ELUCIDATING THE POPULATION STRUCTURE OF INTRODUCED PLANT PATHOGENS < /b >
ELUCIDATING THE POPULATION STRUCTURE OF INTRODUCED PLANT PATHOGENS
Cooperative Research Centre for Tropical Plant Pathology, The University of Queensland 4072 Brisbane, Australia
Background and objectives
The genetic structure of many fungal plant pathogens is unknown. In addition, we know even less about geographic differentiation and migration of pathogen populations in different parts of the world. Most crop plant species have been introduced into Australia in the last two centuries followed by the introduction of numerous plant pathogens. Good historic and recent records exist concerning the introduction and spread of some of these pathogens. Once introduced it is remarkable how rapid most of these fungal pathogens spread and how quickly they adapt to local agricultural practices. Because of limited introductions the genetic basis of these pathogen populations are expected to be rather narrow compared to the populations where they migrated from. The aim of this investigation was to elucidate the population structure of some of the Phytophthora populations introduced into Australia
. Results and conclusion
The homothallic Phytophthora sojae was first identified in Australia in 1979. At that time only race 1 was found and the fungus spread rapidly to most soybean growing regions within a few years while new races developed able to overcome the resistance genes deployed in the local soybean cultivars. The genetic structure of diploid Phytophthora populations is most effectively studied using a number of low copy RFLP markers. Such markers revealed that levels of genotypic diversity were low and one clonal line, or inbred line, dominated the pathogen population over time since the introduction. New races developed locally within this single clonal line, most likely due to mutation. Due to the lack of genetic diversity low levels of outbreeding among different races are very difficult to detect in these monomorphic populations. A small population from the USA showed a much greater level of genetic diversity and identical races to their Australian counterparts occur in different genetic backgrounds providing further evidence for the evolution of virulence within Australia .
Phytophthora cinnamomi is heterothallic and has been in Australia for over a century and is devastating the native vegetation in some parts of the country and causes severe problems in many horticultural and nursery industries. Levels of gene and genotypic diversity again are low after analysis of the populations with low copy RFLP markers. Although both mating occur here the population structure does not support the occurrence of sexual reproduction. Typically a few clonal lines occur at high frequency in the population. Comparison with endemic populations in other parts of the world is often not possible due to the lack of population genetic data from these regions.
1.Drenth, A., Whisson, S.C., Maclean, D.J., Irwin, J.A.G., Obst, N.R. & Ryley M.J., 1996. Phytopathology 86: 163-169.