ALTERED MATING-TYPE IDENTITY IN THE FUNGUS PODOSPORA-ANSERINA LEADS TO SELFISH NUCLEI, UNIPARENTAL PROGENY, AND HAPLOID MEIOSIS

In wild-type crosses of the filamentous ascomycete Podospora anserina, after fertilization, only nuclei of opposite mating type can form dik aryons that undergo karyogamy and meiosis, producing biparental progen y. To determine the role played by the mating type in these steps, the four mat genes were mutagenized in vitro and introduced into a strain deleted for its mat locus. Genetic and cytological analyses of these mutant strains, crossed to each other and to wild type, showed that ma ting-type information is required for recognition of nuclear identity during the early steps of sexual reproduction. In crosses with strains carrying a mating-type mutation, two unusual developmental patterns w ere observed: monokaryotic cells, resulting in haploid meiosis, and un iparental dikaryotic cells providing, after karyogamy and meiosis, a u niparental progeny. Altered mating-type identity leads to selfish beha vior of the mutant nucleus: it migrates alone or paired, ignoring its wild-type partner in all mutant X wild-type crosses. This behavior is nucleus-autonomous because, in the same cytoplasm, the wild-type nucle i form only biparental dikaryons. In P. anserina, mat genes are thus r equired to ensure a biparental dikaryotic state but appear dispensable for later stages, such as meiosis and sporulation.