22 years old (or so they tell their parents). What’s more, some species actually care for their young. The mother and father build an underwater nest for their offspring, which can only breathe via their semi-atrophied gills for the first seven weeks, and the father uses his body to release additional oxygen into the surrounding water, helping them to breathe. So, dual childcare: not such a new idea after all.

[Extra Credit –  Here’s a short video of a lungfish being stalked by a pelican. Spoiler: It ends badly for the lungfish.]

Says Who?

  • Brinkmann et al. (2004) Journal of Molecular Evolution 59: 834-848
  • Fishman et al. (1992) Proceedings of the American Philosophical Society 136(1): 61-72
  • Glass (2008) Respiratory Physiology & Neurobiology 160: 18-20
  • Joss (2006) General and Comparative Endocrinology 148: 285-289
  • Lee et al. (2006) General and Comparative Endocrinology 148: 306-314
  • www.fishbase.org