Kinetics of apical and lateral branching in Aspergillus nidulans and Geotrichum lactis

Abstract

Apical branching was a normal feature of hyphal morphogenesis at the margins of colonies of Aspergillus nidulans and Geotrichum lactis . In G. lactis apical branching occurred in a dichotomous manner. After a hypha had divided apically there was no further wall extension below the point of origin of the branches. Apical branching was preceded in A. nidulans but not G. lactis , by a slight deceleration (18%) in growth rate. Branching was followed in both species by an acceleration in the rate of total hypha formation (i.e. sum of the growth rates of both branches) which persisted until each branch had attained a growth rate identical to that of the original parent hypha; thus there was a doubling in the rate of hypha formation. The growth of lateral branches produced by leading hyphae could be divided into a short exponential phase (doubling times as short as 6–7 min), a phase of increasing growth rate and a phase of constant linear growth rate. The mean linear growth rates of lateral branches of A. nidulans and G. lactis were respectively about 21 and 31% less than the mean growth rates of their leading hyphae. The rate of growth of parent hyphae was not significantly affected by lateral branch formation. Some lateral branches of A. nidulans grew parallel and close to the hyphae from which they arose; thin mycelial strands of three to four hyphae often resulted from this type of growth. The apical region (37 μm from the tip) of a germ-tube of A. nidulans was apparently involved in a negative autotropic response. It is suggested that this region may represent the extension zone of the hypha.