Genetic variability and phenotypic plasticity in flowering phenology in populations of two grasses

QUINN, J. A. (Department of Ecology, Evolution, and Natural Resources, Rutgers University, New Brunswick, NJ 08901-1582) AND J. D. WETHERINGTON (Biometrics Division, DuPont Pharmaceuticals, Wilmington, DE 19880-0721). Genetic variability and phenotypic plasticity in flowering phenology in populations of two grasses. J. Torrey Bot. Soc. 129:96-106. 2002.-Genetic differentiation in flowering phenology has been frequently demonstrated among populations of wide-ranging species. However, fine-tuning of flowering phenology, i.e., responses to yearly fluctuations in the local environment, should best be accomplished via phenotypic plasticity. To test theoretical expectations as to the interaction of genetic differentiation and plasticity, and conditions under which each should be most important, populations of the inbreeding Sporobolus cryptandrus and the outbreeding Panicum virgatum were compared in genetic variation among clones and in amounts and patterns of plasticity in phenology. For each species, three years of data from replicated blocks of cloned genotypes in a common garden near Fort Collins, Colorado, were treated as characters measured in three environments as a way of quantifying "environmental sensitivity" and partitioning the variance in flowering time. The three years provided major differences in length of growing season, temperatures, precipitation, and evaporation rates. The outbreeding species possessed the greatest genetic variation among clones in a population, while the inbreeding species showed less genetic variation within and more between populations. Populations of both species varied significantly in amount and highly significantly in pattern of plasticity. The long-lived, clonal Panicum showed a significantly greater plasticity than the short-lived, non-clonal Sporobolus. The plasticity variance component was the largest component overall for Sporobolus, which is found over the widest range of environments heterogeneous in space and time, and there was a highly significant relation between a population's ecological history and the relative importance of plasticity in flowering phenology. The two species did not show a consistent relationship between amounts of clonal genetic variation and plasticity, emphasizing that experiments on a diversity of species are required to appropriately test theoretical expectations.