Dres, Michele Anne; (2001) Gene flow between host races of the larch budmoth Zeiraphera diniana (Lepidoptera: Tortricidae). Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This project investigates the host race status of larch and pine associated Zeiraphera diniana populations via studies of actual gene flow, hybrid fitness, and mitochondrial DNA. The formation of host races - genetically distinct, host associated biotypes connected by gene flow - is an essential step in sympatric speciation via host shift, and their existence provides strong support for this theory. However, the identification of these forms has been complicated by the use of multiple host race definitions that are often ambigous in an empirical context. Therefore a new, empirically based definition is developed in and employed throughout this thesis. A novel requirement of this definition is that at least 1% actual gene flow (movement of genes via hybridisation) occurs between the races each generation, and leads to backcrossing. Other requirements include sympatry, genetic differentiation, and host fidelity. Previous studies have shown that larch and pine associated Z. diniana meet most host race criteria, but little was known about the level of gene flow between the races. The main findings presented here are that the level of actual gene flow (the movement of genes via hybridisation) between the biotypes is approximately 2.4% per generation, that backcrossing is likely, and therefore that they are indeed host races. The extent of actual gene flow was estimated from the combined probabilities of long range, pheromone-mediated cross attraction (measured in the field), and hybridisation in competitive situations (measured in the laboratory). Evidence of backcrossing was obtained from several experiments. Proportions of non-hybrid, hybrid, and backcross broods hatching, and surviving to final larval instars in the laboratory did not differ. Hybrid females were able to obtain matings with larch males in competitive conditions, and hybrids of both sexes are fertile. In addition, there was no evidence of mitochondrial DNA differentiation between the forms across approximately 800bp of mitochondrial cytochrome oxidase I (partial), tRNA-leucine, and cytochrome oxidase II (partial).

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