ABSTRACT Molecular Microbiology, in press.

Nine genes that are up-regulated during sexual development in the heterothallic oomycete, Phytophthora infestans, were identified by suppression subtractive hybridization. Three genes showed very low but detectable expression in vegetative hyphae and became induced about 10 to 100-fold early in mating, before gametangial initials appeared. The remaining six loci were not induced until later in mating, coincident with the formation of gametangia and oospores, with induction levels ranging from 60 to >100-fold. Only one late-induced gene had any detectable expression during vegetative growth. Five of the genes were single-copy while four were members of multigene families. Sequence analysis revealed that one of the late-expressed genes had similarity with members of the pumilio family of RNA-binding proteins. A second late gene resembled an extracellular glycoprotein from P. megasperma that elicits defense responses in non-host plants, although the P. infestans gene appeared to encode a larger protein. Each of the mating-induced genes were also expressed in a self-fertile strain, which was an apparent heterokaryon. However, while in normal matings two divergently-sized mRNAs were derived from the elicitor-like gene, only one size of transcript was detected in the self-fertile strain. In addition to the mating-induced genes, two extrachromosomal RNA elements were identified.