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 unpredi ctable environmental variability, has led to the existence of seed ban ks-populations of dormant, viable seeds in the soil. Seed banks are th eoretically important to both the demography and genetic structure of plant populations. The presence of seed dormancy can also affect the e volution of traits not directly associated with dormancy and germinati on. Theoretical models have suggested that the existence of dormancy c an influence the rate of evolution of post-germination traits. The eve ntual outcome (e,g., allele frequencies) may be influenced as well, le ading to adaptive syndromes of germination and post-germination traits . Seeds that germinate in different conditions may experience differen t selective regimes for post-germination traits. If there an trade-off s between the fitness of post-germination traits in different environm ents, then seeds that germinate in the environment to which their post -germination traits are adapted will be at a selective advantage. If d ifferences in germination and post-germination traits are genetically based, then potentially adaptive genetic correlations between germinat ion and post-germination traits may evolve. We feel that investigating the ecological and evolutionary importance of these correlations requ ires an empirical approach. As a first step, hen we ask whether the co nditions necessary for such syndromes to arise exist in a particular p lant population. We show that conditions favoring the joint evolution of dormancy and postgermination traits leading to adaptive syndromes e xist in the mustard, Lesquerella fendleri, in central New Mexico. Firs t, Lesquerella experiences the sort of variation in environmental cond itions that would be expected to lead to adaptive trade-offs in the ex pression of post-germination traits for individuals that differ in ger mination traits, Annual precipitation varies greatly from year to year so that germination in drier years would be expected to select for mo re xerophytic traits. Within a year, microenvironmental spatial variat ion exists. Lesquerella growth and reproduction are sensitive to both year-to-year and microenvironmental variation. Second, the seed bank c an affect both the demographic and genetic structure of the population . Dormant seeds remain viable for at least 3 yr and can mitigate the n egative demographic effects of reproductive failure. Allozyme differen ces exist between seeds that germinate in the field and seeds that rem ain dormant, suggesting that the evolutionary potential of the abovegr ound population is influenced by dormancy. Finally, the necessary gene tic and environmental variation is present. Both germination percentag e and post-germination traits (e.g., leaf morphology) vary among and w ithin populations as well as among environmental treatments. Thus, the potential exists for Lesquerella to respond to selective differences between different temporal or spatial environments. We suggest experim ental approaches for assessing the extent to which seed dormancy has a ffected or will affect the evolution of post-germination traits. The c onsequences of past evolution could be explored by asking ''What genet ic 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 ge rmination environment). Exploring whether evolution is currently occur ring would require an assessment of natural selection and the genetic potential for response to selection. Given the difficulty of such stud ies, a reasonable first step would be to explore how evolution can occ ur by performing an artificial selection experiment on dormancy or ger mination percentage. Then, correlated responses of post-germination tr aits 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 adap tive syndromes are documented, questions about the ecological mechanis ms (e.g., necessary frequencies of different year types) and genetic m echanisms (e.g., linkage disequilibrium vs. pleiotropy) can be explore d. We hope to draw attention to seed dormancy, which is an often-ignor ed stage in the life history of plants, and to encourage empirical wor k, which lags far behind theory.