Dispersal may limit the occurrence of specialist wood decay fungi already at small spatial scales

We used species‐specific spore traps to measure airborne dispersal of the wood decay fungus Phlebia centrifuga (spore size 6.5–9 × 2.5–3 μm) up to 1000 m distance from a point source. We fitted two simple dispersal models, an empirical power law model and a semi‐mechanistic diffusion model to the data using the Bayesian approach. The diffusion model provided a better fit than the power law model which underestimated deposition at 3–55 m and overestimated deposition at longer and shorter distances. Model fit improved by allowing overdispersion, suggesting that spores are not dispersed independently but wind can transport spores in groups inside discrete air packages up to considerable distances. Using the fitted diffusion model and available information on the establishment rates of wood‐decay fungi, we examine the distance up to which colonisation from a single fruit body is likely to occur. We conclude that the diluting effect of distance and low establishment success make the occurrence of P. centrifuga dispersal limited possibly already at the distance of tens of metres and very probably at a few hundred metres from the nearest fruit body, despite the fact that under favourable conditions a high proportion of the spores can disperse considerably further. This conclusion is likely to hold generally for those fungal species that inhabit fragmented landscapes, have specialised resource and habitat requirements, and have similar spore size and other dispersal traits as P. centrifuga.     

Beetles provide directed dispersal of viable spores of a keystone wood decay fungus

Wood decay fungi are considered to be dispersed by wind, but dispersal by animals may also be important, and more so in managed forests where dead wood is scarce. We investigated whether beetles could disperse spores of the keystone species Fomitopsis pinicola. Beetles were collected on sporocarps and newly felled spruce logs, a favourable habitat for spore deposition. Viable spores (and successful germination) of F. pinicola were detected by dikaryotization of monokaryotic bait mycelium from beetle samples. Viable spores were on the exoskeleton and in the faeces of all beetles collected from sporulating sporocarps. On fresh spruce logs, nine beetle species transported viable spores, of which several bore into the bark. Our results demonstrate that beetles can provide directed dispersal of wood decay fungi. Potentially, it could contribute to a higher persistence of some species in fragmented forests where spore deposition by wind on dead wood is less likely.     

Testing a mechanistic dispersal model against a dispersal experiment with a wind‐dispersed moss

Wind is the main dispersal agent for a wide array of species and for these species the environmental conditions under which diaspores are released can potentially modify the dispersal kernel substantially. Little is known about how bryophytes regulate spore release, but conditions affecting peristome movements and vibration of the seta may be important. We modelled airborne spore dispersal of the bryophyte species Discelium nudum (spore diameter 25 μm), in four different release scenarios, using a Lagrangian stochastic dispersion model and meteorological data. We tested the model predictions against experimental data on colonization success at five distances (5, 10, 30, 50 and 100 m) and eight directions from a translocated point source during seven two‐day periods. The model predictions were generally successful in describing the observed colonization patterns, especially beyond 10 m. In the laboratory we established spore release thresholds; horizontal wind speed sd > 0.25 m s^?1 induced the seta to vibrate and in relative humidity < 75% the peristome was open. Our dispersal model predicts that the proportion of spores dispersing beyond 100 m is almost twice as large if the spores are released under turbulent conditions than under more stable conditions. However, including release thresholds improved the fit of the model to the colonization data only minimally, with roughly the same amount of variation explained by the most constrained scenario (assuming both vibration of the seta and an open peristome) and the scenario assuming random release. Model predictions under realised experimental conditions suggest that we had a low statistical power to rank the release scenarios due to the lack of measurements of the absolute rate of spore release. Our results hint at the importance of release conditions, but also highlight the challenges in dispersal experiments intended for validating mechanistic dispersal models.     

