YEAST-MYCELIAL DIMORPHISM OF HAPLOID AND DIPLOID STRAINS OF USTILAGO-MAYDIS

Control of the ph of liquid synthetic culture media made possible myce lial growth of both diploid and haploid strains of Ustilago maydis. Wh ereas at neutral ph the fungus grew as a homogeneous population of bud ding yeast-like cells (sporidia), at acid ph it developed in the mycel ial form. Mycelial cells appeared branched and narrower than yeast cel ls. Cell morphology was affected by the carbon and nitrogen sources. W hen the culture medium was removed continuously or intermittently, ver y long, filamentous cells were formed. Colonies of haploid strains dev eloped aerial mycelium ('fuzz' morphology) on acid solid medium. Null 6, bW, and bE mutants behaved in the same way as haploid wild-type str ains. It is suggested that growth at low ph overcomes the control proc esses governed by heterologous bE and bW loci.