X-rays of ancient sunflowers reveal their domestication roots

For his research on the genetics of sunflower adaptation and domestication, Benjamin Blackman usually grows plants in greenhouses and fields. Through a recent collaboration with Michigan State University (MSU), however, he’s been analyzing some older—much older—specimens, too. 

Last spring, Blackman, an assistant professor in the Department of Plant and Microbial Biology, traveled to MSU to help x-ray the ancient remains of sunflower disks. Now held in museum collections at the University of Michigan and the University of Arkansas, the disks were excavated from caves and rockshelters in the Ozarks and Kentucky, and range in age from 3500 years before present (YBP) to 400 YBP. This timeline was established by Blackman’s team through radiocarbon dating of the sample collection.

On the right, Blackman prepares to scan the ancient sunflower remains gathered from the Ozarks and Kentucky. 

The new x-ray scans provide visible evidence of how sunflower’s morphological features were modified by domestication. The collection displays an overall increase in disk size over time, but, contrary to expectation, the youngest disks are not also the largest. For Blackman, this observation raises the question of whether sunflower seed size shows a similar trend. Discovering this pattern is the key to understanding how changing cultivation practices by early sunflower farmers may have differentially impacted the evolution of the two traits.

To achieve this goal, Blackman is collaborating with MSU’s Dan Chitwood, a plant morphologist who has used his X-ray CT scanner to examine a broad array of species, and mathematician Liz Munch, who is developing methods to analyze the disks. Together, the team is applying advanced mathematical models to reconstruct developmental patterns and extract measurements of interest.

On each of the specimens, Blackman and the MSU team are finding and measuring the sizes of any remnant seeds lodged in the disks but more often the scars and bracts they left behind. This data will then be used to extrapolate seed size throughout the dated collection. 

The video on the left shows Ben Blackman and Michelle Quigley’s scans of sunflower remains from the rock shelters in Menifee County, Kentucky. These remains were collected by Volney Jones in 1936 and some of the remains have been dated to 2,900 years old.