OVERCOMING VEGETATIVE INCOMPATIBILITY WITHIN AND BETWEEN PATHOTYPES OF PSEUDOCERCOSPORELLA-HERPOTRICHOIDES BY PROTOPLAST FUSION

Vegetative incompatibility between isolates of Pseudocercosporella her potrichoides was detected using complementation testing on minimal med ium between auxotrophic mutants obtained following uv mutagenesis of c onidia. This analysis confirmed earlier data indicating the occurrence of vegetative incompatibility in this fungus. All combinations of W-t ype with R-type isolates were incompatible. Incompatibility was also s hown between the three W-type isolates tested. Three of the four R-typ e isolates showed full mutual compatibility, while a fourth isolate wa s fully compatible with only one of these strains. This latter isolate exhibited partial compatibility to a second isolate, depending on the auxotrophic phenotype of the strains tested, and was incompatible wit h the third isolate in all strain combinations. Using the protoplast f usion technique it was possible to overcome vegetative incompatibility in all pathotype combinations: W x W, R x R and W x R. Fusion frequen cy in incompatible W x W and W x R crosses was typically ten fold lowe r than in crosses between vegetatively compatible strains. Fusion of i ncompatible R-type isolates was obtained at a similar frequency to com patible strain combinations. Fusion products were not heterokaryotic, since isolated, uninucleate protoplasts were able to regenerate on min imal medium. Regeneration of protoplasts on diagnostic media containin g fungicides confirmed the presence of both parental genotypes in seve ral of the crosses. The use of fungicide resistance in combination wit h auxotrophic requirements allowed the selection of fusion products in crosses with fungicide sensitive wild-type isolates.