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Hyphae are microscopic filaments that elongate and branch to create networks of interconnected tubes. Understanding how they work remains a formidable challenge in experimental mycology. Important advances in hyphal research in the 20th century came from electron microscopy, which revealed clusters of cytoplasmic vesicles in the cell apex, and biochemical studies that identified the cell wall materials that are assembled at the tip. Early genetic experiments on hyphae based on mutant analysis were disappointing and provided little information on the relationship between genotype and phenotype. Progress has come more recently, in the first decades of this century, by combining the techniques of molecular genetics with modern imaging methods. Live-cell imaging has allowed us to study the dynamics of cell components in strains of fungi engineered with plasmids encoding proteins fused to fluorescent probes. This technology has provided significant insights on the growth process and yet the fundamentals of hyphal growth remain elusive.

Cell size is an important variable in the study of cellular growth, metabolism, and the cell cycle. The large size of Amoeba proteus and the ease with which it can be collected and cultured have made it a star in biology education—and it was a model for research on cell biology before the introduction of molecular genetic methods. Measuring the cytoplasmic density of a single amoeba without modern instrumentation seems like a difficult task, but this was done with supreme accuracy in the 1940s. The solution was based on the familiar Cartesian diver that is used to demonstrate Archimedes’s principle. It required the fabrication of a tiny diver that would respond to the additional mass of a cell. Experiments using this method allowed investigators to study changes in size and density associated with feeding, starvation, and cell division. This research is an illustration of the ingenuity of cell biologists in the pre-molecular genetic era of their field, which is often overlooked by contemporary scientists. The consideration of the mass, density, and buoyancy of free-living amoebas encourages a new hypothesis about the evolution of testate amoebas.

A new study of truffle fungi in Patagonia reveals the importance of native bird species in spore dispersal. This has wide implications for the study of animal-fungal symbioses in the Southern Hemisphere.

Like other cells, fungal hyphae show exquisite sensitivity to their environment. This reactiveness is demonstrated at many levels, from changes in the form of the hypha resulting from alterations in patterns of exocytosis, to membrane excitation, and mechanisms of wound repair. Growing hyphae detect ridges on surfaces and respond to restrictions in their physical space. These are expressions of cellular consciousness. Fungal mycelia show decision-making and alter their developmental patterns in response to interactions with other organisms. Mycelia may even be capable of spatial recognition and learning coupled with a facility for short-term memory. Now is a fruitful time to recognize the study of fungal ethology as a distinctive discipline within mycology.

Despite the longstanding use of dried mushrooms and mushroom extracts in traditional Chinese medicine, there is no scientific evidence to support the effectiveness of these preparations in the treatment of human disease. Consumers should evaluate assertions made by companies about the miraculous properties of medicinal mushrooms very critically. The potential harm caused by these natural products is another important consideration. In a more positive vein, the presence of potent toxins and neurotropic compounds in basidiomycete fruit bodies suggests that secondary metabolites with useful pharmacological properties are widespread in these fungi. Major investment in controlled experiments and objective clinical trials is necessary to develop this natural pharmacopeia.

Active discharge of basidiospores in most species of Basidiomycota is powered by the rapid movement of a droplet of fluid, called Buller’s drop, over the spore surface. This paper is concerned with the operation of the launch mechanism in species with the largest and smallest ballistospores. Aleurodiscus gigasporus (Russulales) produces the largest basidiospores on record. The maximum dimensions of the spores, 34 × 28 µm, correspond to a volume of 14 pL and to an estimated mass of 17 ng. The smallest recorded basidiospores are produced by Hyphodontia latitans (Hymenochaetales). Minimum spore dimensions in this species, 3.5 × 0.5 µm, correspond to a volume of 0.5 fL and mass of 0.6 pg. Neither species has been studied using high-speed video microscopy, but this technique was used to examine ballistospore discharge in species with spores of similar sizes (slightly smaller than A. gigasporus and slightly larger than those of H. latitans). Extrapolation of velocity measurements from these fungi provided estimates of discharge distances ranging from a maximum of almost 2 mm in A. gigasporus to a minimum of 4 µm in H. latitans. These are, respectively, the longest and shortest predicted discharge distances for ballistospores. Limitations to the distances traveled by basidiospores are discussed in relation to the mechanics of the discharge process and the types of fruit-bodies from which the spores are released.

