Evidence for the production during mating of factors involved in suppression of asexual sporulation and the induction of ascocarp formation in Pyrenopeziza brassicae

Abstract

A simplified procedure for the extraction of a sex morphogen(s) (SF) from mated cultures of Pyrenopeziza brassicae is described. It is confirmed that inclusion of low concentrations of SF in the culture medium leads to inhibition of asexual sporulation in unmated cultures of P. brassicae and the stimulation of the formation of immature (sterile) apothecia. In mated cultures the formation of both immature and mature apothecia is stimulated by the presence of SF. Using a new rapid bioassay for SF it is established that most SF is produced by mated cultures after four weeks of incubation, that asexual sporulation of P. brassicae decreases with increasing concentrations of SF and that in mated cultures the production of apothecia is dependent on the concentration of SF in the medium. Finally, the period between three and six days of growth is shown to be critical for exposure of cultures to SF to obtain inhibition of asexual sporulation or stimulation of sexual sporulation. Different isolates of P. brassicae exhibit different levels of sensitivity to SF. The characteristics of four classes of sexually deficient mutants are described. Class A mutants are sterile and do not respond to SF. Class B mutants are also sterile, but do respond to SF. Class C mutants are sterile when mated in the absence of SF, but achieve fertility when mated on medium supplemented with SF. Class D mutants do not respond to SF unless mated with a wild-type isolate of opposite mating type. It is concluded that the data are consistent with the hypothesis that during mating in P. brassicae factors are produced that, at low concentrations, lead to a marked reduction in asexual sporulation and promote the development of ascocarps independent of mating.