Yeast-mycelial dimorphism of haploid and diploid strains of Ustilago maydis

1 Departamentos de Genética y Biologia Molecular, Centro de Investigación y de Estudios Avanzados del IPN, Irapuato, Gto, Mexico 2 Ingeniería Genética (Unidad Irapuato), Centro de Investigación y de Estudios Avanzados del IPN, Irapuato, Gto, Mexico 3 Departamento de Histología, Universidad Nacional de México, México, DF, Mexico ABSTRACT Summary: Control of the pH of liquid synthetic culture media made possible mycelial growth of both diploid and haploid strains of Ustilago maydis . Whereas at neutral pH the fungus grew as a homogeneous population of budding yeast-like cells (sporidia), at acid pH it developed in the mycelial form. Mycelial cells appeared branched and narrower than yeast cells. Cell morphology was affected by the carbon and nitrogen sources. When the culture medium was removed continuously or intermittently, very long, filamentous cells were formed. Colonies of haploid strains developed aerial mycelium (‘fuzz’ morphology) on acid solid medium. Null b, bW , and bE mutants behaved in the same way as haploid wild-type strains. It is suggested that growth at low pH overcomes the control processes governed by heterologous bE and bW loci. 5 Jose Ruiz-Herrera. Tel: +52 462 51600. Fax: +52 462 51282. Keywords: Ustilago maydis, dimorphism, pH regulation