Ecology and Population Biology
Three Clonal Lineages of Phytophthora cinnamomi in Australia Revealed by Microsatellites
M. P. Dobrowolski, I. C. Tommerup, B. L. Shearer, and P. A. O'Brien
First and fourth authors: School of Biological Sciences and Biotechnology, Murdoch University, Perth, W.A., 6150, Australia; second author: CSIRO Forestry and Forest Products, Perth, P.O. Box 5, Wembley, W.A., 6913, Australia; and third author: Science and Information Division, Department of Conservation and Land Management, Como, W.A., 6152, Australia. Accepted for publication 30 December 2002.
ABSTRACT Dobrowolski, M. P., Tommerup, I. C., Shearer, B. L., and O'Brien, P. A. 2003. Three clonal lineages of Phytophthora cinnamomi in Australia revealed by microsatellites. Phytopathology 93:695-704. The genetic structure of populations of Phytophthora cinnamomi, a pathogen of an enormous variety of woody plants, was investigated using microsatellites. Three intensively sampled disease sites in southwest Australia were analyzed along with a large culture collection of Australian isolates and some isolates from elsewhere in the world. The mutation in the four microsatellite loci analyzed revealed spatial patterns at the disease sites that correlated with the age of the infestation. Only three clonal lineages were identified in Australian populations and these same clonal lineages were present in worldwide populations, where it is suggested that a limited number of clonal lineages have spread in most regions. No evidence for sexual reproduction between these clonal lineages in Australia has been found even though the pathogen has the opportunity. Instead, mitotic recombination is frequent within the clonal lineages. The implications of this are discussed. Additional keywords: mitotic crossing over, oomycete.
Clonality is common in fungi and oomycetes because of the prominent role that asexual reproduction plays in their life histories (2). The use of molecular techniques for studying population genetic structure has confirmed many examples of this in fungal pathogens, for example, Candida albicans populations in humans (31) and worldwide populations of Phytophthora infestans (17), which are both diploid pathogens. In both these organisms, genetic variation that could not be attributed to sexual reproduction has been found and mitotic recombination suggested as a possible cause because of their diploid nature (17,38). However, conclusive evidence for mitotic recombination in natural populations of clonal fungi is inadequate (22), even though it has been observed in the laboratory (21), usually because of a lack of highly polymorphic genetic markers. Genetic variation generated by mitotic recombination may explain some phenotypic variation arising within clonal lineages of diploid fungi and oomycetes. The diploid oomycete Phytophthora cinnamomi Rands is a root and collar rot pathogen of an enormous variety and number of mainly woody plants (37,39). In southern Australia, the dieback disease it causes has a devastating impact on native ecosystems, because many of the dominant plant species show no resistance. P. cinnamomi is a hermaphrodite that requires the interaction of isolates of opposite mating type (A1 and A2) to stimulate sexual reproduction although it readily reproduces asexually by mycelial growth and motile zoospores (32). It was most likely introduced into Australia following European settlement and into Europe early in the 19th century (4), with Southeast Asia suggested as the origin of the species from biological and historical perspectives (40). Investigations on the genetic structure of P. cinnamomi populations around the world have relied on the use of isozyme polymorCorresponding author: M. P. Dobrowolski E-mail address: dobrowol@murdoch.edu.au

下一页