Plant hybrid zones are common and often represent natural experiments in which the genetically based defenses of plants have been mixed and are ideal for examining the ecological and evolutionary interactions of plants and herbivores as well as the mechanisms of resistance. This study will examine how the gall aphid, Pemphigus betae, selects hosts and responds to narrowleaf cottonwood, Populus angustifolia, and natural hybrids with Fremont cottonwood, P. fremontii. Two interrelated sets of questions will be asked. The first concerning the ecology, evolution and biogeography of this interaction will address a. over many replicate river drainage systems are hybrid plants and zones "sinks" for herbivores such that a few susceptible plants support most of the pest population?, b. if a clone (i.e. a tree and its asexually derived progeny) is susceptible to aphids, will it also be susceptible to other kinds of herbivores such as elk and woodboring beetles ?, and, c. does the presence of extremely susceptible clones prevent herbivores from successfully attacking resistant clones or do susceptible plants simply serve as reservoirs from which they attack the rest of the population? The second question, to address the mechanisms of resistance and susceptibility asks , a. having sucessfully developed molecular genetic techniques (RFLPs) for this system, can the genes for resistance and/or susceptibility be marked and trees screened for resistance in the pure and hybrid zones, b. since individual clones can be genetic mosaics (i.e., different parts of the same tree are genetically different) what are the patterns of mosaicism between hybrid and pure host clones and how are they related to stressful growing conditions?, c. because individual clones can also be developmental mosaics of resistance (i.e., different branches of the same tree or trunks of the same clone vary in resistance as a function of age), how do hybrids and pure hosts vary in their developmental resistance? and, d. because physiological sink-source relationships affect the resistance traits of individual clones, are they also important in affecting patterns of developmental resistance within individual clones.
