Sixteen highly branched (colonial) mutants of
Fusarium graminearum A3/5 were isolated at the
end of 11 Quorn® myco-protein
production fermentations. These ranged from the highly
branched C134-3 to the sparsely branched C145, which was similar to
A3/5 in liquid culture. Although allelic complementation
was observed to occur between some of the mutants,
heterokaryon analysis
revealed that all the highly branched mutants belonged to
a single complementation group. Mixed cultures of A3/5
and six of the
colonial mutants were grown in glucose-, ammonium-,
magnesium- and sulphate-limited chemostat culture. One
mutant (C153) had a
selective advantage over A3/5 in all nutrient
limitations tested; four mutants (C134-1, C134-3, C137-1,
and C135) had a selective
advantage over A3/5 in all except one nutrient
limitation; and one mutant (C139) had a selective advantage
over A3/5 in
magnesium-limited chemostat culture only. Four mutants
(C134-3, C139-1, C153 and C135) had a selective advantage
over A3/5
when the dilution rate was increased above the critical
dilution rate (0·22 h−1)
and nutrients were present in excess. These results
suggest that the growth conditions in the Quorn®
production plant (which is operated as a glucose-stat)
favour mutations that
confer general growth rate advantages, but these mutations
may result in growth disadvantages when nutrient limitations are
imposed. As all of the mutations appear to occur in one
gene or gene cluster, the differing patterns of selective
advantage/disadvantage for the six mutants studied
suggests that regulatory genes may also be involved or
that different sites of
mutation can lead to a variety of pleiotropic effects.