Selection of auxotrophic mutants and heterokaryosis in Alternaria alternata.

Abstract

Auxotrophic mutants were isolated from the Japanese pear pathotype, apple pathotype and saprophyte of Alternaria alternata, respectively, after mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine. Among these mutants, the occurrence of revertants (prototrophs) were detected only from the Met and Tyr mutants of the saprophyte of A. alternata. The two Arg mutants from A. alternata apple pathotype decreased their pathogenicity to apple, due to the decrease of their ability to produce AM-toxin. The other auxotrophic mutants from the apple pathotype and Japanese pear pathotype of A. alternata exhibited almost the same pathogenicity as those of the respective parent isolates. When the spores of Arg mutants from A. alternata apple pathotype were inoculated to apple leaves in the presence of arginine, they recovered pathogenicity and AM-toxin production. Heterokaryons were formed when complementary auxotrophs between the Japanese pear pathotype and the apple pathotype of A. alternata were paired on a minimal medium. All protoplasts prepared from complementing mycelia were heterokaryotic. Both heterokaryons, Cho1 (AK-toxin producer)×Cho3 (AM-toxin producer) and Cho2 (AK-toxin producer)×Arg1 (AM-toxin producer), were pathogenic to Japanese pear and weakly pathogenic to apple.