Castilleja

Abstract: It has been historically difficult to manipulate secondary compounds in living plants to assess how these compounds influence plant-herbivore and plant-pollinator interactions. Using a hemiparasitic plant that takes up secondary compounds from host plants, I experimentally manipulated secondary compounds in planta and assessed their effects on herbivores and pollinators in the field. Here, I show that the uptake of alkaloids in the annual hemiparasite Castilleja indivisa resulted in decreased herbivory, increased visitation by pollinators, and increased lifetime seed production. These results indicate that resistance traits such as alkaloids can increase plant fitness directly by reducing herbivore attack and indirectly by increasing pollinator visitation to defended plants. Thus, selection for production of secondary compounds may be underestimated by considering only the direct effect of herbivores on plant fitness.

Abstract: Summary White pine blister rust disease, caused by the introduced pathogen Cronartium ribicola, has severely disrupted five-needled pine ecosystems in North America. A 100-year effort to manage this disease was predicated in part on the premise that the pathogen utilizes only species of Ribes (Grossulariaceae) as alternate hosts on this continent. The current study presents the first conclusive demonstration that some species in the family Orobanchaceae (Pedicularis racemosa and Castilleja miniata) are functioning as alternate hosts in a natural ecosystem of North America. This finding has implications for improving our understanding of epidemiology, pathogen adaptation and host‐pathogen interactions within white pine blister rust.

Abstract: In some areas of sympatry, reproductively compatible plant species hybridize, but in other areas of sympatry, they do not and they remain reproductively isolated from one another. Explanations offered to explain patterns of hybridization that vary by population have usually focused on genetic or environmental factors. Instead, we examined whether different community contexts might change pollinator preference and constancy and thus influence the likelihood of hybridization among three Indian paintbrush species (Castilleja miniata, C. rhexifolia, and C. sulphurea). To determine whether visitation was context-dependent, we observed pollinator behavior in experimental arrays (constructed using flowering stems of the three Indian paintbrush species) in different contexts. Contexts were defined by which Castilleja species occurred in the immediate neighborhood of the arrays. Specifically, we asked, does visitation to particular species in the arrays depend on context? In general, each Castilleja species was preferred when it matched the surrounding community context, as is predicted by optimal foraging theory. More interestingly, pollinator constancy was weakened in the hybrid context (an area where the three species co-occurred with morphologically intermediate plants), which is likely to increase pollen flow among the species. Reduced pollinator constancy in hybrid zones could set up a positive feedback loop in which more flower diversity is created through hybridization, decreasing pollinator constancy, and leading to more hybridization. This self-reinforcing mechanism could lead to "hybridization hot spots" and to a patchy distribution of hybrid populations. We expect that this mechanism may be important in other animal-pollinated plant hybrid zones.

Abstract: I examined how the performance of Castilleja wightii (Scrophulariaceae), a generalist root parasite, is affected by the availability of different combinations of host species. In this greenhouse study, I focused on pairs of hosts consisting of either two leguminous host individuals (Lupinus arboreus; Fabaceae), two non-nitrogen-fixing hosts (Eriophyllum stachaedifolium; Asteraceae), or one individual of each of these species. Castilleja growth and reproductive performance were greatly improved by the simultaneous attack of two distinct host species, even though Castilleja grown with two Lupinus hosts had significantly higher nitrogen content. Different combinations of host species also strongly affected the growth of aphid colonies feeding on the Castilleja used in this experiment. Across all treatments, the growth of aphid colonies was positively correlated with the nitrogen content of the parasitic plants, which, in turn depended on the combination of hosts attacked. Aphid colonies feeding on parasites ...