A genetic linkage map of the fungus phycomyces blakesleeanus for gene identification by map-based cloning

Abstract

The Mucoromycotina is one of the basal lineages in the fungi, all of which are poorly understood. Mucormycosis is a life threatening infection caused by fungi of the order Mucorales, the most common species of which is Rhizopus oryzae. Certain genetic traits in fungal genomes are thought to influence their ability to cause disease. This fungus is closely related to Phycomyces blakesleeanus, for certain common properties like taxonomic classification and including response to environmental signals. In view of these common properties, Phycomyces presents itself as an important model to study pathogenic determinants of Rhizopus infection. P. blakesleeanus is a zygomycete fungus classified in the subphylum Mucoromycotina, studied because of its environmental sensing abilities and responses, and its ability to synthesize the pigment beta-carotene. Light is an environmental signal that modulates many aspects of fungal biology including the ability to cause disease. There are eight unknown light-sensing mad genes in Phycomyces. The inability to transform DNA into Phycomyces has blocked the identification of genes in this fungus. In this research, a genetic map based on 100 molecular markers, assigned to 121 progeny, was generated in a cross between two wild type strains NRRL1555 and UBC21. The map comprised 1037.8 cM spread over 11 linkage groups. The map was then used as the starting point for the choice of markers for the map-based identification of madC, by crossing madC mutants to strain UBC21. madC was identified as a new gene required for fungal responses to their environment. The madC gene encodes a Ras GTPase activating protein. These findings indicate that the Ras signal transduction pathway plays role in light sensing. Because both light sensing and Ras signaling are required for virulence in other fungal species, a future direction would be to test if mutation of the madC gene has an effect on disease caused by pathogenic members of the Mucoromycotina. By identifying the mad light-sensing genes of Phycomyces, this research can be potentially translated into therapeutics by drug discovery of those genes with roles in both light-sensing and pathogenicity.