Selfing in the context of self-incompatibility: The mixed-mating system of the fungus Cryphonectria parasitica

The spatial and temporal dynamics of fungal breeding systems remain virtually unstudied, relative to the long tradition of work in plant breeding systems. Given the dearth of information on the natural breeding systems of fungi, it was my goal to conduct a detailed examination of the mating system of the fungus Cryphonectria parasitica.; Interest in the mating system of C. parasitica was generated by an initial study of a single population that was found to have a mixed mating system (both outcrossing and selfing) (Milgroom et al., 1993). Although this has been documented in many plants, it is the first and only report of mixed mating in a fungus. To determine if mixed mating is common to C. parasitica populations, I studied four additional populations, using both phenotypic and molecular markers, and found that all had mixed mating systems (outcrossing rates of {dollar}sim{dollar}75%).; Biparental inbreeding (due to mating between relatives, versus inbreeding due to selfing) was investigated qualitatively in three of the above four populations, and was also studied quantitatively in the earlier-cited population (Milgroom et al. 1993), and found to contribute significantly to the genetic structure of the populations studied.; The importance of self-fertilization to the breeding system of C. parasitica motivated laboratory-based studies of the mating system, to better understand the phenomenon of self-fertilization and to substantiate earlier but undocumented reports of a self-incompatibility system. Using a classical genetics approach, I show herein that C. parasitica has a bipolar (one-locus, two-allele) self-incompatibility system similar to all other self-incompatible ascomycetes that have been studied. Molecular biological studies of the mating-type (MAT) locus, which confers self-incompatibility, resulted in: (1) the identification and partial cloning of the two idiomorphs at the MAT locus; (2) that although all isolates (being haploid) have chromosomal copies of only one or the other idiomorph, many carry copies, presumably extrachromosomal, of the alternate idiomorph, albeit at very low titer; and (3) that self-fertility occurs by a mating-type switching mechanism whereby the chromosomal idiomorph in a portion of the fungal thallus is replaced by an extrachromosomal copy of the alternate idiomorph.