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Genetic Interaction of the Fusiform Rust Fungus with Resistance Gene Fr1 in Loblolly Pine

First author: Research Geneticist, U.S. Department of Agriculture (USDA) Forest Service, Southern Institute of Forest Genetics, 23332 Highway 67, Saucier, MS 39574; second author: Department of Forestry, North Carolina State University (NCSU), 840 Main Campus Drive, 2500 Partners II Bldg., NCSU Centennial Campus, Raleigh 27695; and third author: Research Geneticist and Project Leader, USDA Forest Service, Southern Institute of Forest Genetics, 23332 Highway 67, Saucier, MS 39574

We propose a method for defining DNA markers linked to Cronartium quercuum f. sp. fusiforme avirulence (Avr) genes. However, before this method can be successfully employed, a spore competition study was needed to determine the genetic composition of single pycnial drops and multiple drops on single galls when using the standard inoculation procedure, whether virulent (avr1) basidiospores ever predispose some resistant (Fr1/fr1) trees to infection by avirulent (Avr1) basidiospores, and whether avr1 and Avr1 basidiospores equally infect susceptible (fr1/fr1) trees. Results of this study suggest that multiple infections within a single gall are common using the concentrated basidiospore system, resulting on average in >4 infection events per tree. Due to multiple infections within a single gall, an individual pycnial drop cannot be assumed to consist of spores from only a single haploid pycnium. Roughly 57% of the drops harvested were found to consist of more than one haploid genotype, most likely due to the physical mixing of spores from genetically different pycnia. Most importantly, although multiple infections do occur in the formation of a single gall, there is no evidence to suggest that the genetics of the proposed gene-for-gene interaction are compromised. Only avr1 basidiospores were observed to cause infection on Fr1/fr1 trees, whereas both avr1 and Avr1 basidiospores were observed to cause infection on fr1/fr1 trees, albeit not at equal frequencies.

Additional keywords: gametothalli , infection haplotype , Pinus taeda .