Evidence for segregation of somatic incompatibility during hyphal tip subculture of Rhizoctonia solani AG 4

Field isolates of Rhizoctonia solani are presumed to consist of heterokaryo tic cells, but the number of different nuclear types present in each cell i s unknown, nor is it known if there is any regulation of the heterokarotic state. To investigate the nature of heterokaryosis in AG 4, hyphal tip subc ultures of two strains were examined using vegetative compatibility as a ma rker. Upon subculture, each isolate could be placed into one of two VCGs, m embers of which were compatible within the group but incompatible with thos e of the other group and the parent strain. No other phenotype could distin guish these groups from each other or the parents. Pairing of hyphal tip is olates never resulted in regeneration of the parental strain. Examination o f nuclei in the tip cells of incompatible strains by DAPI staining and fluo rescence microscopy showed variation in nuclear numbers in different parts of the fungal colony and a significant tendency for some adjacent branch ti ps to have a similar number of nuclei. These results suggest a mosaic of nu clear, and perhaps mitochondrial, types within the colony, and that subcult ures of components of the mosaic retain their differences. This variation i mplies that regulation of nuclear number within a colony is loosely control led.