ill.; Thesis (Ph. D.)--Ohio State University, 2001; Includes bibliographical references (leaves 100-114); Advisor: Andrea Wolfe, Evolution, Ecology and Organismal Biology Program
ABSTRACT Penstemon subgenus Dasanthera is a small group of 16 taxa distributed at high elevations in western North America. Hybridization is common in the subgenus when two or more species occur in sympatry. In this study, I examined evolutionary trends and the importance of hybridization and gene flow to the evolution of the subgenus. Phylogenetic relationships among members of Penstemon subgenus Dasanthera were assessed using ITS and matK sequence data, and inter-simple sequence repeat (ISSR) markers to elucidate biogeographic relationships and morphological trends in the subgenus. These data support previous hypotheses suggesting that the Cascade-Sierra Nevada lineage is derived from the northern Rocky Mountain lineage. Furthermore, there is a shift in growth form from deciduous perennials to evergreen, woody subshrubs concurrent with migration to the Cascade-Sierra Nevada mountains. Within the Cascade-Sierra lineage, P. newberryi and P. rupicola display a series of floral shifts that may represent adaptations for hummingbird pollination. The molecular data presented here also demonstrate the utility of dominant marker data for reconstructing phylogenetic relationships among closely-related species. To examine the potential outcomes of hybridization in the subgenus, ISSR and morphological markers were used to assess hybrid zone structure and patterns of gene flow between Penstemon davidsonii and P. rupicola on Wizard Island in Crater Lake National Park, Oregon (USA). On Wizard Island, a hybrid zone was found on the southwest portion of the island, in addition to unintrogressed populations of both species throughout the island. I surveyed three hybrid subpopulations from different localities in the hybrid zone to examine population structure throughout the hybrid zone. In each subpopulation, plants were categorized based on morphological characters diagnostic for P. davidsonii and P. rupicola. Using molecular data, the proportion of P. davidsonii-typical and P. rupicola-typical ISSR bands was calculated for each putative hybrid. Comparisons were made between hybrid categories and unintrogressed populations of P. davidsonii and P. rupicola using an approximation of Fisher's Exact Test to test for directionality of gene flow. These results indicate an asymmetrical pattern of gene flow and the potential for introgressive hybridization in these species. The observed asymmetry in the pattern of gene flow on Wizard Island could be the result of either endogenous selection pressures, in the form of pre-pollination barriers (ethological isolation) or post pollination barriers (pollen-tube growth rate or seed-siring ability), or exogenous selection in the form of differential fitness of hybrids in hybrid zones. I tested for differences in both pollinator visitation rates and pollinator behavior during visitation, and differential seed siring ability for P. davidsonii and P. rupicola on the observed pattern of gene flow on Wizard Island. Although the same suites of pollinators visit both P. davidsonii and P. rupicola, the frequency of visitation varies between the two species. Penstemon rupicola is visited primarily by pollen-collecting insects, including syrphid flies and sweat bees, that probably do not serve as effective iii pollinators, whereas P. davidsonii is visited primarily by leafcutter bees that may be good pollinators. From these studies, it is not clear how pollinator visitation affects gene flow in hybrid zones. Fruit set and seed set data demonstrate that P. rupicola shows a greater seed-siring ability relative to P. davidsonii for heterospecific crosses. However, seed germination rates were higher for P. davidsonii than for P. rupicola for all cross types, and highest for heterospecific crosses. These results suggest that P. davidsonii serves as a better maternal parent with regard to seed germination. The data presented suggest that the factors influencing hybrid zone structure on Wizard Island are complex, and probably include a combination of pre-pollination barriers, post-pollination barriers, and selection for fitness traits in the hybrid zone. iv
ABSTRACT Penstemon subgenus Dasanthera is a small group of 16 taxa distributed at high elevations in western North America. Hybridization is common in the subgenus when two or more species occur in sympatry. In this study, I examined evolutionary trends and the importance of hybridization and gene flow to the evolution of the subgenus. Phylogenetic relationships among members of Penstemon subgenus Dasanthera were assessed using ITS and matK sequence data, and inter-simple sequence repeat (ISSR) markers to elucidate biogeographic relationships and morphological trends in the subgenus. These data support previous hypotheses suggesting that the Cascade-Sierra Nevada lineage is derived from the northern Rocky Mountain lineage. Furthermore, there is a shift in growth form from deciduous perennials to evergreen, woody subshrubs concurrent with migration to the Cascade-Sierra Nevada mountains. Within the Cascade-Sierra lineage, P. newberryi and P. rupicola display a series of floral shifts that may represent adaptations for hummingbird pollination. The molecular data presented here also demonstrate the utility of dominant marker data for reconstructing phylogenetic relationships among closely-related species. To examine the potential outcomes of hybridization in the subgenus, ISSR and morphological markers were used to assess hybrid zone structure and patterns of gene flow between Penstemon davidsonii and P. rupicola on Wizard Island in Crater Lake National Park, Oregon (USA). On Wizard Island, a hybrid zone was found on the southwest portion of the island, in addition to unintrogressed populations of both species throughout the island. I surveyed three hybrid subpopulations from different localities in the hybrid zone to examine population structure throughout the hybrid zone. In each subpopulation, plants were categorized based on morphological characters diagnostic for P. davidsonii and P. rupicola. Using molecular data, the proportion of P. davidsonii-typical and P. rupicola-typical ISSR bands was calculated for each putative hybrid. Comparisons were made between hybrid categories and unintrogressed populations of P. davidsonii and P. rupicola using an approximation of Fisher's Exact Test to test for directionality of gene flow. These results indicate an asymmetrical pattern of gene flow and the potential for introgressive hybridization in these species. The observed asymmetry in the pattern of gene flow on Wizard Island could be the result of either endogenous selection pressures, in the form of pre-pollination barriers (ethological isolation) or post pollination barriers (pollen-tube growth rate or seed-siring ability), or exogenous selection in the form of differential fitness of hybrids in hybrid zones. I tested for differences in both pollinator visitation rates and pollinator behavior during visitation, and differential seed siring ability for P. davidsonii and P. rupicola on the observed pattern of gene flow on Wizard Island. Although the same suites of pollinators visit both P. davidsonii and P. rupicola, the frequency of visitation varies between the two species. Penstemon rupicola is visited primarily by pollen-collecting insects, including syrphid flies and sweat bees, that probably do not serve as effective iii pollinators, whereas P. davidsonii is visited primarily by leafcutter bees that may be good pollinators. From these studies, it is not clear how pollinator visitation affects gene flow in hybrid zones. Fruit set and seed set data demonstrate that P. rupicola shows a greater seed-siring ability relative to P. davidsonii for heterospecific crosses. However, seed germination rates were higher for P. davidsonii than for P. rupicola for all cross types, and highest for heterospecific crosses. These results suggest that P. davidsonii serves as a better maternal parent with regard to seed germination. The data presented suggest that the factors influencing hybrid zone structure on Wizard Island are complex, and probably include a combination of pre-pollination barriers, post-pollination barriers, and selection for fitness traits in the hybrid zone. iv