Date of Degree
PhD (Doctor of Philosophy)
Andrew A. Forbes
Phytophagous (plant-feeding) insects are extremely species-rich and typically display tight host associations (meeting and mating on or near their host plant) with one or a small number of hosts. This specialized lifestyle can promote diversification through assortative mating, ultimately leading to genetically differentiated host races (host associated differentiation; HAD). It has been shown that HAD can cascade up to the parasitic wasps (parasitoids) that utilize the phytophagous insects as hosts. Cascading HAD occurs when there is genetic differentiation among parasitoids as a result of differential host plant use by their host insects. Thus, host switching can promote parasitoid diversification as well.
Here, I present three studies designed to help understand aspects of parasitoid shifts to novel hosts and environments. All of the studies in this dissertation utilize the Rhagoletis complex of flies and their associated parasitoids. Specifically, I address i) the role of subtle trait variation and environmental context in predicting successful parasitoid host shifts; ii) whether parasitoid host discrimination (a trait that can influence host shifts) is an innate or learned behavior; and iii) whether contemporary patterns of host shifts among parasitoids are echoed by historical host shifts in cophylogenetic analyses of host and parasitoid genera?
Towards my first aim, I present a phenomenological model developed to predict successful host shifts by parasitoids. The simulations of the model explore how environmentally mediated traits can affect successful parasitoid colonization of a new host. For my second aim, I hypothesize that behaviors impacting parasitoid host plant preferences host shifts will be genetically based rather than a learned behavior. Shifting to a new host plant has been shown to cause reproductive isolation in phytophagous insects because of strong fidelity with their host plant. Parasitoids, however, have no direct contact with the host plant as they develop entirely within the host insect. The differences in life history traits could result parasitoid host shifts being driven by random changes in host preferences. I present preliminary results suggesting that parasitoids preferentially respond to their ancestral host plant’s olfactory cues, suggesting that host preferences have a genetic basis. Finally, I present a cophylogenetic analysis of Rhagoletis hosts and their parasitoids. I find that cospeciation is the most common coevolutionary event, although there is evidence of recent host shifting that contributes to current parasitoid species diversity. The results of these studies can help us understand how host shifts can act as a potential mechanism driving diversification in parasitoids.
Insects are tremendously diverse, amounting to 60-80% of all animals on earth. About 45% of all described insects are plant-feeding (phytophagous) or parasitic insects. Phytophagous insects and the parasitic wasps (parasitoids) that feed on them are often highly specialized, utilizing one or a small number of hosts. It has been hypothesized that the species diversity of phytophagous insects is due to their ability to adopt new host plants (host shift). It is possible that host shifts can promote diversity among parasitoids as well. My dissertation research focuses on parasitoids of Rhagoletis fruit flies, and addresses three questions regarding how shifts to new host environments affects parasitoid species diversity. First, I present a mathematical model developed to predict successful host shifts by parasitoids based on environmentally mediated traits. I find that small changes in a single trait mediate the outcome of comeptition. Second, I hypothesize that behaviors impacting parasitoid host shifts will be genetically-based rather than learned behaviors. I find that parasitoids preferentially respond to their ancestral host plant’s olfactory cues, suggesting that host preferences are genetic. Finally, I use a coevolutionary analysis of several parasitoid genera to identify the relative imporance of host shifts on parasitoid species diversity. The results suggest that species diversity is primarily a result of parasitoid cospeciation with their host insects, with some recent host shifting events contributing in the near term. The insights gained from these studies of host-parasitoid systems can help us understand mechanisms driving global patterns of biological diversity.
publicabstract, Coptera, Diachasma, evolutionary ecology, host-parasitoid relationships, Rhagoletis, Utetes
Copyright 2015 Gabriela Hamerlinck