Hormonal regulation of sexual reproduction and mating type change in heterothallic Pythium splendens

Abstract

When different mating types of Pythium splendens were paired on opposite sides of a polycarbonate membrane, numerous oospores were produced by (+) but not (−) isolates, indicating that sexual reproduction of this fungus is a result of selfing of the (+) isolate in response to the stimulation by a hormone produced by the (−) isolate. Hormonal regulation of sexual reproduction in P. splendens was greatly affected by culture age and different isolates also differed in ability to produce hormone and to form oospores in response to the hormone. Four single-conidium subcultures of (+) isolate 461 of Pythium splendens produced oospores in sectors on fresh V-8 agar after being stored in sterile distilled water for 48–93 months. Single-spore cultures obtained from conidia produced on oospore sectors consisted of 293 (+), 3 (−) and 3 (±) type. All the 36 single-propagule colonies obtained from a stored culture originated from spherical structures smaller than conidia produced on V-8 agar. The mating type distribution of these colonies were 33 (+), 1 (−) and 2 (±). Single-conidium cultures from the ± type behaved either as (+) or (−). Hormone production and reception of the (−) variant were characteristic of the (−) wild type. Results suggested that oospore formation in single cultures by aged P. splendens is due to the appearance of the opposite mating type during the storage period and the (±) type is a transitional state in the process of mating type change.