A HOMOLOG OF THE YEAST SHE4 GENE IS ESSENTIAL FOR THE TRANSITION BETWEEN THE SYNCYTIAL AND CELLULAR STAGES DURING SEXUAL REPRODUCTION OF THE FUNGUS PODOSPORA-ANSERINA

The Podospora anserina cro1 gene was identified as a gene required for sexual sporulation, Crosses homozygous for-the crol-1 mutation yield fruiting bodies which produce few asci due to the formation of giant p lurinucleate cells instead of dikaryotic cells after fertilization, Th is defect does not impair karyogamy, but meioses of the resultant poly ploid nuclei are most often abortive, Cytological studies suggest that the primary defect of the mutant is its inability to form septa betwe en the daughter nuclei after each mitosis, a step specific for normal dikaryotic cell divisions, Tile cro1-1 mutant would thus be unable to leave the syncytial vegetative state while abiding by the meiotic prog ramme, cro1-1 also shows defects in ascospore germination and growth r ate, GFP-tagging of the CRO1 protein reveals that it is a cytosolic pr otein mainly expressed at the beginning of the dikaryotic stage and at the time of ascospore maturation, The CRO1 protein exhibits significa nt similarity to the SHE4 protein, which is required for asymmetric ma ting-type switching in budding yeast cells, Thus, a gene involved in a symmetric cell divisions in a unicellular organism plays a hey role at the transition between the syncytial (vegetative) state and the cellu lar (sexual) state in a filamentous fungus.