~~The induction by A2 strains of Phytophthora parasitica and Phytophthora palmivora of sexual reproduction in an A 1 strain of P. parasitica has been studied using synthetic medium with and without sterol, the strains being separated by a polycarbonate membrane. Production of the inducing a 2 hormone by the A2 strain does not appear to require sterol, but sterol greatly enhances the response to the inducing hormone by the A1 strain as well as being required for subsequent oospore development.
I N T R O D U C T I O NSexual reproduction in heterothallic species of Phytophthora requires the interaction of strains of two mating types, designated A1 and A2. This interaction has been studied by KO (1978KO ( , 1980 by having the paired complementary strains separated by a polycarbonate membrane. The membrane is not penetrated by the hyphae, but KO has shown that some substance must be produced by the A2 strain which diffuses through the membrane and induces reprodygtion in the A1 strain, leading to development of antheridia and oogonia and production of selfed oospores. KO has called this inducing substance a2 hormone. The A1 strain also produces an a1 hormone which induces oospore formation in A2 strains.Sexual reproduction in Phytophthora requires the presence of sterols in the medium. The fungi are unable to synthesize sterols, and it has been argued that the required sterols are metabolized to steroid hormones which control sexual development (Elliott, 1982; Elliott 8z Sansome, 1977). The distinction between induction and subsequent sexual development is important. The later stages of development (meiosis and oospore formation) show specific steroid requirements (Elliott 8z Sansome, 1977). The use of KO's system now enables us to examine whether the induction process is also sterol dependent. KO's experiments provide no information on this point as V8 juice medium was used throughout. We report experiments in which the strains were grown on synthetic medium, with and without sterols, which were intended to show whether the production of a hormone requires the presence of sterol.
M E T H O D SThe strains used were: A1 mating type -Phytophthora parasitica (P. nicotianae var. parasitica) P991; A2 mating type -P. parasitica P73 1 and P. palrnivora P255 (IMI 189732).The medium contained, per litre: 10 g sucrose, 1.5 g monosodium glutamate monohydrate, 0.5 g KH,PO,, 0.25 g MgS0,.7H,O, 0.1 g CaCl,, 1 ml trace element solution (Elliott 8c Knights, 1981), 1 mg thamin hydrochloride, 1.0 g Tween 80, 0 or 10 mg cholesterol or sitosterol, 10.0 g Oxoid agar no. 3. The Tween 80 and sterol were dissolved in acetone and added after autoclaving (1 ml of each solution per 100 ml medium).Medium was dispensed in 35 mm plastic Petri dishes, 3 ml per dish, inoculated centrally and incubated at 25 OC.Cultures of the required age (generally 6 d for both mating types) were removed whole from the small plastic dishes and placed one above the other in 9 cm glass Petri dishes with a 47 mm polycarbonate membrane (CPR 0.2 pm; Nuclepore Corporati...