A PUTATIVE RHAMNOGALACTURONASE REQUIRED FOR SEXUAL DEVELOPMENT OF NEUROSPORA-CRASSA

In previous work, the asd-1 (ascus development) gene of the filamentou s fungus Neurospora crassa was identified as a gene expressed preferen tially during the sexual cycle and shown to be essential for normal se xual development. The asd-1 gene has been sequenced and further charac terized. It contains two introns, the first of which is in-frame and i nefficiently or differentially spliced. The predicted ASD-1 protein ha s extensive homology with rhamnogalacturonase B of Aspergillus aculeat us, which cleaves the backbone within the ramified hairy regions of pe ctin. In homozygous asd-1 crosses, sexual development is initiated and large numbers of normal-sized asci are formed. Ascospore delineation does not occur, however, and no sexual progeny are produced. As most a sd-1 asci contain eight nuclei, the two meiotic divisions and subseque nt mitotic division typical of normal crosses seem to occur, but the h aploid nuclei are not partitioned into ascospores. In wild-type crosse s, the ASD-1 protein is present in large amounts in croziers and young asci, but it is only faintly detectable in more mature asci containin g developing ascospores. Models to explain the possible role of a rham nogalacturonase in sexual development are presented.