Classical and molecular genetics of Bremia lactucae, cause of lettuce downy mildew

Lettuce downy mildew caused by Bremia lactucae has long been a model for understanding biotrophic oomycete–plant interactions. Initial research involved physiological and cytological studies that have been reviewed earlier. This review provides an overview of the genetic and molecular analyses that have occurred in the past 25 years as well as perspectives on future directions. The interaction between B. lactucae and lettuce (Lactuca sativa) is determined by an extensively characterized gene-for-gene relationship. Resistance genes have been cloned from L. sativa that encode proteins similar to resistance proteins isolated from other plant species. Avirulence genes have yet to be cloned from B. lactucae, although candidate sequences have been identified on the basis of motifs present in secreted avirulence proteins characterized from other oomycetes. Bremia lactucae has a minimum of 7 or 8 chromosome pairs ranging in size from 3 to at least 8 Mb and a set of linear polymorphic molecules that range in size between 0.3 and 1.6 Mb and are inherited in a non-Mendelian manner. Several methods indicated the genome size of B. lactucae to be ca. 50 Mb, although this is probably an underestimate, comprising approximately equal fractions of highly repeated sequences, intermediate repeats, and low-copy sequences. The genome of B. lactucae still awaits sequencing. To date, several EST libraries have been sequenced to provide an incomplete view of the gene space. Bremia lactucae has yet to be transformed, but regulatory sequences from it form components of transformation vectors used for other oomycetes. Molecular technology has now advanced to the point where rapid progress is likely in determining the molecular basis of specificity, mating type, and fungicide insensitivity.