The sexuality and population structure of Pythium sylvaticum

Sexual characteristics of 36 isolates of Pythium sylvaticum were investigated. Fourteen of 15 heterothallic males were also homothallic while only 2 of 21 females were homothallic. Four male and five female groups of isolates were delineated by similar reacting patterns under five different temperature and light conditions. Mature oospores formed in crosses varied with both male and female isolates and were not correlated with sexual strengths of reacting isolates. Three-to-one ratios of males to all other progeny occurred among 198 single-oospore progeny of two crosses involving non-homothallic parents. Most progeny were also homothallic. A pattern for the distribution of sexual characteristics of natural populations of P. sylvaticum is proposed based on characteristics of the 36 isolates.