The Invisible Colours of Flowers

Determining flower symmetry.

In 1742 the famous botanist, Carolus Linnaeus, found a remarkable toadflax plant on an island near Stockholm. Normal toadflax (Linaria vulgaris) flowers have distinctive upper and lower petals but, in this specimen, the petals all looked the same, making the flower radially symmetrical. Linnaeus was the founding father of plant and animal classification. His classification system for plants was based on flower structure, and so the unusual toadflax should have belonged to a totally new species. Yet, in every other respect, it resembled common toadflax. Linnaeus concluded that this peculiar plant had arisen by the “transformation” of common toadflax into a new species - a radical idea at a time when species were thought to be fixed, timeless acts of creation. Even though the flowers themselves were quite attractive, he named the new form Peloria, Greek for ‘monster’, because the disturbing biological implications of their discovery made them monstrous.

Despite the apparent complexity of flower shape, all flowers are actually one or other of two possible types. Those, like roses and tulips, where a line can be drawn across the centre of the flower, in many directions, to give two mirror images. These flowers are said to have ‘radial’ (multiple planes of) symmetry. In contrast, flowers like peas, toadflax and snapdragons, have ‘bilateral’ (a single plane of) symmetry as there is only one direction in which a line can be drawn across the centre of the flower to give two mirror images.

In flowers with bilateral symmetry, a particular gene is switched on in one region of the very young flower bud. This region provides a reference point that allows the developing flower organs to develop into different shapes based on their positions relative to the reference point. If the key gene is inactivated, by a mutation, then no reference point is established and the flower organs develop equally, resulting in flowers with radial symmetry.

The genetic basis for the bizarre change in the toadflax flowers, observed by Linnaeus, remained a mystery for 250 years. We now know the “monstrous” form is not a new species but is caused by a naturally occurring mutation of a single gene (called cycloidea), which normally establishes the asymmetry in very young toadflax flower buds. In Peloria, this gene is inactive, and asymmetry is not established, leading to radially symmetric flowers.

Mutations that convert flowers with bilateral symmetry to radial symmetry have been seen in several species, including Gloxinia and Saintpaulia, and much of the research on this gene has been done using Antirrhinum majus (snapdragon). These mutations are particularly interesting to scientists because radial symmetry in flowers is thought to be a more primitive flower form, in evolutionary terms, than bilateral symmetry. Bilateral symmetry also allows the development of flowers that are specially adapted to animal pollinators so, by studying these mutations, scientists are able to understand more about plant development, the evolution of plants, and their co-evolution with animal pollinators.

The full story of Linnaeus’s discovery of Peloria is described in a recent book, The Art of Genes (1999, OUP) by Enrico Coen.