Clinal variation in seed traits influencing life cycle timing in Arabidopsis thaliana.

Early life-history transitions are crucial determinants of lifetime survival and fecundity. Adaptive evolution in early life-history traits involves a complex interplay between the developing plant and its current and future environments. We examined the plant's earliest life-history traits, dissecting an integrated suite of pregermination processes: primary dormancy, thermal induction of secondary dormancy, and seasonal germination response. We examined genetic variation in the three processes, genetic correlations among the processes, and the scaling of germination phenology with the source populations' climates. A spring annual life history was associated with genetic propensities toward both strong primary dormancy and heat-induced secondary dormancy, alone or in combination. Lineages with similar proportions of winter and spring annual life history have both weak primary dormancy and weak thermal dormancy induction. A genetic bias to adopt a spring annual strategy, mediated by rapid loss of primary dormancy and high thermal dormancy induction, is associated with a climatic gradient characterized by increasing temperature in summer and rainfall in winter. This study highlights the importance of considering combinations of multiple genetically based traits along a climatic gradient as adaptive strategies differentiating annual plant life-history strategies. Despite the genetic-climatic cline, there is polymorphism for life-history strategies within populations, classically interpreted as bet hedging in an unpredictable world.