Conidial dispersal in Stachybotrys chartarum in response to low-velocity airflow was studied using a microflow apparatus. The maximum rate of spore release occurred during the first 5 min of airflow, followed by a dramatic reduction in dispersal that left more than 99% of the conidia attached to their conidiophores. Micromanipulation of undisturbed colonies showed that micronewton (microN) forces were needed to dislodge spore clusters from their supporting conidiophores. Calculations show that airspeeds that normally prevail in the indoor environment disturb colonies with forces that are 1000-fold lower, in the nanonewton (nN) range. Low-velocity airflow does not, therefore, cause sufficient disturbance to disperse a large proportion of the conidia of S. chartarum.

Analysis of airflow patterns around models of mushrooms and fresh fruiting bodies in a low speed wind tunnel showed division of the airstream at the leading edge of the cap, an increase in wind speed a few millimeters above and below the cap, and the development of a mantle of very slow-moving air closer to the lower surface. Mushroom caps with a pronounced bell shape offered the greatest interrup- tion to airflow and showed the maximum reduction in wind speed beneath their lower surface. Calcula- tions suggest that the measured decrease in wind speed may reduce the number of spores that are blown back into the cap, promoting dispersal from the fruiting body.

Melanins are complex black polymers of resonance stabilized cyclic subunits (including in- doles, phenols, hydroxynaphthalenes) and are noto- riously intractable to chemical analysis. Cell walls of numerous fungi are melanized, rendering many spores, vegetative hyphae, and certain fruit bodies opaque. Melanin deposition protects the pigmented cell from physical and biological stresses, excludes poisons, and helps limit leakage of certain metabo- lites. By forming a physical barrier between the cell and its surroundings, the production of a melanized wall layer has a profound effect upon the interactions between pathogenic fungi and their hosts. In many plant and animal diseases caused by melanotic fungi, melanin biosynthesis is an important determinant of pathogenicity and virulence. The mode of action of melanin in these diseases is the subject of intensive research, and a variety of mechanisms that link fun- gal melanin and disease have been discovered.

Viscous drag causes the rapid deceleration of fungal spores after high-speed launches and limits discharge distance. Stokes' law posits a linear relationship between drag force and velocity. It provides an excellent fit to experimental measurements of the terminal velocity of free-falling spores and other instances of low Reynolds number motion (Re10 m/s and travel as far as 2.5 m (Re>100).

Gilled mushrooms are produced by multiple orders within the Agaricomycetes. Some species form a single array of unbranched radial gills beneath their caps, many others produce multiple files of lamellulae between the primary gills, and branched gills are also common. In this largely theoretical study we modeled the effects of different gill arrangements on the total surface area for spore production. Relative to spore production over a flat surface, gills achieve a maximum 20-fold increase in surface area. The branching of gills produces the same increase in surface area as the formation of free-standing lamellulae (short gills). The addition of lamellulae between every second gill would offer a slightly greater increase in surface area in comparison to the addition of lamellulae between every pair of opposing gills, but this morphology does not appear in nature. Analysis of photographs of mushrooms demonstrates an excellent match between natural gill arrangements and configurations predicted by our model.

Background Spore discharge in the majority of the 30,000 described species of Basidiomycota is powered by the rapid motion of a fluid droplet, called Buller's drop, over the spore surface. In basidiomycete yeasts, and phytopathogenic rusts and smuts, spores are discharged directly into the airflow around the fungal colony. Maximum discharge distances of 1–2 mm have been reported for these fungi. In mushroom-forming species, however, spores are propelled over much shorter ranges. In gilled mushrooms, for example, discharge distances of

Many fungal pathogens penetrate plant leaves from a specialized cell called an appressorium. The rice blast pathogen Magnaporthe grisea can also penetrate synthetic surfaces such as poly(vinyl chloride). Previous experiments have suggested that penetration requires an elevated appressorial turgor pressure. In the present report we have used nonbiodegradable Mylar membranes, exhibiting a range of surface hardness, to test the proposition that penetration is driven by turgor. Reducing appressorial turgor by osmotic stress inhibited penetration of these membranes. The size of the turgor deficit required to inhibit penetration was a function of the surface hardness. Penetration of the hardest membranes was inhibited by small decreases in appressorial turgor, while penetration of the softer membranes was sensitive only to large decreases in turgor. Similarly, penetration of the host surface was inhibited in a manner comparable to penetration of the hardest Mylar membranes. Indirect measurements of turgor, obtained through osmotically induced collapse of appressoria, indicated that the infection apparatus can generate turgor pressures in excess of 8.0 MPa (80 bars). We conclude that penetration of synthetic membranes, and host epidermal cells, is accomplished by application of the physical force derived from appressorial turgor.