Seasonal and Diurnal Patterns of Spore Release Can Significantly Affect the Proportion of Spores Expected to Undergo Long-Distance Dispersal

Many of the fungal pathogens that threaten agricultural and natural systems undergo wind-assisted dispersal. During turbulent wind conditions, long-distance dispersal can occur, and airborne spores are carried over distances greater than the mean. The occurrence of long-distance dispersal is an important ecological process, as it can drastically increase the extent to which pathogen epidemics spread across a landscape, result in rapid transmission of disease to previously uninfected areas, and influence the spatial structure of pathogen populations in fragmented landscapes. Since the timing of spore release determines the wind conditions that prevail over a dispersal event, this timing is likely to affect the probability of long-distance dispersal occurring. Using a Lagrangian stochastic model, we test the effect of seasonal and diurnal variation in the release of spores on wind-assisted dispersal. Spores released during the hottest part of the day are shown to be more likely to undergo long-distance dispersal than those released at other times. Furthermore, interactions are shown to occur between seasonal and diurnal patterns of release. These results have important consequences for further modelling of wind-assisted dispersal and the use of models to predict the spread of fungal pathogens and resulting population and epidemic dynamics.     

Fern and lycopod spores rain in a cloud forest of Hidalgo,Mexico

The aim of this study was to determine the composition of the “spore rain” of ferns and lycopods in a cloud forest. We tested whether the canopy impedes spore dispersal to surrounding areas and how spore dispersal is affected by rainfall. The spores were captured with a modified Bush–Gosling trap placed at 30 cm above ground level in forested and non-forested sites from March 2009 to February 2010. We collected 2462 fern spores from 158 morphospecies of which 76 were identified to species level. Thirty-seven species were found exclusively in the spore rain, and 39 were found as sporophytes as well (local component). Mean daily spore density (spores m^?2) was calculated to find the sporulation period for each species. Twenty species showed seasonal patterns of sporulation. The highest spore density was found at the forested site (70 morphospecies and 1856 spores), of which 39 morphospecies (1482 spores) corresponded to the local vegetation. Fifty-five taxa were shared between the forested and non-forested site. In the non-forested site, 605 spores were captured belonging to 64 species. The density of spore rain between sites was significantly different. The rainfall amount was the same at both sites, with a dry period in March, April, and July 2009, and February 2010. There was a negative effect of rainfall on spore rain. The main sporulation occurred in the dry season with strong winds. Although the canopy inhibits airborne dispersal of fern spores, a small amount of spores can disperse beyond the canopy and reach surrounding areas. The rainfall might wash spores to ground and favor the colonization and the establishment of new populations.     

Relationship between basidiospore size,shape and life history characteristics: a comparison of polypores

Although spore dispersal is the predominant mode of spread in fungi, the functional ecology of offspring (spore) size and shape has received little attention. We investigated the relationship between spore size (volume) and shape and various life history parameters among 303 European polypore species. In an analysis of variance, basidiocarp size, nutritional mode (parasitic vs. saprotrophic) and host (conifer vs. deciduous) accounted for a significant part of the variation in spore size. Species producing large basidiocarps also produce large spores, parasites produce larger spores than saprotrophs, and species colonizing deciduous trees generally produce larger spores than those colonizing conifers. There was a correlation between spore size and shape, with larger spores being more spherical. The most important factors accounting for variation in spore shape were rot type (white rot vs. brown rot) and nutritional mode, with white rot species and parasites having more spherical spores compared to brown rotters and saprotrophs.     

Size matters for violent discharge height and settling speed of Sphagnum spores: important attributes for dispersal potential

Background and Aims

Initial release height and settling speed of diaspores are biologically controlled components which are key to modelling wind dispersal. Most Sphagnum (peat moss) species have explosive spore liberation. In this study, how capsule and spore sizes affect the height to which spores are propelled were measured, and how spore size and spore number of discharged particles relate to settling speed in the aspherical Sphagnum spores.

Methods

Spore discharge and spore cloud development were filmed in a closed chamber (nine species). Measurements were taken from snapshots at three stages of cloud development. Settling speed of spores (14 species) and clusters were timed in a glass tube.

