Here's to sunflowers on sand dunes

By Jeff Mitton

My destination was Colonnade Arch, a scenic, buttressed alcove on the rim of the Green River Canyon, west of Canyonlands National Park.

I was driving on a 37-mile long dirt road to my campsite, when I noticed something that struck me as odd. A small set of sand dunes south of the road supported a vigorous population of sunflowers. I found that these sunflowers have an interesting history, and evolutionary biologists know them well.

When I returned to Boulder with photographs of the population, I shared them with my colleague Nolan Kane and his graduate student Silas Tittes. Kane is a recognized authority on the genomics of sunflowers, and his graduate student is conducting research on hybridization between wild and domesticated sunflowers. Kane not only knew about these sunflowers, he had a sample of DNA from the population.

Western sunflowers, H. anomalus, as well as paradox and desert sunflowers, are younger than we are. They evolved when humans were migrating from our geographic origin in southern Africa to Europe and AsiaSunflowers are in the genus Helianthus, which has about 70 species, with all but three of them native to North America. Probably the best known is the common sunflower, Helianthus annuus, for it grows in all of the 48 contiguous states and it is the state flower of Kansas. The common sunflower was first domesticated in eastern North America more than 4,000 years ago, and it was also cultivated in Mexico shortly afterward. It is found on the eastern plains and it seems to have spun off a variant that is a successful weed along roads, trails and bicycle trails in Boulder.

Due to their diversity, agricultural history and importance, evolutionary geneticists have developed an abiding interest in the evolution of sunflowers. Loren Rieseberg at the University of British Columbia has led this work. He and his colleagues, using genetic data, inferred an evolutionary tree for Helianthus. Quite unexpectedly, the tree revealed that three species did not exhibit the mode of speciation shared by most other sunflowers. The evolutionary tree shows that most closely related species exhibit a pattern of bifurcation, or divergence from a single common ancestor. But the genetic data also revealed that three species were produced by recent hybridization between two distantly related sunflowers.

The genus Helianthus arose around 5 million years ago, while the common and prairie sunflowers evolved between 1.5 and 2 million years ago. Although they are far apart on the evolutionary tree, these species hybridized repeatedly between 60,000 and 200,000 years ago. Unassailable genetic evidence shows that hybridization yielded three species that occupy very different environments today.

The species that I found growing on sand dunes is the western sunflower, Helianthus anomalus. It is specifically adapted to sand dunes, where it grows from 3 feet to 6 feet tall. Its leaves are modified to store water, and its roots grow particularly fast, so that it can reach water deep below the surface. Because it occupies the fringes of sand dunes, it faces little competition from other plants.

Another product of hybridization is paradox or puzzle sunflower, H. paradoxus. It occupies salt marshes and the shores of saline ponds in western Texas and New Mexico. This species is rare and is protected in some places.

The desert sunflower, H. deserticola, is adapted to the desert floor and its native distribution is limited to deserts in Nevada, Arizona and Utah, where it grows up to 16 inches tall. Crosses between the parental species, common and prairie sunflowers, produced plants very similar to H. deserticola, which more adapted to desert conditions than either parental species.

Western sunflowers, H. anomalus, as well as paradox and desert sunflowers, are younger than we are. They evolved when humans were migrating from our geographic origin in southern Africa to Europe and Asia. It is entirely possible that more sunflower species are evolving today.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

Nov. 3, 2015