Browsing by Subject "germination"

Environmentally cued development is widespread across the natural world. Many organisms rely on abiotic and biotic cues to undergo developmental transitions like budburst, flowering, and mating at the appropriate times of year. The study of the timing of these transitions is known as phenology. Because the timing of development determines the environment experienced by the next life history stage, it has the potential to affect evolutionary processes that occur after development. Further, because the timing of development can filter the environment experienced by the next life history stage, it can be considered a form of “habitat tracking.” In this dissertation, I use manipulative laboratory and field experiments to quantify how germination phenology can alter the postgermination environment, show that the postgermination environment can itself be genetically determined, show that germination phenology is a form of habitat tracking, and test how germination phenology can affect trait expression, natural selection, and fitness.

In my first chapter, using the ecological and genetic model species Arabidopsis thaliana, I showed that when the timing of development is genetically controlled, and the timing of development affects the environment experienced by the next developmental stage, then the environment experienced after development can itself be inherited and can evolve. Further, I demonstrated that germination phenology is a form of “habitat tracking”, by enabling seeds to establish seedlings in a subset of the full environmental conditions available. In my second chapter, using ecologically diverse A. thaliana genotypes, I show that the timing of germination can affect natural selection on postgermination traits, and that postgermination traits can affect selection on germination phenology. In my third chapter, using two plants native to North Carolina, Phacelia purshii and P. fimbriata, I show that populations can vary naturally in their propensity to germinate in response to different environmental cues, that populations preferentially germinate in habitats that are beneficial for seedlings, and when placed in new geographic locations, populations may use phenology to track novel but beneficial environmental conditions.

My dissertation placed the common process of cued development into the well- established theoretical framework of habitat tracking and habitat selection. By doing so, I was able to generate and test novel predictions about potential consequences of phenological cueing that have not yet been explored—namely, that the post- development environment itself can be inherited, that the magnitude and frequency of natural selection can vary with changes in habitat tracking, that habitat tracking itself may evolve in response to traits expressed and environments experienced after development, and that habitat tracking through phenology may be an important mechanism that organisms can use to cope with climate change.

Theoretical and empirical studies have consistently shown that the optimal timing of seed germination reduces exposure to physical stress and minimizes competitive interactions with neighbors. However, this research has not accounted for facilitative (positive) interactions among plants, which become more pronounced as environmental stress increases. Facilitation is more likely to occur early in a plant's life when it is more susceptible to stress. In seasonal environments, the stress a given individual experiences can change throughout the year, and some years are more stressful than others. These sources of temporal variation in stress will dictate the facilitation-competition balance that individuals experience. However, it remains unclear how this balance affects the optimal timing of germination. My dissertation research asks how the timing of germination responds to neighbors, how those responses affect the facilitation-competition balance individuals experience, and how that balance in turn affects fitness and demography. More generally, it asks how the timing of germination and other types of emergence affect the facilitation and competition that individuals experience throughout their lives.

I used laboratory, greenhouse, and field experiments to examine how the timing of germination in the winter annual Arabidopsis thaliana (Brassicaceae) responds to cues of neighbors and how those responses affect interactions with neighbors. I then developed a mathematical model of population growth in an annual plant to examine how intraspecific facilitation and competition over ontogeny affect the optimal degree of investment in dormancy (i.e., delayed germination) in variable environments.

My experiments revealed that seeds of A. thaliana typically delay germination in response to neighbors and that these responses can promote facilitative interactions and reduce competitive ones with neighbors. Selection against delayed germination, which occurs because of stress later in the season, can be mitigated by facilitation. Further, delaying germination can be beneficial by increasing the difference in sizes between seedlings and their neighbors, which may promote resource partitioning. In the theoretical study, I found that increasing the degree of investment in the fraction of dormant seeds (i.e., delaying germination) can promote the persistence of populations that experience both facilitation and competition in variable environments. This occurs because increased dormancy prevents high juvenile densities that promote facilitation and consequently limit reproduction in large populations. The findings of this research indicate that plant-plant interactions depend strongly on temporal context, and they reveal that the facilitation-competition balance determined by temporal variation in stress plays a key role in how germination and dormancy traits will evolve in variable environments.

This dissertation examines the processes that generate phenotypic variation in life cycles in seasonal environments. Collectively, a life cycle describes the stages an organism passes through during a generation. The timing, or phenology, of these transitions is often influenced by both environmental and allelic variation. Using the model organism Arabidopsis thaliana and both empirical and modeling approaches, I examine how correlations between life-cycle transitions, environment-dependent allelic effects, and epistasis generate patterns of life-cycle variation both within and between generations. In my first chapter, I use experiments to determine that many combinations of genetic, environmental, and developmental factors can create similar germination phenotypes, that maternal effects can influence phenotypes more than genetic differences, and that cross-generational effects can reduce variation in germination timing despite variation in flowering and dispersal time. In my second chapter, I use a modeling approach to consider the entire life cycle. I find that environmental variation is a major driver of phenotypic variation, and that considering the known geographic distribution of allelic variation across the range improves the match of model predictions to phenotypes expressed in natural populations. Specifically, variation in dormancy generated in the previous generation is predicted to cause life-cycle differences within a location, and the geographic distribution of allelic variation in dormancy interacts with local climatic environments to canalize an annual life history across the range. Finally, I test if allelic and environmental variation that affects early life stages can influence the environment experienced during reproduction. This environment determines both the time available for reproduction and the environment experienced during senescence. By implementing simple survival rules for flowering plants in the model, I show that time available for a plant to reproduce depends on earlier phenological traits and varies widely from year to year, location to location, and genotype to genotype. If reproductive trade-offs that underlie the evolution of senescence are environmentally sensitive, these results suggest that genetic variation in earlier life-stage transitions might shape senescence rates and whether they are environmentally responsive. In sum, my dissertation demonstrates the importance of pleiotropy, environment-dependent allelic expression, and epistasis in defining life-cycle variation, and proposes a novel way of predicting these relationships and complex life cycles under seasonal conditions.