Using genetic crosses between single chs mutants of Ustilago maydis inocula
ted into maize (Zea mays) seedlings, two classes of double mutants affected
in genes coding for chitin synthetases were isolated: chs3/chs4, and chs4/
chs5. Analysis of the mutants showed almost no change in their phenotype co
mpared with wild-type strains. Growth rate, effect of stress conditions, di
morphic transition and mating were not affected. The only salient differenc
es were increased sensitivity to osmotics at acid pH, and decrease in chiti
n synthetase activity, especially when measured with CO2+, and in chitin co
ntent. Most significant was a decrease in virulence, although this appeared
to be due a factor unrelated to CHS genes. These data can be taken as furt
her evidence that multigenic control of chitin synthetase in fungi operates
as a safety mechanism to guarantee fungal viability in changing and hostil
e environmental conditions.