Abstract
Dormancy allows organisms to persist in temporally variable environments by suspending development during unfavorable conditions. The timing and expression of dormancy can vary widely among species, populations, and even among offspring from the same parent. Understanding how this variation arises is key to explaining adaptation to environmental heterogeneity. This dissertation examines how environmental variability, maternal effects, and population connectivity shape dormancy strategies and genetic diversity in Daphnia, using experiments and population genomic analyses across multiple biological scales.
In chapter 1, I ask whether variation in dormancy duration exists, and whether variation in the termination in dormancy is maternally affected. Using mesocosm-reared Daphnia pulex, I found that siblings from the same ephippium hatch at similar times even when separated, indicating maternal effects on dormancy duration. I also found that many ephippial embryos hatch early, prior to exposure to cues consistent with environmental change. This early emergence carries life history costs, including delayed reproduction and reduced sexual investment. In chapter 2, I investigate how dormancy strategies differ between species occupying habitats with different levels of environmental predictability. I compare dormancy investment and termination between D. pulex and D. obtusa, two species occupying habitats with different levels of environmental predictability. I found that D. obtusa invests more heavily in sexual reproduction, but both species respond similarly to cues that terminate dormancy. Cold exposure promotes hatching, desiccation suppresses emergence, and low levels of early hatching occur in both species, suggesting conserved mechanisms of dormancy termination despite different investment strategies. In chapter 3, I ask whether genetic diversity is maintained or lost within populations that live in environments that fluctuate through time. I examine genetic diversity in D. obtusa populations across eastern North America using samples taken from small pool environments. I find that multiple deeply diverged mitochondrial haplotypes coexist locally. Additionally, nuclear diversity reveals that the clonal lineages turnover within pools, migration exists between pools, and suggests that there is interbreeding between lineages of different mitochondrial haplotypes. These results suggest that dormancy and intermittent dispersal counteract genetic drift in highly dynamic environments.
Together, this work shows that dormancy is a flexible trait shaped by maternal effects, habitat predictability, and connectivity, with important consequences for both life history variation and the maintenance of genetic diversity.