Key Results

The maximum discharge speed measured was 3·6 m s⁻¹. Spores reached a maximum height of 20 cm (average: 15 cm) above the capsule. The cloud dimensions at all stages were related positively to capsule size (R² = 0·58–0·65). Thus species with large shoots (because they have large capsules) have a dispersal advantage. Half of the spores were released as singles and the rest as clusters (usually two to four spores). Single spores settled at 0·84–1·86 cm s⁻¹, about 52 % slower than expected for spherical spores with the same diameters. Settling speed displayed a positive curvilinear relationship with spore size, close to predictions by Stokes'' law for spherical spores with 68 % of the actual diameters. Light-coloured spores settled slower than dark spores. Settling speed of spore clusters agrees with earlier studies. Effective spore discharge and small, slowly settling spores appear particularly important for species in forested habitats.

Conclusions

The spore discharge heights in Sphagnum are among the greatest for small, wind-dispersed propagules. The discharge heights and the slow settling of spores affect dispersal distances positively and may help to explain the wide distribution of most boreal Sphagnum species.     

A network involving Rho-type GTPases, a paxillin and a formin homologue regulates spore length and spore wall integrity in the filamentous fungus Ashbya gossypii

Fungi produce spores that allow for their dispersal and survival under harsh environmental conditions. These spores can have an astonishing variety of shapes and sizes. Using the highly polar, needle-shaped spores of the ascomycete Ashbya gossypii as a model, we demonstrated that spores produced by this organism are not simple continuous structures but rather consist of three different segments that correlate with the accumulation of different materials: a rigid tip segment, a more fragile main spore-compartment and a solid tail segment. Little is currently known about the regulatory mechanisms that control the formation of the characteristic spore morphologies. We tested a variety of mutant strains for their spore phenotypes, including spore size, shape and wall defects. The mutants that we identified as displaying such phenotypes are all known for their roles in the regulation of hyphal tip growth, including the formin protein AgBni1, the homologous Rho-type GTPases AgRho1a and AgRho1b and the scaffold protein AgPxl1. Our observations suggest that these proteins form a signalling network controlling spore length by regulating the formation of actin structures.     

Deposition of Erysiphe graminis Conidia on a Barley Crop

Naturally released Erysiphe graminis conidia were trapped (on horizontal slides, on vertical sticky cylinders and in suction traps) in a barley crop infected with powdery mildew and the numbers of single spores and of clumps of different sizes deposited on the traps were counted. The efficiencies of impaction calculated from deposits and wind speed measurements were higher than expected from mean wind speed measurements. The values were consistent with the hypothesis that spores were removedpredominantly in gusts. More than half the conidia were removed in clumps of two or more spores. The measurements suggest that clumps were more effectively deposited than single spores. The measurements demonstrate that spore release mechanisms can influence spore deposition significantly, especially close to the source.     

EXCHANGES OF INFORMATION,ENERGY & MATERIALS IN SYMBIOSES

We investigated patterns of spore dispersal in the giant kelp Macrocystis pyrifera by collecting 80 independent measurements of spore dispersal from isolated individuals and isolated groups of individuals over a two‐year period. Our results indicate that giant kelp spores routinely disperse both short (i.e. a couple meters) and long (i.e. hundreds to thousands of meters) distances depending on the oceanographic conditions. One consequence of spore dispersal over short distances is self‐fertilization (i.e., fertilization between male and female gametophytes derived from the same sporophyte). Field experiments designed to test the effects of self‐fertilization on lifetime fitness in Macrocystis revealed significant inbreeding depression. Birth rates in self‐fertilized populations were ca. 50% of those produced from outcrossing, which lead to significant differences in cohort size that persisted up through the adult stage. In contrast to outcrossed populations, very few individuals produced from selfing became reproductively mature, and those that did were significantly less fecund than outcrossed individuals. By contrast, long‐range dispersal of spores leads to increased rates of outcrossing. However, long‐range dispersal is typically accompanied by massive dilution of spores, leading to low densities of spore settlement. Sparse spore settlement decreases the overall chance of fertilization in the microscopic gametophyte generation thereby reducing the potential for colonization of the macroscopic sporophyte stage. Large population size of adult sporophytes coupled with the synchronous release of spores in response to environmental cues can help offset the effects of spore dilution and extend the distances over which giant kelp is able to colonize.     

