John Webster

Aquatic hyphomycetes and other fungi in living aquatic and terrestrial roots of Alnus glutinosa

Sixty-six fungal endophytes, mainly Deuteromycotina, were isolated from the aquatic and terrestrial root systems of Alnus glutinosa . Twelve are mostly known from aquatic habitats; among them, Heliscus lugdunensis was isolated from ca 50% of all bark and xylem pieces examined. The mean colonization frequency by aquatic species in immersed roots was as high as 30% as opposed to 12% in soil roots and the total number of species found was also larger in aquatic roots. Results of cluster and correspondence analysis indicated that the aquatic and soil root samples are characterized by two different endophyte populations. The significance of water-submerged roots as substrata for aquatic hyphomycetes is discussed.

Phylogenetic analysis of Puccinia distincta and P. lagenophorae, two closely related rust fungi causing epidemics on Asteraceae in Europe

Phylogenetic analyses of the ITS1-5.8S-ITS2 region of the ribosomal RNA gene cluster were carried out with two short-cycled (aecial/telial) European rusts on Asteraceae, Puccinia distincta causing the current pan-European epidemic on Bellis perennis, and P. lagenophorae causing a similar disease on Senecio spp., as well as the macrocyclic P. obscura which alternates between B. perennis (pycnial and aecial host) and Luzula spp. (main host). All three species formed a well-resolved cluster when compared with the ITS sequences of a range of other rust fungi, using both parsimony and distance methods. The sequences of P. distincta and P. lagenophorae differed from each other at three positions whereas P. obscura differed from P. distincta at 37 points. Together with consistent morphological and epidemiological differences across Europe, these data support the recognition of P. distincta as a separate species from P. lagenophorae. Both may be derived from P. obscura, although the precise evolutionary history remains obscure.

Mannitol and hexoses are components of Buller's drop

The mechanism of ballistospore self-propulsion by basidiomycetes involves the hygroscopic adsorption of water vapour by a drop of liquid (Bullers drop) on the hilar appendix of the spore until it makes contact with another expanding drop, the adaxial drop on the face of the spore above it. Aliquots of liquid from Bullers drop have been collected from Itersonilia perplexans using micropipettes. Analysis of the droplets by microfluorescence assays indicated the presence of mannitol and hexoses. Using the same technique, washings from basidiospore deposits of I. perplexans, containing the evaporated remnants of Bullers drops and the adaxial drops carried away by the spores on discharge, were further shown to contain mannitol and hexoses. Glc analyses of washings from spore deposits of a range of basidiomycetes also indicated the presence of mannitol and hexoses. Calculations based on the estimated concentrations of mannitol and hexoses in Bullers drop from Itersonilia show that the concentration of solutes is sufficient to bring about the condensation of water vapour from a saturated atmosphere and thus to cause the growth of Bullers drop at rates previously measured. When brought close to, but not in contact with, the surface of an agar plate, crystals of pure mannitol rapidly deliquesced. Mannitol and hexoses contribute significantly to the hygroscopic nature of Bullers drop in basidiomycetes.

Vapour as the source of water in Buller's drop

Evidence is presented in support of the hypothesis that the expansion of Bullers drop on basidiospores is by condensation of water vapour around hygroscopic material extruded from the hilar appendix. Earlier work had shown that detached ballistospores of Itersonilia could still develop Bullers drop which could attain 60% of the volume of the spore, but with no measurable decrease in spore size. Using time-lapse photomicrography it was shown that in 8 basidiomycetes including Uredinales, Homo- and Heterobasidiomycetes, drop expansion was not associated with reduction in basidiospore dimensions. The rate of drop expansion in a near-saturated atmosphere over water-agar was similar in 5 basidiomycetes. In Auricularia auricula-judae the rate of drop expansion could be controlled by lowering the water activity of the agar by the addition of 1–3% mannitol. When liquid which condensed around basidiospores of Agaricus bisporus on a cooled microscope slide was evaporated, a hygroscopic residue remained. In a theoretical treatment of the expansion of a drop by condensation from a saturated atmosphere around hygroscopic material it is shown that the expansion of a drop to a radius of 4 μm in 10–14 s could be achieved by a lowering of the water vapour pressure at the surface of the drop equivalent to a relative saturation deficit of about 1·5 × 10 −3 . This suggests that a low solute concentration of the order of 0·08 m in the drop would be sufficient. It is also shown that the shape of the drop expansion curves is consistent with the physical principles involved.

