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The genus Cercospora consists of numerous important, apparently asexual plant pathogens. We designed degenerate primers from homologous sequences in related species to amplify part of the C. apii, C. apiicola, C. beticola, C. zeae-maydis and C. zeina mating type genes. Chromosome walking was used to determine the full length mating type genes of these species. Primers were developed to amplify and sequence homologous portions of the mating type genes of additional species. Phylogenetic analyses of these sequences revealed little variation among members of the C. apii complex, whereas C. zeae-maydis and C. zeina were found to be dissimilar. The presence of both mating types in approximately even proportions in C. beticola, C. zeae-maydis and C. zeina populations, in contrast to single mating types in C. apii (MAT1) and C. apiicola (MAT2), suggests that a sexual cycle may be active in some of these species.

Interspecific pairings between MAT-1 and MAT-2 strains of seven species of Ceratocystis from conifer hosts resulted in partial interfertility in many cases. Perithecia from such crosses typically produced few ascospores, which were often misshapen and intermixed with cellular debris, and a low ascospore germination percentage was seen. Intraspecific pairings of strains of opposite mating type generally produced many perithecia with many ascospores of a high germination percentage. Mycelial morphology, cycloheximide resistance, and isozyme markers were used to confirm that many of the interspecific pairings produced viable hybrid progeny. Tester strains of C. pinicola, C. coerulescens, and C. laricicola frequently crossed with other species. Because MAT-2 strains of the Ceratocystis species studied are capable of selfing due to unidirectional mating-type switching, MAT-2 tester strains that fruited poorly or were self-sterile had to be selected. In a few cases, these MAT-2 strains were induced to self by pairing with MAT-1 or MAT-2 strains of other species. Hybrids yielding ascospores formed only in crosses of strains of opposite mating type, but many interspecific pairings of strains of the same or opposite mating type resulted in the production of sterile perithecia. Partial interfertility between species based on a low number of misshapen ascospores was a good indicator of biological species, and such delineations corresponded well with species delimited by morphological and physiological phenotypes. Copyright 1998 Academic Press.