Detection of arbuscular mycorrhizal fungal spores in the air across different biomes and ecoregions

Aerial dispersal of fungal spores is common, but the role of wind and air movement in dispersal of spores of arbuscular mycorrhizal (AM) fungi is largely unknown. Several studies have examined the possibility of AM fungal spores being moved by wind vectors without observing spores taken from the air environment. For the first time this study observed the presence of AM fungal spores in the air. The frequency of AM fungal spores in the air was determined in six North American biomes composed of 18 ecoregions. Multiple samples were taken from both the air and the soil at each location. AM fungal spores were found in high abundance in the soil (hundreds of spores per gram of soil), however, they were rarely found in the air (most samples contained no AM fungal spores). Furthermore, only the Glomus morphotype was found in the air, whereas spores in the soil were taxomomically more diverse (Glomus, Acaulospora, Gigaspora, Scutellospora morphotypes were observed). The proportion of Glomus spores in the air relative to Glomus spores in the soil was highest in more arid systems, indicating that AM fungi may be more likely to be dispersed in the air in such systems. Nonetheless, the results indicate that the air is not likely a dominant mode of dispersal for AM fungi.     

Natural folding of airborne fungal spores: a mechanism for dispersal and long-term survival?

Analysis of numerous air samples has indicated that dormant, viable fungal spores are highly present, which suggests that aerial dispersion is important for fungi. Whereas the majority of the spores may travel only very short distances, there is indication that a notable number of them cover much longer distances. Harmomegathy is a terminology coined by Wodehouse (1935) describing the natural folding of pollen to accommodate controlled and reversible water loss. Here, we discuss evidence that this concept may also apply to airborne fungal spores that face similar challenges and have to survive periods of drought and low temperatures while retaining viability to germinate after deposition upon a suitable moist substrate. In fact, (air)dried conidia, appear collapsed, survive for much longer times compared to spores in liquid, that deteriorate in time. This indicates that for some types of fungal spores, true dormancy is reached in the desiccated state. For these airborne spores this might be regarded as a pre-adaptation that supports long-distance transport of viable cells through air. We state that spores are naturally folded during transport in air if the humidity is low enough. We hypothesize that this is a pre-adaptation supporting release, dispersal and survival of airborne spores. Moreover, the smaller size of dry naturally-folded spores may also be relevant, e.g. for the opportunistic pathogenic fungus Aspergillus fumigatus reduced spore size supports deposition within the alveoli in the lung.     

Dispersal of Erysiphe graminis and Lycopodium clavatum spores near to the source in a barley crop

In experiments to study dispersal of spores in a crop of barley, a 4-m wide strip of the cultivar Zephyr (a mildew-susceptible variety) was a source of mildew (Eyrsiphe graminis) conidia. Small suction traps, previously calibrated in a wind tunnel, were used to measure spore concentration within and above the crop. Large concentrations of conidia were measured in the crop at the downwind edge of the Zephyr strip but these decreased rapidly with distance downwind. At 1 m concentrations were halved and by 4 m they were no greater than the background concentration measured in the crop upwind of the source. Next to the source, concentrations were much greater within than above the crop and net spore movement (flux) was upwards out of the crop; by 4 m downwind concentrations were greater above the crop and spore flux was reversed. Lycopodium clavatum spores were released in the same crop from a line of point sources. Concentrations also decreased rapidly downwind but, with no background of spores, numbers remained greater within than above the crop further from the source than for E. graminis. Even so by 7 m downwind concentrations in the crop had declined to less than those above. Deposition of L. clavatum spores onto horizontal glass slides in the crop agreed with that expected by settling. However, impaction onto vertical cylinders among plants was much greater than predicted. The reason is not known although turbulent air-flow around plants may, in some way, enhance impaction. Many E. graminis conidia near the source were deposited in clumps. This prevented any accurate prediction of deposition rates as fall speeds of clumps (necessary for prediction) were not known. Not surprisingly, deposition on horizontal slides often exceeded that expected from settling of single spores although it was not always greatest where clumps predominated. The proportion of spores deposited on vertical cylinders and horizontal slides located among plants ranged from 0–02 to 0–27 and from 0–019 to 0–127 of the area dose, respectively. Although these may seem to be small trapping efficiencies, the same deposition rates in a crop with many leaves and stems would rapidly filter most spores from air in the crop and can explain why concentrations were observed to decline so rapidly.     

