UBC Theses and Dissertations
Evaluating the presence and introgression of the hybrid forest pathogen Cronartium x flexili Allen, Kiah
The recent discovery of a hybrid forest fungal pathogen, Cronartium x flexili, suggested to have arisen from sexual hybridization between the introduced C. ribicola and the native C. comandrae, was surprising because the parental species do not share hosts. Although the pathosystems of both parental species are well described, the impact of their hybridization is yet unknown. The purpose of this study is to determine the occurrence, level of hybridization and introgression of the hybrid pine stem rust Cronartium x flexili. A total of 831 samples from dikaryotic aecia of C. ribicola and C. comandrae were collected from 13 sites across British Columbia and Alberta and analysed both genetically and morphologically over two sampling seasons. Microscopic and genetic methods, including PCR, qPCR, and genotyping by sequencing (GBS), were used to identify hybrid samples. The results of these analyses indicate that C. x flexili is either no longer prevalent in areas where it was previously found, or else prevalent at such low frequencies that it has evaded detection in the sampling effort of this study. Two previously collected and extracted C. x flexili DNA samples were examined using a fixed loci analysis and did not demonstrate evidence of introgression, indicating that this hybrid does not facilitate gene flow between the introduced C. ribicola and the native C. comandrae. This suggests that when a hybridization event occurs between the parental Cronartium spp., first generation (F1) hybrids result but are not fertile and do not occur perennially. Some evidence suggests that this could be due to a number of factors including: low relative fitness of C. x flexili compared to the parental species if ‘hybrid breakdown’ occurs as explained by the Bateson-Dobzhansky-Muller incompatibility (BDMI) model; variations in local climate factors influencing life cycle parameters; or sexual incompatibility with the parental species. This work adds to the limited literature on the genetics of hybrid forest fungal pathogens and improves our understanding of the evolutionary mechanisms occurring when allopatrically evolved forest fungal species hybridize.
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