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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