Control of hyphal orientation in colonies of Mucor hiemalis

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This is a study of factors controlling the development of hyphae of Mucor hiemalis on a plane uniform agar surface, whereby a single germ tube elongating linearly gives rise to a colony enlarging symmetrically on an approximately uniform front through 360° from its centre. Most branches arise closely behind an existing elongating hyphal tip, and only a small number are formed on older parts of hyphae. The observed distances between successive points of of origin of hyphae fit a γ distribution with estimated shape parameter 1.5 and an estimated scale parameter of 0.011μm. The observed angles at which branches diverged from their parents' axes ranged within a normal distribution (mean 56° σ 17°). The first formed hyphae extend at about 20 μm per h for about 1 h after germination. Subsequently some stop growing permanently or temporarily; variation in the rates of elongation at the remaining tips, and at those branches formed after this first hour fits a half normal curve (mean 85 μm h−1).

Growing hyphal tips repel each other, the maximum distances between tips at which this interaction has been seen range from about 10 μm to about 20 μm. No appreciable reorientation has been seen when living tips come close to dead ones, or if they approach agar sites from which another growing tip has been removed. The effects of moving growing tips close to older parts of the mycelium are less regular. Half of 68 observed cases stopped elongating, approximately one quarter (16) grew away from the older hyphae before making contact, approximately one quarter (18) crossed the older hyphae with no visible change in direction.

Analysis of patterns produced by computer simulation indicates that the circular shape is the expected result of any system having the observed variation in branching and rates of elongation. No other controlling factor is needed to explain the patterns seen. In particular there is no evidence that orientating interactions between hyphal tips have any biologically important effect on colony shape. The amount to which the observed variation could be increased or reduced without affecting the growth patterns in any biologically important way is being studied.

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Present address: Institute for Marine Environmental Research, The Hoe, Plymouth.

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