Ecology and biological control of an apomictic invasive plant, \ud Chondrilla juncea (Asteraceae)

Graduation date: 2009A study consisting of three parts was undertaken to investigate how variation in species interactions, population genetic structure, epidemiological parameters, and plant breeding system may influence the ecology and biological control of an apomictic invasive plant, Chondrilla juncea (Asteraceae). Interactions between natural enemy species may modify their net effect on host plants, but little research has been done to examine how plant resistance influences species interactions in biological weed control. First, we performed common garden experiments with clonal accessions of C. juncea to compare a rust-susceptible weed biotype with a rust-resistant biotype, both of which are invasive in western North America. Inoculations with two biological control organisms, a rust fungus pathogen (Puccinia chondrillina) and an eriophyid gall mite (Eriophyes chondrillae), were applied separately and in combination to test if plant performance is modified by antagonistic or facilitative species interactions. We found no significant rust x mite interaction effects for several plant traits (shoot length, shoot dry biomass, fecundity, rosette growth, and rosette senescence), and therefore the two natural enemies appear to have independent and complementary effects on plant performance. However, rust inoculation reduced the total dry biomass of mite galls in the rust-susceptible biotype by indirect competition due to decreased growth of diseased shoots, but not in the rust-resistant biotype. Our results indicate that rust disease may have the potential to modify mite gall epidemiology and relative plant performance in mixed populations of resistant and susceptible C. juncea biotypes.\ud In the second part of the study, a monitoring survey of C. juncea was conducted in southwest Oregon during 2006 to investigate natural associations in the distribution of damage caused by P. chondrillina, E. chondrillae, and the cecidomyiid gall midge Cystiphora schmidti. Mite galling was the most aggregated type of damage both within and among field sites. Analysis of an allometric relationship of shoot growth (biomass versus stem area) indicates that rust disease reduced C. juncea growth but only at high stand density, and in general rust disease was the most randomly distributed type of damage within sites. There was no evidence from the field survey to support a species interaction between the gall mite and the rust pathogen, but negative associations were detected between midge galling and the damage caused by both of the other natural enemies, which may indicate species interaction via rust and/or mite interference with C. schmidti oviposition. However, this potential antagonism was found only at high stand density with relatively high levels of damage caused by each natural enemy, and so the overall impact of biological control probably is not strongly diminished by enemy interference. \ud The third part of the study was a mathematical model of inheritance and genetic equilibrium in meiotic diplospory, which is an unusual mechanism of gametophytic apomixis occurring in invasive populations of C. juncea and also in triploid dandelions (Taraxacum officinale). Crossing-over during prophase I of meiotic diplospory has the potential to generate genotypic diversity in apomictic lineages because it involves homologous recombination without fertilization ("subsexual reproduction"). I developed a stage-structured matrix model of inheritance in subsexual reproduction using a wide range of probabilities for recombination and mutation to assess the range of equilibrium conditions under which genotypic diversity is stable in obligately apomictic lines. The analysis supports predictions of genotypic diversity when rates of crossing-over and mutation are not highly imbalanced. Therefore subsexual reproduction remains mathematically plausible as a contributing factor to the quantitative variation in disease resistance and ecological traits of invasive C. juncea found in this study and others