Towards a model of the genetic architecture of Phytophthora ramorum susceptibility in coast live oak
Richard S. Dodd1, Daniel Hüberli2, Vladimir Douhovnikoff2, Tamar Harnik2, Zara Afzal-Rafii3, and Matteo Garbelotto2
The appearance of SOD symptoms on coast live oak trees is spatially uneven within stands and is unrecorded over a large part of the natural range of this species. Among the many factors that could play a role in this uneven distribution of disease is genetic-based variation in levels of host susceptibility both at the local and at the population-wide levels. We have begun a series of studies to investigate whether variations in field observations of disease symptoms may be associated with genetic structure of coast live oak. 1. To investigate variation in susceptibility, we designed an in vitro system to assay response of excised shoots to inoculation of Phytophthora ramorum. Inoculum was placed under the bark and the excised shoots were grown on the greenhouse bench for about 3 weeks. Response to inoculation was assessed by measuring the size of the resulting lesion. We tested populations from southern California, from an east-west transect in central California and from northern populations in Marin County. All shoots responded to the inoculation by producing lesions, but those from southern California produced only small lesions that tended to be surrounded by a sharply demarcated boundary layer, suggesting a reaction by the plant to the pathogen. There was significant variation among individuals in all populations and the repeatability of this differential response is being tested. 2. An AFLP molecular study of range-wide populations of coast live oak is underway. Our preliminary data indicate a marked differentiation of southern California populations from those of central and northern California. Further inoculation studies are needed to test whether the north-south segregation of populations observed from the molecular data corresponds with a similar geographical division in responsiveness to inoculation with P. ramorum. 3. DNA fingerprints, using a large number of AFLP primers, are being analyzed for all inoculated individuals and also for a large number of field site individuals ranked as showing serious signs of infection, moderate infection and no signs of infection, to look for possible associations between the AFLP banding patterns and degree of infection.
1Department of ESPM, 145 Mulford Hall, University of California, Berkeley CA 94720; (510) 643-1635 dodd@nature.berkeley.edu
2Department of ESPM, 151 Mulford Hall, University of California, Berkeley CA 94720
3CNRS, Université d'Aix-Marseille III, 13397 Marseille, France
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