Resistance and phenology in Brassica rapa: Multiple factors influence damage and its fitness impacts

Genetically based resistance traits are well established for Brassica rapa (Brassicaceae). This work builds on the existing understanding of resistance and examines interactions between phenology and resistance, fitness impacts of simultaneous above- and below-ground damage, and frequency dependent selection on resistance.; Chapter 1 reports work examining two populations in Orange County, California. Plants from a moister, higher herbivory site flowered earlier in the field and greenhouse due to genetic differences between populations in flowering time. When plants from these populations were infected with clubroot fungus (Plasmodiophora brassicae), dose-response curves for infection and death rates showed that plants from the early flowering population were more resistant and more tolerant to infection (Chapter 2). Flowering time may underlie these differences. Another experiment found additive fitness effects when clipping was added to infection.; Chapter 3 reports findings of a field experiment using phenotypically manipulated plants (resistant = undamaged, susceptible = clipped) that detected frequency dependent selection on cost-free resistance. Resistance was favored at all frequencies, and a compounding benefit to resistance was seen in an increase in selection favoring resistance as frequency of resistance increased.; Correlations between resistance (to aphids (Brevicoryne brassicae ), snails, and seed weevils) and phenology (leaf number, days to flowering, and flowering duration) and fitness impacts of resistance and phenology were examined in a quantitative genetics field experiment using paternal half-sib families (Chapter 4). Aphid and weevil resistance were genetically based and positively correlated. Snail resistance was not genetically based. Aphid damage correlated positively with leaf number; possibly aphids choose larger plants. Weevil damage correlated negatively with days to flowering; earlier flowering exposes pods to more of the weevil's attack period. Interactions between phenology and one herbivore's attack did not affect subsequent herbivores. Seed mass (fitness) was affected by aphids, days to flowering, and leaf number, but not by their interactions. Correlations between phenological variables and resistance suggest that selection on phenology could affect evolution of resistance. Here, selection deriving from aphid damage, leaf number, or days to flowering would favor joint evolution of high leaf number, early flowering, and high aphid resistance due to aphid resistance-leaf number and leaf number-days to flowering correlations.