Genetics of Plasmodial Compatibility and Heterokaryosis in Didymium iridis

SUMMARY“Phenotypic” identicalness for certain compatibility loci appears to determine whether or not 2 Didymium iridis plasmodia will fuse when brought into contact. Based on almost 5,000 fusion tests, it has been firmly established that if Plasmodium X fuses with Y and Y fuses with Z, then X will also fuse with Z.Mass-spore F1 plasmodia almost always display the same compatibility phenotype as their parent Plasmodium. Mass-spore-derived plasmodia are probably always heterokaryotic, containing a large number of different nuclear types.Two types of heterokaryons, other than those involving plasmodial compatibility alleles, were created. One type consisted of 2 different sets of plasmodial color factors while the other brought together 2 sets of mating type alleles.Three different homothallic isolates of D. iridis were studied, and all intra-isolate plasmodial combinations were compatible, but all inter-isolate ones were incompatible. It is concluded that the homothallic, as well as the heterothallic isolates, display a plasmodial compatibility system.