The evolution of reproductive isolating barriers that prevent gene flow between species is essential to the process of speciation. One such barrier is intrinsic postzygotic isolation, which proceeds as hybrid sterility or inviability, and is commonly attributed to Dobzhansky-Muller genic incompatibilities. Here, deleterious interlocus interactions occur between incompatible alleles of complementary genes when brought together in the genome of a hybrid. Although these hybrid incompatibilities are widespread, having been identified in mammals, fish, plants and fungi, still relatively little is known about the nature of the genes involved. In the model plant species Mimulus, a Dobzhansky-Muller incompatibility exists between two populations of the yellow monkey flower, Mimulus guttatus, in which the interaction between a single gene from a copper tolerant population, Copperopolis, and a small number of polymorphic genes from a second non-tolerant population, Cerig-y-drudion, results in hybrid necrosis in the F1. Hybrid necrosis, a form of hybrid inviability with phenotypic characteristics strongly similar to those of plants responding to pathogen attack, is a common barrier preventing hybridization in plants. As well as being of interest in terms of evolution, hybrid necrosis has practical implications in plant breeding as it prevents the combining of desirable traits from related species in commercial cultivars. In the cross between Copperopolis and Cerig-y-drudion, copper tolerance, conferred by a single major gene, and hybrid necrosis are tightly linked but the independent or synonymous nature of the gene(s) in the Copperopolis population that contribute to these two characteristics is unknown. A key aim of this thesis was to establish the nature of the single gene in Copperopolis that contributes to hybrid necrosis with regards to its linkage to copper tolerance. The gene for hybrid necrosis was found to be tightly linked to, but discrete from, the gene controlling copper tolerance. Three candidate genes for this hybrid necrosis locus were indentified: a Jumonji-domain containing protein with probable function as a methyltransferase, a glycosyltransferase and a possible phosphatase. Interestingly, the latter two have potential functional roles in the plant immune system. The second key aim of this thesis was to perform the first investigation into the small number of genes in the Cerig-y-drudion population that contribute to the crossing barrier. Two QTLs for hybrid necrosis were identified. One QTL on Chromosome 9 is responsible for around 20% of the hybrid necrosis whilst the second QTL on Chromosome 12 acts as an enhancer of the first QTL causing an additional 10% of necrosis. Interestingly both these QTLs contain R genes, further implicating the possible involvement of the plant immune system in this crossing barrier
