Yeast cell phenotype and morphology result from the interaction of two
processes, polarized growth, on the one hand, and cytokinesis with su
bsequent division of the cell wall, on the other. Imbalance of the equ
ilibrium of these two principal processes may lead to the occurrence o
f yeast morphological mutants that generate rough colonies. Two types
of such mutants exist, Rgh and Rpm. The Rgh mutants have ellipsoid cel
ls and either form chains or generate clumps at high frequency. These
changes in heterothallic strains of saccharomyces occur due to recessi
ve mutations in any of 11 nonlinked genes. In homothallic strains of s
accharomyces, the trait Rgh depends on the complementary interaction o
f one dominant and two recessive genes. Rpm mutants generate rough col
onies, which contain elongated cells that give rise to pseudomycelium.
The mutants obtained are recessive and occur in six nonlinked genes.
The Rpm mutants represent an example of the manifestation of mycelium
- yeast dimorphism in fungi, and this case may be regarded as a simple
morphogenetic model.