Parasitic and pathogenic lifestyles have evolved repeatedly in eukaryotes (93). Several parasitic eukaryotes represent deep phylogenetic lineages, suggesting that they feature unique molecular processes for infecting their hosts. One such group is formed by the oomycetes. Traditionally, due to their filamentous growth habit, oomycetes have been classified in the kingdom Fungi. However, modern molecular and biochemical analyses suggest that oomycetes have little taxonomic affinity with filamentous fungi but are more closely related to brown algae (heterokonts) in the Stramenopiles, one of several major eukaryotic kingdoms (2, 56, 93). The most notable and beststudied oomycete species is Phytophthora infestans, the Irish famine pathogen. P. infestans causes late blight, a devastating and reemerging disease of potatoes and tomatoes (4, 17, 18, 82, 90–92). Other oomycetes include destructive plant and animal pathogens, as well as saprophytes that are beneficial to the environment (56). Despite their peculiar phylogenetic affinities and economic importance, oomycetes were chronically understudied at the molecular level. However, in recent years, increased awareness of the evolutionary history of oomycetes as unique eukaryotic microbes resulted in the emergence of a group of researchers specializing in oomycete genetics. The evolution of this community has been driven by the biological properties of oomycetes, which require alternative methodologies. Technical developments, such as routine DNA transformation, use of reporter genes, and genetic manipulation using gene silencing have facilitated the discovery and functional analyses of several interesting genes. With recent efforts in genomics and functional genomics and the resulting resources, genetic research on oomycetes has entered an exciting phase. This review provides an overview of oomycete biology, with a particular emphasis on molecular genetics. It describes the molecular and genomic resources available for these organisms and discusses the current status of, and future perspectives in, basic studies of pathogenic oomycetes. Unfortunately, too little is known about the molecular genetics of the pathogenicity of oomycetes
other than Phytophthora to warrant a comprehensive review. Therefore, the section on infection mechanisms focuses on Phytophthora. For additional information on various aspects of oomycete biology, the reader is referred to a number of recent reviews (4, 38, 39, 43, 82, 91, 92, 99, 100).
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