Dorken, Marcel MD

Darwinian fitness is determined by the outcome of allocations of energy and nutrients to plant growth, survival, and reproduction. Allocations to reproduction differ from allocations to growth and survival because the products of reproduction (pollen, ovules, and fruits) are dispersed away from the plant and so the resources used to produce them cannot be re-allocated. As a result, and because the total resource pool available to a plant is limited, allocations to reproduction can reduce future growth, reproduction, and/or survival, yielding fitness costs of reproduction. This study provides a novel approach to detecting the costs of reproduction through multiple reproductive currencies, including both biomass and nitrogen allocations and their temporal effects on current and future plant performance. My results demonstrate the environmental dependence of the costs of reproduction and show that under certain conditions, males can bear greater costs than females, contrary to the prevailing view.

Author Keywords: Cost of reproduction, dioecy, flower removal, life-history trade-offs, photosynthetic machinery, sexual dimorphism

Heterosis, expressed as phenotypic superiority over parental species, typically peaks in first generation hybrids (F1s), while later generations (F2 +) exhibit lower fitness. The decrease in hybrid fitness is called hybrid breakdown. The overall incidence of hybrid breakdown in invasive hybrid zones remains poorly understood. The Laurentian Great Lakes (LGL) region contains a hybrid zone comprised of: native Typha latifolia, Typha angustifolia, and hybrid Typha × glauca. F1 T. × glauca display heterosis and are invasive, while later generation hybrids are relatively rare. To investigate possible hybrid breakdown, I compared seed germination and plant growth of backcrossed and advanced-generation (F2) hybrids to F1s and T. latifolia. I found evidence for hybrid breakdown in F2s and backcrossed hybrids, expressed as reduced growth and germination rates. Expression of hybrid breakdown in F2s and backcrosses may explain their relative rarity in the LGL hybrid zone.

Author Keywords: Advanced-generation hybrids, Backcrossed hybrids, Hybridization, introgression, Invasive species, plant competition