The possible involvement of osmotically generated hydrostatic pressure in driving actin-rich extensions of the cell surface was examined using cultures of chick neurons. Estimation of the excess internal osmotic pressure of chick neural tissue by vapor pressure deficit osmometry, and of the excess internal hydrostatic pressure in cultured chick neurons using a calibrated pressure pipette, gave upper limits of 10 mosM and 0.1 atmosphere (1 atmosphere = 101325 Pa), respectively. Increases in the osmolality of the medium surrounding cultured neurons by addition of sucrose, mannitol or polyethylene glycol by amounts that should eliminate any internal pressure not only failed to arrest the growth of filopodia but caused them to increase in length up to twofold in 3–5 min. Lamellipodia remained unchanged following hyperosmotic shifts of 20 mosM, but higher levels caused a small decrease in area. Reduction of osmolality by the addition of water to the culture fluid down to 50% of its normal value failed to show any detectable change in either filopodial length or lamellipodia area. These observations argue against an osmotic mechanism for growth cone extension and show that the growth of filopodia, in particular, is unlikely to be driven by osmotically generated hydrostatic pressure. In contrast to the short-term effects on growth cone morphology, the slower elongation of the neuritic cylinder showed a consistent osmotic response. Growth rates were reduced following addition of osmolytes and increased in rate (as much as sixfold) following addition of water to the culture medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Ballistospore discharge is a feature of 30 000 species of mushrooms, basidiomycete yeasts and pathogenic rusts and smuts. The biomechanics of discharge may involve an abrupt change in the center of mass associated with the coalescence of Buller's drop and the spore. However this process occurs so rapidly that the launch of the ballistospore has never been visualized. Here we report ultra high- speed video recordings of the earliest events of spore dispersal using the yeast Itersonilia perplexans and the distantly related jelly fungus Auricularia auricula. Im- ages taken at camera speeds of up to 100 000 frames/ s demonstrate that ballistospore discharge does in- volve the coalescence of Buller's drop and the spore. Recordings of I. perplexans demonstrate that al- though coalescence may result from the directed col- lapse of Buller's drop onto the spore, it also may in- volve the movement of the spore toward the drop. The release of surface tension at coalescence pro- vides the energy and directional momentum to pro- pel the drop and spore away from the fungus. Anal- yses show that ballistospores launch into the air at initial accelerations in excess of 10 000 g. There is no known analog of this micromechanical process in an- imals, plants or bacteria, but the recent development of a surface tension motor may mimic the fungal bi- ology described here.

The water molds Achlya bisexualis Coker and Saprolegnia ferax (Gruithuisen) Thuret (Class: Oomycetes) normally grow in the form of slender hyphae with up to 0.8 MPa (8 bar) of internal pressure. Models of plant cell growth indicate that this turgor pressure drives the expansion of the cell wall. However, under conditions of prolonged osmotic stress, these species were able to grow in the absence of measurable turgor. Unpressurized cells of A. bisexualis grew in the form of a plasmodium-like colony on solid media, and produced a multinucleate yeast-like phase in liquid. By contrast, the morphology of S. ferax was unaffected by the loss of turgor, and the mold continued to generate tip-growing hyphae. Measurements of cell wall strength indicate that these microorganisms produce a very fluid wall in the region of surface growth, circumventing the usual requirement for turgor.

The potato famine of 1845–1846 had a devastating effect on Ireland. DNA analysis of herbarium specimens has allowed identification of the strain of plant pathogen responsible.

When hyphae of the water mold Achlya were subjected to osmotic stress, imposed with polyethylene glycol (PEG)-300 or sucrose, turgor pressure fell in proportion to the increase in external osmotic pressure. There was no evidence of turgor regulation, even over a period of days, yet the extension rate was unaffected until turgor was reduced to less than a third of the normal level of 0.6-0.8 MPa (6-8 bars). Measurements of the pressure at which the hyphae burst indicate that they respond to osmotic stress by softening their apical cell walls, sustaining extension growth despite reduced turgor pressure. The effect of osmolytes excluded by the wall was very different; superfusion of growing hyphae with PEG-6000 or dextran-6000 reduced turgor and stopped extension but did not induce wall softening. Furthermore, the hyphae did not resume growth during an hour or more of continuous exposure to these substances. Although the two classes of osmolytes have the same effect on turgor, they may induce different strains within the cell wall; this might then affect the capacity of the organism to detect the drop in turgor or to soften its cell wall. The interplay between turgor and wall strength supports the proposition that turgor supplies the driving force for extension and that production of the standard hyphal form requires a balance between hydrostatic pressure and a resistive cell wall.

