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 charac terize vegetative compatibility, heterokaryosis, and asexual gene exch ange in the fungal biological control agent Colletotrichum gloeosporio ides f.sp. aeschynomene (Cga). Several paired auxotrophic strains prod uced heterokaryotic colonies, but conidia recovered from the colonies comprised only the parent phenotypes. Analysis of mycelial plugs remov ed from along the colony radius demonstrated that heterokaryosis was l imited to the colony center. In host inoculation studies, only parenta l phenotypes were recovered from cocolonized lesions of the weed host Aeschynomene virginica. Based on pairings with nit mutants, 13 Cga iso lates were vegetatively compatible but were vegetatively incompatible with 13 C. gloeosporioides isolates obtained from other hosts. Althoug h heterokaryosis was demonstrated with Cga, no putative diploids or re combinant phenotypes were detected. Host specialization, vegetative co mpatibility, 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 risk s associated with the importation of an exotic biocontrol fungus or re lease of a genetically modified strain for biological control. (C) 199 4 Academic Press, Inc.