Enjoying the multiple colors and forms of columbines

By Jeff Mitton

The blue columbine, Aquilegia coerulea, has the distinction of being the state flower, perhaps because it is one of the most beautiful and striking in the mountains.

The central male and female structures are long, numerous and brilliant yellow. Each of the five petals has two structures, a white blade and a blue nectar spur. Outside of the ring of petals are five sepals that look like an outer ring of blue petals. The flowers are large, about 2 inches across and almost 3 inches long.

The genus Aquilegia, which has about 70 species in Europe, Asia and North America, is distinguished by its spurs, which are recognized as an evolutionary novelty that enabled an adaptive radiation, or a rapid proliferation of species.

Consequently, most of the species are relatively young and only slightly differentiated, so hybridization has been reported for many of the species.

Our blue columbine’s typical color and form are familiar to many, but this species has a surprising diversity of colors and forms.The nectar spurs vary immensely among species, from 1 to 12 cm long. Each spur has a nectar gland at its base, so nectar rewards are efficiently harvested by pollinators with tongue lengths approximating nectar spur lengths. Among species, nectar spurs increase in length as the pollinator guilds transitioned from bumblebees to hummingbirds and then to hawkmoths, which have the longest tongues.

Nectar spurs seem to have played a central role in Aquilegia's adaptive radiation, for they restrict the pollinators and therefore play a role in the evolution of reproductive isolation, which defines species boundaries.

Our blue columbine's typical color and form are familiar to many, but this species has a surprising diversity of colors and forms. A hike along the Homestead Trail in Hermit Park Open Space, near Estes Park, provides access to populations containing three forms of columbines.

The most striking form has an entirely blue flower lacking spurs. This form was given the name daileyae by University of Colorado professor William Weber, and today it is recognized as a variety. Its petals are entirely blue and are not as round at the margin nor cupped at the base as are typical white petals. The lack of spurs is also striking. The transformation of the petals from white to blue is controlled by variation at a single gene.

Another form seen beside the Homestead Trail is entirely blue but has the spurs that are found on the typical blue columbine. Once again, the variation of normal spurs versus no spurs is controlled by a single gene. So the entire range of variation depicted in the photo is controlled by just two genes.

Scott Hodges and his Ph.D. student Nathan Derieg, at the University of California Santa Barbara, are examining the developmental genetics of the columbines at Hermit Park.

Given the important role that the nectar spurs play in attracting some pollinators and discouraging others, I imagine that adjacent plants could be pollinated by different guilds of pollinators. Columbines with spurs might be pollinated hawkmoths, while the spurless daileyae might be pollinated by bumblebees.

Spurless forms also appear in other species of columbines.

Blue columbines have yet another color morph that I have not seen at Hermit Park or any other place in the Front Range. Some Aquilegia coerulea retain the yellow pistil and stamens, but the petals, sepals and nectar spurs are entirely white. A population of predominantly white columbines is on Douglas Pass, north of Grand Junction.

But the white form becomes predominant in Wyoming, so that most of the plants have white flowers in the Ferris Mountains, the Wind River Range and the Teton Range. Imagine a stand of ghostly columbines.

I always enjoy finding a patch of columbines, for their showy flowers really brighten a forest or meadow. But I also appreciate how their obvious flower color and form variation remind all of the importance of genetic variation.

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.

April 10, 2015