When certain fungi infect grasses and cereals, they develop a specialized structure called the appressorium. This microscopic structure inflates on the surface of the grass leaf, then generates enough force to push through its cuticle and cell wall to tap the juices within. The development of a technique to measure the forces generated by the appressorium should help in understanding how this infection platform works.

Mushrooms are the fruit bodies of certain fungi and usually develop from two compatible colonies. A study of one such fungus, however, shows that during a particular period the mushrooms were mosaics — that is, they formed from several genetically distinct populations of cells. The reasons remain mysterious.

Temperature measurements from the spore-producing gills, spines, and tubes underneath fruit-body caps revealed cooling in 18 basidiomycete species growing in mixed deciduous woodland. The temperature of cultured fruit bodies of Lentinula edodes and Pleurotus ostreatus fell upon exposure to low velocity airflow, consistent with an evaporative mechanism of cooling. The mechanism of ballistospore discharge characteristic of basidiomycete fungi is dependent on condensation of water from the air surrounding the spores onto the spore surface. The current model for this process predicts that condensation, and therefore spore discharge, is enhanced by cooling of the fruit body.

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The distinctive shapes of basidiomata in the bird's nest fungi reflect differences in the mechanism of splash discharge. In the present study, peridiole discharge was examined in Nidularia pulvinata using high-speed video. Nidularia pulvinata produces globose basidiomata that split open at maturity to expose 100 or more peridioles within a gelatinous matrix. Each peridiole contains an estimated 7 million spores. The impact of water drops splashed the peridioles horizontally from the fruit body, along with globs of mucilage, at a mean velocity of 1.2 m s−1. Discharged peridioles travelled for a maximum horizontal distance of 1.5 cm. This launch process contrasts with the faster vertical splashes of peridioles over distances of up to one metre from the flute-shaped fruit bodies of bird's nest fungi in the genera Crucibulum and Cyathus. Peridioles in these genera are equipped with a funicular cord that attaches them to vegetation, placing them in an ideal location for ingestion by browsing herbivores. The absence of cords in N. pulvinata and its use of a sloppy discharge mechanism suggest that it is more likely to be dispersed by animals feeding on the forest floor.

The bird's nest fungi (Basidiomycota, Agaricales) package millions of spores into peridioles that are splashed from their basidiomata by the impact of raindrops. In this study we report new information on the discharge mechanism in Crucibulum and Cyathus species revealed with high-speed video. Peridioles were ejected at speeds of 1–5 m per second utilizing less than 2 % of the kinetic energy in falling raindrops. Raindrops that hit the rim of the basidiome were most effective at ejecting peridioles. The mean angle of ejection varied from 67 to 73° and the peridioles travelled over an estimated maximum horizontal distance of 1 m. Each peridiole carried a cord or funiculus that remained in a condensed form during flight. The cord unravelled when its adhesive surface stuck to a surrounding obstacle and acted as a brake that quickly reduced the velocity of the projectile. In nature, this elaborate mechanism tethers peridioles to vegetation in a perfect location for browsing by herbivores.

The use of molecular bar-coding and consensus on nomenclatural practices has encouraged optimism about the future of fungal taxonomy and systematics. There are, however, profound deficiencies in our understanding of fungal diversity and broader problems with the taxonomic enterprise that deserve greater attention. For 250 years mycologists have tried to reconcile fungal diversity with the Linnean fantasy of a divine order throughout nature that included unambiguous species. This effort has failed and today's taxonomy rests on an unstable philosophical foundation. Rather than persisting with the present endeavour, it may be more fruitful to abandon the notion of fungal species pending further basic research. In the meantime, mycologists should consider tagging collections with digital codes and assigning these operational taxonomic units to higher taxonomic ranks whose objective reality is corroborated by strong phylogenetic evidence.

Osmotic pressures (II) of aqueous solutions of polyethylene glycols (PEGs) of average relative molecular weight (M(r)) between 200 and 10,000 were measured using vapor pressure deficit osmometry. The relationships between molarity and II were described with high precision by second order polynomials for each of the PEGs studied. In contrast to previous reports, equivalent weights of different polymers in solution did not generate the same II; low M(r) PEGs generated a higher II than the higher M(r) PEGs. The effect of PEGs upon II represents an interaction between concentration and M(r).