Symbiotic associations between animals and microbes are widespread in nature, yet the factors controlling the abundance and distributions of particular symbionts are mostly unknown. Vertically transmitted (VT) symbionts can invade host populations by providing net benefits to hosts. While most beneficial symbiotic interactions that have been characterized are nutritional, other major pathways likely exist that facilitate symbiont transmission within host populations. Natural enemies, for example, are important selective forces in shaping the life history of many invertebrates and hosts may benefit from collaborations with microbes to aid in their defense.In this dissertation I have addressed the role of VT, facultative (= secondary) symbionts (SS) of Acyrthosiphon pisum in mediating interactions with an important natural enemy, the parasitic wasp, Aphidius ervi. I found that, in a common genetic background, two A. pisum SS (R- and T-type SS) confer resistance to A. ervi, by causing mortality to developing wasp larvae. Defensive mutualisms with microbes provide a mechanism for the spread and persistence of VT symbionts.A. pisum superinfected with both R- and T-type SS were found to be more resistant to parasitism that those singly infected with either SS. Despite this added benefit to resistance, R + T-type superinfections were rare in a survey of A. pisum symbionts, likely attributable to severe fecundity costs. R-type densities increased dramatically in superinfected hosts and over-proliferation of SS may result in poor aphid performance. Thus, interactions among the symbionts themselves also likely play a critical role in determining the distributions of symbionts in nature.I also found that four T-type isolates from A. pisum, and one from another aphid species, all conferred resistance to parasitism in the same A. pisum host background. The levels of resistance varied greatly among isolates, including one that conferred nearly complete resistance. A single T-type isolate was also found to confer similar levels of resistance in five A. pisum backgrounds. These results indicate that SS-mediated resistance is a general phenomenon in A. pisum and that the SS isolate is more important in determining the level of resistance than is the aphid genotype or interaction between isolate and aphid genotype.
| Identifer | oai:union.ndltd.org:arizona.edu/oai:arizona.openrepository.com:10150/194228 |
| Date | January 2005 |
| Creators | Oliver, Kerry M. |
| Contributors | Hunter, Martha S., Hunter, Martha S., Moran, Nancy A., Madisson, David R., Papaj, Daniel R. |
| Publisher | The University of Arizona. |
| Source Sets | University of Arizona |
| Language | English |
| Detected Language | English |
| Type | text, Electronic Dissertation |
| Rights | Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. |
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