The ecology and evolution of life history trait divergence in Panicum hallii

Exploring the process of local adaptation is important to understand how environmental variation can drive phenotypic and genetic differentiation as a response to selection. One hypothesis is that local adaptation results from genetic trade-offs in which locally favored alleles reduce fitness elsewhere to provide environment specific fitness advantage. However, studies that aimed to explain local adaptation in plants were mostly done by transplanting seedlings and avoided the important aspects of divergence in earlier life stages. In this thesis, I assess local adaptation from seed and seedling perspective using Panicum hallii, a C4 perennial grass endemic to North America. This is a wonderful system to study local adaptation as it spreads to a wide range of climatic conditions. By combining field and lab-based experiment, I showed that the seed and seedling transition is important for adaptation. The pattern of genetic architecture at seed and seedling traits suggests that the differentiation was primarily due to genetic pleiotropy or linkage disequilibrium. I also discovered the evidence of antagonistic pleiotropy at two loci for seed size and dormancy traits (Chapter-1). I later tested whether the seed size and dormancy trait combination evolved as correlational selection by doing a complex reciprocal transplantation at two representative habitats. My data showed that both seed size and dormancy traits are important for adaptation to native habitats, suggesting that correlational selection is important for adaptation to different habitats. I also showed that soil surface disturbance had a significant effect on the recruitments, plants adapted to high competitive habitat was favored in undisturbed plot and the opposite was observed for the plants adapted in dry, calcareous soil (Chapter-2). Finally, I looked at natural diversity population to estimate the impact of different seed size strategies for adaptation in different habitats. Our data exhibited that the ecotypic differentiation in P. hallii were strongly associated with seed size and seed size was under strong selection pressure for adapting at variable sites (Chapter-3). Overall, this dissertation showed that seed and seedling traits are extremely important for adaptation in different habitats and the variation in these traits significantly impact life history strategies in P. hallii.