Can Dormancy Affect the Evolution of Post‐Germination Traits? The Case of Lesquerella Fendleri
Authors: Evans Ann S. , Cabin Robert J. 
Published: May 2006

Seed dormancy, which is thought to have evolved in response to unpredictable environmental variability, has led to the existence of seed banks–populations of dormant, viable seeds in the soil. Seed banks are theoretically important to both the demography and genetic structure of plant populations. The presence of seed dormancy can also affect the evolution of traits not directly associated with dormancy and germination. Theoretical models have suggested that the existence of dormancy can influence the rate of evolution of post—germination traits. The eventual outcome (e.g., allele frequencies) may be influenced as well, leading to adaptive syndromes of germination and post—germination traits. Seeds that germinate in different conditions may experience different selective regimes for post—germination traits. If there are trade—offs between the fitness of post—germination traits in different environments, then seeds that germinate in the environment to which their post—germination traits are adapted will be at a selective advantage. If differences in germination and post—germination traits are genetically based, then potentially adaptive genetic correlations between germination and post—germination traits may evolve. We feel that investigating the ecological and evolutionary importance of these correlations requires an empirical approach. As a first step, here we ask whether the conditions necessary for such syndromes to arise exist in a particular plant population. We show that conditions favoring the joint evolution of dormancy and post—germination traits leading to adaptive syndromes exist in the mustard, Lesquerella fendleri, in central New Mexico. First, Lesquerella experiences the sort of variation in environmental conditions that would be expected to lead to adaptive trade—offs in the expression of post—germination traits for individuals that differ in germination traits. Annual precipitation varies greatly from year to year so that germination in drier years would be expected to select for more xerophytic traits. Within a year, microenvironmental spatial variation exists. Lesquerella growth and reproduction are sensitive to both year—to—year and microenvironmental variation. Second, the seed bank can affect both the demographic and genetic structure of the population. Dormant seeds remain viable for at least 3 yr and can mitigate the negative demographic effects of reproductive failure. Allozyme differences exist between seeds that germinate in the field and seeds that remain dormant, suggesting that the evolutionary potential of the aboveground population is influenced by dormancy. Finally, the necessary genetic and environmental variation is present. Both germination percentage and post—germination traits (e.g., leaf morphology) vary among and within populations as well as among environmental treatments. Thus, the potential exists for Lesquerella to respond to selective differences between temporal or spatial environments. We suggest experimental approaches for assessing the extent to which seed dormancy has affected or will affect the evolution of post—germination traits. The consequences of past evolution could be explored by asking "What genetic and phenotypic differences exist between individuals that germinate and those that remain dormant?", while controlling for factors that influence germination (maternal genotype, maternal environment, and germination environment). Exploring whether evolution is currently occurring would require an assessment of natural selection and the genetic potential for response to selection. Given the difficulty of such studies, a reasonable first step would be to explore how evolution can occur by performing an artificial selection experiment on dormancy or germination percentage. Then, correlated responses of post—germination traits could be examined. Empirical studies such as these are necessary in order to better understand the role of seed banks in plant ecology and evolution. Then, once associations that can be interpreted as adaptive syndromes are documented, questions about the ecological mechanisms (e.g., necessary frequencies of different year types) and genetic mechanisms (e.g., linkage disequilibrium vs. pleiotropy) can be explored. We hope to draw attention to seed dormancy, which is an often—ignored stage in the life history of plants, and to encourage empirical work, which lags far behind theory.