Cell wall permeability and its relationship to spore release in Achlya intricata

Dynamically, the process of sporangial emptying in the zoosporic fungus Achlya intricata best fits a model based on the regulated dissipation of an osmotically generated pressure potential. Two testable predictions, derived from this hypothetical discharge mechanism, are (a) that the sporangial cell wall must be capable of maintaining a concentration gradient of some solute to depress the luminal osmotic potential, since the mature sporangium lacks a plasmalemma; and (b) that the emptying process should be inhibited by water stress. The permeability of the sporangial wall was estimated by monitoring the uptake of polyethylene glycol (PEG) molecules of different molecular weights. An aqueous solution of PEG-2000 contained molecules of the critical hydrodynamic volume, corresponding to a maximum pore size of ∼2.0 nm in the sporangial wall. Complete control (and inhibition above a critical solute concentration) over the emptying process was gained using polymers with Mrs in excess of 2000. The pressure potential within the sporangium was estimated, during discharge, to be ∼ + 10−3 MPa. This value of pressure potential is consistent with that predicted from a mathematical model previously published.

A particle plating method for analysis of fungal community composition and structure

An indirect method, particle plating, is described which enables the active fungal community in detritus to be described and analysed. Detritus was serially washed and then homogenized. Particles, with a mean volume of 0·04 mm 3 , were sieved from the homogenate, plated and incubated. Changes in the colonization and frequency of species shows that particles of this size behave as quadrats. A derivation of the binomial theorem is used to calculate confidence intervals for the colonization of the resource. Particle plating is more sensitive to changes in fungal frequencies than an equivalent direct observational technique.

Pythium acanthophoron, a mycoparasite, rediscovered in India and Britain

A description of Pythium acanthophoron is given. It has spiny oogonia, but no zoosporangia were observed. It can grow as a mycoparasite on Fusarium solani and on Pythium myriotylum (two fungal pathogens causing rhizome rot of ginger) and a range of other fungi.

Puccinia distincta , cause of the current daisy rust epidemic in Britain, in comparison with other rusts recorded on daisies, P. obscura and P. lagenophorae

Three British rust fungi reported to form aecia on daisy ( Bellis perennis ) are compared. A recently discovered species, possibly introduced from Australia, is identified as Puccinia distincta. It is a short-cycled - opsis form lacking uredinia, with aecia and telia confined to Bellis and its aeciospores capable of reinfecting the same host. This rust is compared with a macrocyclic heteroecious eu -form, P. obscura , which produces pycnia and aecia on Bellis and uredinia and telia on Luzula. Puccinia distincta is probably derived from and correlated with P. obscura because both species have the same aecial host, and because their teliospores are closely similar in morphology and dimensions. Comparison is extended to P. lagenophorae , a common rust with aecia and telia on Senecio spp., which has been reported to form aecia on Bellis following inoculation with aeciospores from Senecio. The repeated failure of our own reciprocal inoculation experiments using aeciospores from the two different hosts and differences in teliospore morphology between P. distincta and P. lagenophorae lead us to conclude that the current rust epidemic on daisies is caused by P. distincta which is distinct from P. lagenophorae.

Mechanism of sporangial emptying in Saprolegnia

In common with other zoosporic fungi, sporangial emptying in Saprolegnia is disrupted by water stress. During superfusion of sporangia with polyethylene glycol (PEG-8000) solution (⩾ 0·2%, w/v), the direction of zoospore motion was reversed, the zoospores moving toward the basal septum and collecting in the base of the sporangium. With continued superfusion with PEG-8000, a few zoospores escaped slowly from the sporangium by flagellar locomotion. Rapid expulsion of the spores occurred upon return to control superfusion with 1 m m -CaCl 2 . Statistical analysis of zoospore velocity data suggested that the rapid emptying following PEG-8000 treatment resulted from the ‘flushing-out’ of the PEG solution from the sporangial lumen, rather than by the restoration of the natural discharge mechanism. These data, along with observations on the behaviour of zoospores during release from secondary sporangia, support the hypothesis that two mechanisms of zoospore motion interact during sporangial emptying in Saprolegnia : (a) an osmotically generated pressure-driven flow of the zoospores and (b) flagellar locomotion. The first mechanism appears to be very similar to the more fully characterized discharge process in the closely related genus Achlya .

Dynamics of sporangial emptying in Achlya intricata

The dynamics of sporangial emptying in the zoosporic fungus Achlya intricata were characterized by analysis of photographic and video records of spore release. The mature sporangium is a semirigid cell wall enclosing loosely packed unicellular spores. There is a highly significant linear relationship between sporangial volume and the number of spores produced. During discharge, the spores exit the sporangium in a pulse-like fashion, the motion of the sporangial contents being punctuated by the passage of each successive spore through the constricting papilla. The plot of the number of spores released against time (“emptying profile”) is a rectangular hyperbola indicating that the velocity of spore release decreases during the emptying process. However, each individual spore accelerates during its passage through the sporangial lumen. Spore velocity shows a strong temperature dependence and is higher in larger sporangia than in smaller ones. Puncture at any point along the surface of the mature sporangium results in the release of spores through the artificial opening. Dynamically, sporangial emptying in A. intricata shares many features in common with Phytophthora cactorum and best fits a model based on the regulated dissipation of an osmotically generated pressure potential. This hypothesis is supported by mathematical modeling and the results of previous experiments.