Re-establishment of latitudinal clines and local adaptation within the invaded area suggest rapid evolution of seed traits in Argentinean sunflower (Helianthus annuus L.)

Abstract

Invasive plants represent a valuable model system for studying contemporary evolution and predicting evolutionary responses to global climate change. Rapid adaptation to climate during range expansion has been recently recognised as a major factor in biological invasions. In this study, by using complementary approaches (common garden studies and the presence of parallel geographic clines), we tested for rapid, adaptive evolution of seed traits in wild sunflower (Helianthus annuus L.). Seeds from 22 wild sunflower populations from native (North America) and invasive (Argentina and Australia) groups were grown in a common garden for 2 years (experiments) and used for evaluating genetic differences in seed traits. Seed germination at two times after harvest, seed mass, and size (length and width) were recorded. In addition, 25 climatic variables were used to characterize the local environment of each population and to evaluate the geographic variation in the traits. Seeds from the invasive group showed larger mass and size and higher germination (lower seed dormancy) than seeds from the native group. Latitudinal cline explained most of the group variation in seed dormancy, but not in seed mass or size. Invasive sunflower from Argentina (but not from Australia) re-established the latitudinal cline observed in the native group. We provide evidence that support rapid, adaptive evolution (< 70 years) of seed dormancy in the invasive Argentinean sunflower in response to warmer environments found in Argentina, suggesting that crop wild relatives can quickly evolve in response to novel abiotic conditions.