Partial Interfertility Among theCeratocystisSpecies on Conifers

Abstract

Interspecific pairings betweenMAT-1andMAT-2strains of seven species ofCeratocystisfrom 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 ofC. pinicola, C. coerulescens,andC. laricicolafrequently crossed with other species. BecauseMAT-2strains of theCeratocystisspecies studied are capable of selfing due to unidirectional mating-type switching,MAT-2tester strains that fruited poorly or were self-sterile had to be selected. In a few cases, theseMAT-2strains were induced to self by pairing withMAT-1orMAT-2strains 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.