Some components of the air spora in Jamaica and their possible medical application

A knowledge of the pollen and fungal spores which comprise the air spora is useful as a preliminary approach to the problem of respiratory allergy. Therefore, this study of the qualitative and quantitative aspects of the air spora was done. Fungal spores were found to be numerically dominant, comprising 97.73% whilst pollen comprised 0.40% of the total material observed. A small number of types made up the majority of the fungal air spora, namely, Cladosporium, the Sporobolomycetaceae group, Diatrype, Glomerella, hyaline and coloured basidiospores, and septate fusiform spores. Seasonal periodicity studies on twenty-five fungal types showed that a high number of spores were trapped for sixteen during wet months, four during cooler months, and that five showed no seasonal trends. Mean diurnal periodicity studies for the year on the same twenty-five spore types showed that all had a maximum number of spores trapped at some time during the day. Investigation of the effect of rainfall on the numbers of spores released showed that the amount and duration of rainfall, the time of day rain occurs, and the length of the dry period preceding rain were of varying importance to particular spore types.     

The high-resolution architecture and structural dynamics of Bacillus spores

The capability to image single microbial cell surfaces at nanometer scale under native conditions would profoundly impact mechanistic and structural studies of pathogenesis, immunobiology, environmental resistance, and biotransformation. Here, using in vitro atomic force microscopy, we have directly visualized high-resolution native structures of bacterial endospores, including the exosporium and spore coats of four Bacillus species in air and water environments. Our results demonstrate that the mechanisms of spore coat self-assembly are similar to those described for inorganic and macromolecular crystallization. The dimensions of individual Bacillus atrophaeus spores decrease reversibly by 12% in response to a change in the environment from fully hydrated to air-dried state, establishing that the dormant spore is a dynamic physical structure. The interspecies distributions of spore length and width were determined for four species of Bacillus spores in water and air environments. The dimensions of individual spores differ significantly depending upon species, growth regimes, and environmental conditions. These findings may be useful in the reconstruction of environmental and physiological conditions during spore formation and for modeling the inhalation and dispersal of spores. This study provides a direct insight into molecular architecture and structural variability of bacterial endospores as a function of spatial and developmental organizational scales.     

Patterns of unobstructed splash dispersal

Unobstructed splash dispersal patterns were measured in the absence of rain over mown grass using a fluorescent tracer, and a colorimetric method was used indoors in still air. When drops fell into a thin horizontal water film 0–1 mm deep, the volume of the incident drops dissipated as splash droplets was similar to the volume splashed from the film, irrespective of the distance of fall of the drops. Drop size, angle of inclination and distance of fall had significant effects on the volume of drops splashed from an inclined surface. The effects of rigidity, inclination and nature of surface were found to be significant when drops impacted onto surfaces with or without a wax covering and either rigidly or loosely supported. When splash- and dry-air-dispersed Lycopodium spores were simultaneously released, many more splashed spores were caught close to the source, but the dispersal gradient of splashed spores was steeper than that of dry-air-dispersed spores. Splash-dispersed spores were caught on slides, cylinders and rotorods but trap efficiency could not be evaluated.     

