Sigma Xi lecture highlights plant evolution

Although flowers are not generally considered to be particularly fast-moving creatures, Joan Edwards, professor of biology, challenged this preconception over the course of her two-part Sigma Xi lecture. In her talk, Edwards discussed plants’ rapidity in two very different contexts: the incredible rate at which flowers have evolved and speciated, and the lightning-fast physical motions certain plants perform to spread seed.

Edwards opened her lecture by citing Darwin’s “abominable mystery”: How did flowering plants evolve so quickly? Darwin dubbed their evolution as such because it conflicted with his notion of gradual evolution. He postulated the “lost island theory” or “mountaintop hypothesis” to explain the rapid emergence of flowers – perhaps they had evolved in some isolated environment like a desert island or remote mountain and were at some point abruptly introduced to the rest of the world.

Edwards hinted that this theory is not as far-fetched as it initially appears, citing genetic studies which confirm divergence from a common ancestor. Mitochondrial, chloroplastic and nuclear DNA all indicate common ancestry. Although this common ancestor may not have been as geographically isolated as Darwin’s proverbial insular plant, it nonetheless diverged so quickly that flowers constituted the vast majority of all plant species within tens of millions of years of its emergence.

Edwards attempted to answer Darwin’s question by focusing on genetic and physiological reasons for rapid speciation. She pointed to the small number of genes that control the development of flowers’ organs. Flowers consist of four organs, each of which is a modified leaf whose transformation is governed by a specific set of genes. Although we do not see much resemblance between flowers and leaves, slight mutations in these four genes cause flowers to develop into small clusters of ordinary leaves. Slight genetic variation affects profound physical change in specific areas of the flower, so flowers can quickly evolve to target specific vectors of genetic exchange to the extent that members of the same species can target vastly different pollinators, a process that was accelerated by the co-evolution of flowers and pollinators.

To illustrate this point, Edwards referenced two flowers she studies as part of her research: the wood lily and goldenrod. Both flowers illustrate the influence of pollinators on flowers’ characteristics. The wood lily, primarily visited by butterflies, has evolved to stand perfectly upright to form a landing platform for the insects like the butterfly, whose large wings collect more pollen when it can land flat on the flower.

Preliminary qualitative studies by Allison Gardner ’10, one of Edwards’s thesis students, indicate that the dramatic extent of speciation of goldenrod and aster – there are 22 species of goldenrod and 18 species of asters in Berkshire County alone – can be attributed to the propensities of individual pollinators to prefer particular species.

The second part of the lecture focused on the incredibly rapid movements certain plants undergo to spread gametes and the ways in which their methods have evolved to favor specific pollinators. She began by speaking generally about plants’ motion and presenting several high-speed videos of rapid plant motion.

Edwards discussed the Canadian dogwood, whose flowers have hairs that trigger explosive distribution of pollen. With a camera capable of 100,000 frames per second, the biology department observed a maximum acceleration of 19,600 meters per second per second, which qualified the Canadian dogwood as the fastest plant in the Guinness Book of World Records.

Because many different insects visit the flower, its trigger hairs have evolved to be precisely calibrated. Insects that weigh more than .01 grams are heavy enough to open it, whereas insects that weigh less cannot. Fittingly, larger pollinators have a tendency to prefer dogwood. Furthermore, they also collect much more pollen. The Canadian dogwood has evolved to ensure that only large, efficient pollinators receive pollen, and not waste any on what Edwards described as “moochers.”

However, large insects also have a tendency to eat pollen, rather than depositing it on other flowers. Thus, rather than leaving pollen easily accessible, the flower conceals it until a landing insect of sufficient size triggers an explosive burst that completely coats it in pollen, which prevents the insect from eating it.

Edwards concluded by discussing her studies of sphagnum moss, a highly prevalent plant that covers approximately 1 percent of all Earth’s land surface. Although sphagnum moss is not a flower, the physiological process by which it launches its spores is fitting. Outer cells of spore capsules dry up and collapse inwards, building up four to five atmospheres of pressure within the spore capsule until it explodes. Spores are relatively light, so their terminal velocity is too low for any significant amount of travel. The structure of the spore capsule compensates for this deficiency by creating air vortices that propel spores to such heights that they can travel kilometers via air currents.