Ann S. Evans

Can dormancy affect the evolution of post-germination traits? The case of Lesquerella fendleri

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 difference 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., as well as among environmental treatments. Thus, the potential exists for Lesquerella to respond to selective diffences between different temporal or spatial environments.

GENETIC EFFECTS OF GERMINATION TIMING AND ENVIRONMENT: AN EXPERIMENTAL INVESTIGATION

Seeds of many species do not germinate immediately after dispersal, but instead may remain indefinitely in a dormant but viable state. Although it is well established that seeds often exhibit diversified patterns of dormancy and germination, the causes and consequences of this variation remain poorly understood. In this study, we investigate the extent to which seed genotypes of the desert mustard Lesquerella fendleri differentially germinate and establish under experimental conditions in a greenhouse. We used a two‐way factorial design to compare genotypes of Lesquerella plants derived from seeds that germinated and established at different times and under different soil water regimes. Overall allozyme allele frequencies of Lesquerella plants varied significantly with both germination time and initial soil water availability. Single‐locus heterozygosity analyses revealed that seeds sown into initially low water conditions produced plants that were significantly more heterozygous than plants derived from seeds experiencing constantly high water conditions, but heterozygosity did not differ significantly among plants originating from early‐ and late‐germinating seeds. This is the first study to experimentally demonstrate that germination timing and environment can significantly affect the genetic structure of emerging plant populations. The study suggests that germination and survival behavior may (1) play an important role in generating and maintaining the genetic structure of natural plant populations and (2) set the stage for subsequent evolution.