Mating-type heterokaryosis and selfing in Cryphonectria parasitica

Selfing in the chestnut blight fungus, Cryphoneetria parasitica, occurs by two different genetic mechanisms. Most self-fertile isolates of C parasitica are heterokaryotic for mating type, and the progeny from selfing segregate for mating type. Further, we resolved mating-type (MA T) heterokaryons into homokaryons of both mating types by isolating uninucleate asexual spores (conidia). However, because ascospore progeny, with rare exceptions, are not MA T heterokaryons, C parasitica must lack a regular mechanism to maintain heterokaryosis by selfing. We hypothesize that heterokaryon formation may occur either because of recurrent biparental inbreeding, or by mating-type switching, possibly one involving some kind of parasexual process. The second mechanism found for selfing in C. parasitica occurred less frequently. Three single-conidial isolates (MAT-1 and MAT-2) selfed and produced progeny that did not segregate for mating type. It is currently not known if meiosis occurs during ascospore formation by this mechanism. (C) 2003 Elsevier Inc. All rights reserved.