Dispersal of Septoria nodorum Pycnidiospores by Simulated Rain and Wind

The influence of wind on the splash dispersal of Septoria nodorum pycnidiospores was studied in a raintower/wind tunnel complex with single drops or simulated rain falling on spore suspensions or infected stubble with windspeeds of 1.5 to 4 m/sec. When single drops fell on spore suspensions (depth 0.5 mm, concentration 7.8 × 10⁵ spores/ml) most of the spore-carrying droplets collected on fixed photographic film between 0–4 m downwind (windspeed 3 m/sec) were >200 μm in diameter. However, most spores were carried in droplets with diameter > 1000 μm, 70 % of which carried more than 100 spores. When simulated rain fell on infected stubble most of the spore-carrying droplets collected beyond 1 m downwind (windspeeds 1.4 and 4 m/sec) were <200 μm in diameter and none were >600 μm; most of these droplets carried only one spore. The distribution of splash droplets (with diameter >100 μm) deposited on chromatography paper showed a maximum at 40–50 cm upwind of the target but many more droplets were deposited 20–30 cm downwind, when single drops fell on a spore suspension (concentration 1.2 × 10⁵ spores/ ml) containing fluorescein dye with a windspeed of 2 m/sec; droplets were collected up to 3 m downwind but not more than 70 cm upwind. With a windspeed of 3 m/sec, numbers of sporecarrying droplets and spores collected on film decreased with increasing distance downwind; most were collected within 2 m of the target but some were found up to 4 m. When simulated rain fell on infected stubble, increasing the windspeed from 1.5 to 4 m/sec greatly increased the number of spores deposited more than 1 m downwind. At 1.5 m/sec none were collected beyond 2 m downwind, whereas at 4 m/sec some were collected at 4 m. A few air-borne S. nodorum spores were collected by suction samplers at a height of 40 cm at distances up to 10 m downwind of a target spore suspension on which simulated rain fell.     

Spore rain in relation to regional sources and beyond

While patterns of spore dispersal from single sources at short distances are fairly well known, information about ‘spore rain’ from numerous sources and at larger spatial scales is generally lacking. In this study, I sampled spore rain using a novel method consisting of 0.25–0.5 m² cotton cloth traps at nine sites in the boreo‐nemoral vegetation zone in eastern Sweden during two seasons, using Sphagnum spores as a model. Traps were located in various landscapes (mainland, islands). Additional trapping was done in an arctic area (Svalbard) without spore production. Spore densities were tested against distance from the nearest source and area of sources (open peatlands) within different radii around each site (5, 10, 20, 50, 100, 200, 300, 400 km). The cloth method appeared reliable when accounting for precipitation losses, retaining approximately 20–60% of the spores under the recorded amounts of precipitation. Estimated spore densities ranged from 6 million m^?2 and season within a large area source, via regional deposition of 50 000–240 000 spores m^?2, down to 1000 m^?2 at Svalbard. Spore rain for all sites was strongly related to distance from the nearest source, but when excluding samples taken within a source peatland, the amount of sources within 200 km was most important. Spores were larger at isolated island sites, indicating that a higher proportion originated from distant, humid areas. Immense amounts of Sphagnum spores are dispersed across regional distances annually in boreal areas, explaining the success of the genus to colonise nutrient poor wetlands. The detectable deposition at Svalbard indicates that about 1% of the regional spore rain has a trans‐ or intercontinental origin. The regional spore rain, originating from numerous sources in the landscape, is probably valid for most organisms with small diaspores and provides a useful insight in ecology, habitat restoration and conservation planning.     

Deposition Gradients Near to a Point Source in a Barley Crop

By measuring deposits of droplets downwind from a source in a barley crop during crop growth, gradients of deposition were established. Droplets were generated using a May spinning disc, which approximated to a point source. Droplets were labelled with thiabendazole so that deposit could be measured photometrically. Droplet diameter was approximately 20 micrometres, a size similar to spores of barley powdery mildew, an important foliar pathogen. Gradients of deposition were influenced by the density of the crop, by wind speed and by air turbulence. At any one time these three factors could interact to change the gradient substantially. Exponential and power law equations fitted the data equally well although due to experimental variation neither gave a very good fit. It is suggested that over the first few metres of dispersal from the source an exponential equation can be used to model the gradient. This has the advantage that it can be extrapolated to give an estimate of “self-infection” i.e. the spores deposited on the plant on which they were produced.