ABSTRACT Fungal Genetics and Biology 21, 188-197.

Mating interactions in Phytophthora infestans, a heterothallic oomycete, are regulated by sexual compatibility loci and a system of sexual preference. The development of gametangial associations in this species were examined using matings between strains containing a beta-glucuronidase (GUS) transgene and nontransformed strains. The GUS gene enabled the facile assessment of whether oospores resulted from hybridization between isolates or from self-interactions and allowed measurements of the sexual preference of strains. Depending on the strains being mated, from 5 to 99% of the total oospores formed resulted from outcrossing. In such hybrid pairings, A1 and A2 field isolates generally acted as predominant females and males, respectively. Nevertheless, genetic analyses indicated that sexual preference and compatibility type were determined by unlinked loci. The sexual preference of most strains varied depending on the relative preference of their mating partners. Both sexual preference and the frequency of outcrossing versus self-interactions varied with the position of gametangia within the mating culture.