Yellow butterfly milkweed

This is peak season for butterfly milkweed (Asclepias tuberosa). Rettenmund Prairie has a huge number of plants, with quite a wide variation in color, but none pure yellow. I discussed Woodford's work in a post last year that showed that color in this species is due to a combination of yellow carotenoids and red anthocyanins. Some populations in Minnesota and southwestern United States seem to completely lack the reds and are pure yellow. As far as  I know, yellow populations have not previously been reported in Wisconsin.

Last year Kathie found a yellow butterflyweed in one of our planted prairies (the Pocket Prairie) and this year another of the same color has shown up in a remnant prairie (Unit 7) nearby. Adjacent, as the photo shows, is the more typical red-orange variety.

Adjacent yellow and red-orange butterflyweed populations
in a prairie remnant at Pleasant Valley Conservancy

Woodson's work on the biogeography of color variation in butterflyweed is fascinating. With a group of graduate students, he drove over 45,000 miles in eastern and central United States, on the lookout for roadside populations. Flower color varied strikingly in different parts of the country. The yellow varieties were found primarily in the southwest, although also in Minnesota.

Woodson, who was based in St. Louis, discusses one drive he made along Route 66 between St. Louis and Oklahoma City. Somewhere in western Missouri flower color made a sharp change from red to yellow. "When I first encountered it in 1958, along old U. S. Highway 66 between Joplin, Missouri, and Oklahoma City-a single day's drive of only 260 miles-I could scarcely believe my eyes. The variation...is amazing. West of Oklahoma City the yellows pre- dominate..."

Visually Woodson quantified the flower colors by means of a standard color chart, and followed this up in the lab with spectrophotometric measurements of extracted pigments. The latter provided a neat way of comparing different populations. The photo here, taken from his paper in Evolution, shows the comparison. The two pigment types could be separated by extraction, since the carotenoids are water insoluble but soluble in methanol, whereas the anthocyanins are water soluble and extractable with dilute hydrochloric acid. Although no genetics has been done, it seems reasonable that pigment concentration and  hence flower color are genetically controlled.

To see quite a range of flower colors in butterfly weed (although not the yellow), take a stroll right now through Rettenmund Prairie.