They found that the chitinase A genes in the evening primrose species that reproduce sexually were different in four places from the ancestral gene, while the genes in the asexual species had not changed.
She also looked at how well 12 different genotypes of the same asexual evening primrose species weathered attacks by powdery mildew.
This suggests that common pathogens of evening primrose plants are an important selective agent for these plants," Hersch-Green said.
These warm-weather primroses are the Oenothera (pronounced ``ee-NOTH-er-a'') primroses, including the Mexican primrose (Oenothera Berlandieri) and the evening primrose (Oenothera Hookeri), which are native to Mexico, California and the Southwest.
The evening primrose has butter yellow flowers that are visible now.