Sorbose and the Hyphal branching rhythm in neurospora crassa : the influence of sorbose on growth, morphology, period and temperature compensation

Abstract

The glucose analogue sorbose has an interesting effect on the growth of the fungus Neurospora crassa. On sorbose Neurospora crassa wild type (wt) grows slower and with a marked difference in morphology. The appearance of a noncircadian hyphal branching rhythm with a lack of temperature compensation is a known effect of sorbose grown wt Neurospora crassa. The aim of this study was to determine if previous results reported by Feldman on the phenomenon of the hyphal branching rhythm and its lack of temperature compensation, were reproducible. The hyphal branching rhythm is a specific rhythm which appears when wt Neurospora crassa goes into hypherbranching morphology. This results in seemingly timed “wave fronts” where the hyphal branching is most pronounced. The hyphal branching rhythm is in sharp contrast to the free running, circadian and temperature compensated conidiation rhythm of Neurospora crassa in constant dark conditions (DD) and a constant temperature range between 18°C and 32°C. Neurospora crassa `s conidiation rhythm is the timed and rhythmic execution of spore formation which is visually observable and can be seen as a rhythmic formation of orange hives of spores in a glass tube. The conidiation rhythm disappears in constant light conditions (LL). The rhythm is governed by an internal genetic /transcriptional biological clock or oscillator where the key element is the gene frequency (frq) and its product, the protein FREQUENCY (FRQ). In 1974 Feldman reported that the period of a wt Neurospora crassa strain grown on sorbose/sucrose media was poorly temperature compensated and not influenced by LL conditions. Feldman found that the period length of the hyphal branching rhythm increased from 21.1 h to 93.1 h over a range of constant temperatures from 30°C to 20°C. The control media without sorbose produced a steady conidiation rhythm with circadian properties. The results from this thesis confirm Feldman’s results that the hyphal branching rhythm is poorly temperature compensated and that the period length of the hyphal branching rhythm increase with decreasing constant temperature. Contributing to the hyphal branching rhythm are morphological changes, such as tight colonial growth, extensive hyphal branching, few aerial hyphae and sporadic conidiation. The hyphae of Neurospora crassa grown on sorbose vi are shorter, thicker and more deformed than hyphae from wt Neurospora crassa growing on media without sorbose. This is thought to be the results of alteration in cell wall composition, mainly the depletion of β -1,3 -glucan in the cell wall. The question whether the hyphal branching rhythm is a biological rhythm controlled by a circadian rhythm like that which is controlling the conidiation rhythm has been examined by using the knock out mutant frq 10 in DD conditions at 29°C. On minimal media without sorbose frq 10 grew fast, and with normal mycelial spreading, but without rhythmic conidial bands. Grown on sorbose, frq 10 showed the hyphal branching rhythm as the wt Neurospora crassa. It therefore appears that the hyphal branching rhythm is independent of the frq gene and its products.