  Ecological selection forces for weedy and domesticated traits have influenced
the evolution of seed shape in Setaria resulting in similarity in seed shape
that reflects similarity in ecological function rather than reflecting
phylogenetic relatedness. Seeds from two diploid subspecies of Setaria viridis,
consisting of one weedy subspecies and two races of the domesticated
subspecies, and four other polyploidy weedy species of Setaria. We quantified
seed shape from the silhouettes of the seeds in two separate views. Differences
in shape were compared to ecological role (weed vs. crop) and the evolutionary
trajectory of shape change by phylogenetic grouping from a single reference
species was calculated. Idealized three-dimensional models were created to
examine the differences in shape relative to surface area and volume. All
populations were significantly different in shape, with crops easily
distinguished from weeds, regardless of relatedness between the taxa.
Trajectory of shape change varied by view, but separated crops from weeds and
phylogenetic groupings. Three-dimensional models gave further evidence of
differences in shape reflecting adaptation for environmental exploitation. The
selective forces for weedy and domesticated traits have exceeded phylogenetic
constraints, resulting in seed shape similarity due to ecological role rather
than phylogenetic relatedness. Seed shape and surface-to-volume ratio likely
reflect the importance of the water film that accumulates on the seed surface
when in contact with soil particles. Seed shape may also be a mechanism of
niche separation between taxa.
