The Use of Vegetative Compatibility and Heterokaryosis to Determine Potential Asexual Gene Exchange in Colletotrichum gloeosporioides

Auxotrophic and nitrate nonutilizing (nit) mutants were used to characterize vegetative compatibility, heterokaryosis, and asexual gene exchange in the fungal biological control agent Colletotrichum gloeosporioides f.sp. aeschynomene (Cga). Several paired auxotrophic strains produced heterokaryotic colonies, but conidia recovered from the colonies comprised only the parent phenotypes. Analysis of mycelial plugs removed from along the colony radius demonstrated that heterokaryosis was limited to the colony center. In host inoculation studies, only parental phenotypes were recovered from cocolonized lesions of the weed host Aeschynomene virginica. Based on pairings with nit mutants, 13 Cga isolates were vegetatively compatible but were vegetatively incompatible with 13 C. gloeosporioides isolates obtained from other hosts. Although heterokaryosis was demonstrated with Cga, no putative diploids or recombinant phenotypes were detected. Host specialization, vegetative compatibility, and lack of a demonstrated parasexual cycle may serve to genetically isolate Cga from other populations of C. gloeosporioides. Evidence of asexual gene exchange can be used to assess potential risks associated with the importation of an exotic biocontrol fungus or release of a genetically modified strain for biological control.