I spent the day getting some new hanging baskets of flowers for my porch, hoping the shade of the porch doesn't send these plants to an early grave, and cleaning said porch of the mountains of tree sperm (a.k.a. pollen) so that I could enjoy the weather without looking like I engaged in a losing battle with a horny pine tree. This is normally more time than I generally spend with the reproductive structures of plants (whole plants yes, specifically plant genitalia no), and so it put me in the flower frame of mind. Hence, today's post.
The evolution of flower shape and symmetry is of particular interest and importance because it can affect pollinator behavior. Affect pollinator behavior and you affect who is successful at reproducing. All of a sudden our cute, and even sexy, talk of flowers becomes a talk of genetics. A new paper in PLoS Genetics takes a somewhat novel approach to this question: Classic art.
Vincent van Gogh's Sunflower paintings are well known and duplicated many times by many artists. When you look at the paintings you notice the vibrant colors, the angles, and the detail (down to differences in petal structure). Now, I'm not an artist, art critic, or art historian. Heck, I'm proud that I can draw stick figures that don't look like they have suffered head trauma or just got a prescription for Viagra. However, I do notice elements about van Gogh's Sunflower paintings that make me go "huh." Particularly the symmetry of the flowers. Look closely. Do those look like the sunflowers that you are used to seeing? As it turns out, when van Gogh painted his famous sunflower pictures in France in 1888 and 1889, he was painting mutant sunflowers. These flowers were double-flowered (dbl) sunflower mutants in which the disc florets develop bilateral symmetry rather than their typical radial symmetry.
Actinomorphic flowers exhibit radial symmetry; No matter which diameter you halve it along the halves will be equal. This type of floral symmetry is considered to be the ancestral state. Zygomorphic flowers exhibit bilateral symmetry; They can be divided by only a single plane into two mirror-image halves. This type of floral symmetry has arisen several times during the evolution of flowering plants. Those species exhibiting this anatomy tend to be from more speciose clades, likely because the symmetry increases pollinator specificity. Common (or wild type) sunflowers have a composite flower head that has a single whorl of large, flattened yellow ray florets on the outer perimeter with hundreds of disc florets that can produce seeds. Contrast this to the dbl mutant which exhibits flowers with multiple bands of yellow florets and much fewer internal disc florets. You've got that all worked out in your mind's eye, right? Well, as described by the authors and in the terms we just defined, Vincent van Gogh's sunflowers exhibit zygomorphic symmetry rather than the actinomorphic symmetry characterized in the sunflowers we are used to.
So what causes this (genetically speaking) and how do you find it? Remember good ole Gregor Mendel and his pea plant experiments? Well, that's how you find it. These scientists crossed the wild type variety of sunflower with the double-flowered variety. At first they thought a single, dominant gene was responsible for the change in the flower, but subsequent crosses revealed a third flower type of intermediate form. This intermediate form results from a gene recessive to both the wild type and the dbl variety. That means that something less-simple is going on.
Their next step was to sequence the genes. It is known that the genetic control of floral symmetry involves CYCLOIDEA (CYC)-like TCP transcription factors. The gradient of the expression of this gene can control the different types of petals that form. The genetic sequencing showed that dbl mutants had an insertion into the promoter region of a CYC-like gene (HaCYC2c). This is normally expressed in the wild type flowers but in dbl flowers it is expressed throughout the inflorescence causing it to lose actinomorphy. This same gene was disrupted in other types of sunflower mutants.
Very long story short: They found the genes that caused the van Gogh's mutant sunflowers. Does this change how you view the painting? Perhaps. Perhaps not. But it does solve a decades-long question, and in my book that is truly neat.
Read the paper here:Chapman, Mark A., et al. (2012) Genetic analysis of floral symmetry in Van Gogh's Sunflowers reveals independent recruitment of CYCLOIDEA genes in the Asteraceae. PLoS Genetics: 8(3), e1002628 (DOI: 10.1371/journal.pgen.1002628)
Brush up on your Sunflower's painting history at the Van Gogh Gallery
Read more over at Science Daily's article "Scientists Reveal Genetic Mutation Depicted in Van Gogh's Sunflower Paintings"
(images via Pacific Bouquets & Fine Gifts, Inc and Fantasy Art History, respectively)