The factors Governing the Maintenance of Gynodioecy in Sidalcea oregana ssp. spicata (Malvaceae)

DOCTORATE DISSERTATION:Gynodioecious populations contain both hermaphrodite and female plants. This dimorphism results in an inequality in the potential genetic contributions of the sex morphs. Females can contribute genes only through ovules, whereas hermaphrodites can contribute through both pollen and ovules. Thus, females must experience a compensatory advantage to be maintained. This advantage could be realized through higher seed production or production of higher quality seeds. The focus of this dissertation research was to evaluate the factors that could be operating to maintain females in gynodioecious Sidalcea oregano ssp. spicata (Malvaceae) at Sagehen Creek Field Station, in the Sierra Nevadas. I investigated the importance of both pollination and resource allocation in determining patterns of seed production by the sex morphs. The sex morphs receive equivalent and sufficient levels of pollination but females produced 10 percent more seeds than hermaphrodites. The sex morphs allocated similar total amounts of resources to reproduction on a whole-plant basis but females allocated a greater proportion to seeds while hermaphrodites allocated proportionately more to flowers. To study the factors that influence seed and seeding quality in the greenhouse and in the field I used a breeding design which allowed me to separate the effects of maternal sex and inbreeding. After the first year of growth in the greenhouse a selfed hermaphrodites offspring had an expected cumulative fitness of only 74 percent that of an outcrossed offspring. When levels of out-crossing were equivalent, offspring of females were 91 percent more fit than progeny of hermaphrodites. It is unlikely that resource provisioning differences created this substantial fitness differential; seeds produced by the sex morphs did not differ significantly in biomass, nitrogen or phosphorus content. It is possible that this fitness differential was due to genetic (cytoplasmic) maternal effects. Under different conditions of resource availability, however, sex-specific resource allocation could result in provisioning differences that do affect fitness. I manipulated resource availability by adding NPK fertilizer to plants in the field. The sex morphs responded differently to these conditions in terms of flowering time, seed production, and per seed allocation. Hermaphrodites prioritized pollen allocation over seed allocation. Sex-specific allocation patterns can be interpreted as fitness optimizing strategies, and may play an important role in the maintenance of gynodioecy in Sidalcea, especially if resources vary.