Adaptation of Seed Oil to Germination Temperature in Helianthus annuus

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For many species of plants, seed oils perform the essential function of providing energy and carbon to germinating seeds before photosynthesis begins. Since the timing and rate of germination have been shown to strongly affect the fitness of many species of plants, the composition of seed oils could be under strong selection insofar as they affect these characters. One very striking and consistent pattern in seed oil composition is that plants growing at higher latitudes have lower proportions of saturated oil in their seeds than plants growing at lower latitudes. The pattern holds whether one looks among species or within species that have broad latitudinal ranges. Based on biochemical, physiological, energetic, and ecological lines of evidence, it is hypothesized that germination temperature is the primary selective agent on the proportion of saturated oil in seeds. Seeds with a lower proportion of saturated oil that germinate at cooler temperatures are predicted to have better germination performance than seeds higher in saturated oils. This performance advantage should lead to improved competitiveness and higher fitness. At warmer germination temperatures, relative fitnesses of seeds with high and low proportions of saturated oil should reverse. Since germination temperature decreases at higher latitudes, this hypothesis could account for the latitudinal gradient in the proportion of saturated oil in seeds. The experiments in this proposal will test whether the performance, competitiveness and fitness of seeds with different proportions of saturated oils are affected by germination temperature as predicted. Specially bred high and low saturated-oil lines of a wild sunflower, Helianthus annuus, will be used to test the predictions. The results of the experiments will help us understand the evolutionary significance of a nearly ubiquitous pattern in seed oil composition, and it will more generally help us understand why seed-oil compositions vary to such a great extent. The results may also have agricultural implications because they will suggest where plants with desired proportions of saturated and unsaturated seed oils occur and possibly how to breed seeds that are better adapted to the zones where they are planted. Finally, the work proposed here will be the foundation for future work testing whether the patterns and processes in Helianthus seed oils hold more generally and whether the genetic bases of saturated oil proportions are consistent.