Peter A. McGee

Lack of global population genetic differentiation in the arbuscular mycorrhizal fungus Glomus mosseae suggests a recent range expansion which may have coincided with the spread of agriculture.

The arbuscular mycorrhizal fungus Glomus mosseae is commonly found in agricultural fields. The cosmopolitan species is found in Africa, Europe, America, Asia and Australia. Three hypotheses may explain this worldwide distribution: First, speciation occurred before the continents separated 120 Ma; second, the distribution is a result of human‐mediated dispersal related to agriculture and finally, the morphologically defined species may encompass several local endemic species. To test these hypotheses, three genes were sequenced from 82 isolates of G. mosseae originating from six continents and the resulting sequences analysed for geographical subdivision and estimation of migration between continents. Coalescent analyses estimated divergence and age of mutations. Bayesian coalescent modelling was used to reveal important past population changes in the global population. The sequence data showed no geographical structure, with identical genotypes found on different continents. Coalescence analyses indicated a recent diversification in the species, and the data could be explained by a recent population expansion in G. mosseae. The results of this study suggest that speciation and the range expansion happened much later than continental spread and that human activity may have had a major impact on the dispersal and the population structure of the fungus.

Some endophytic fungi reduce the density of pustules of Puccinia recondita f. sp. tritici in wheat

The interaction between Puccinia recondita f. sp. tritici (now widely referred to as P. triticina) and endophytic fungi in wheat was examined in laboratory experiments to determine whether the presence of fungal endophytes suppresses leaf rust disease caused by this fungus. Endophytes and cell-free washings from culture plates of the endophytes reduced the density and size of pustules in a susceptible cultivar when inoculated 3, 7 and 14 d prior to the pathogen. Disease at 12 d was reduced when the endophytes were inoculated simultaneously up to 50 mm from the fungus. Interactions between endophytes and this Puccinia are most probably mediated by defence mechanisms induced in the host plant.

Some Chytridiomycota in soil recover from drying and high temperatures

Rhizophlyctis rosea was found in 44% of 59 soil samples from national parks, urban reserves and gardens, and agricultural lands of eastern New South Wales, Australia. As some of the soils are periodically dry and hot, we examined possible mechanisms that enable survival in stressful environments such as agricultural lands. Air-dried thalli of R. rosea in soil and pure cultures of R. rosea, two isolates of Allomyces anomalus, one isolate of Catenaria sp., one of Catenophlyctis sp. and one of Spizellomyces sp. recovered following incubation at 90 degrees C for two days. Powellomyces sp. recovered following incubation at 80 degrees. Sporangia of all seven fungi shrank during air-drying, and immediately returned to turgidity when rehydrated. Some sporangia of R. rosea released zoospores immediately upon rehydration. These data indicate that some Chytridiomycota have resistant structures that enable survival through periodic drying and high summer temperatures typical of soils used for cropping. Eleven Chytridiomycota isolated from soil did not survive either drying or heat. Neither habitat of the fungus nor morphological type correlated with the capacity to tolerate drying and heat.

Simulated fire reduces the density of arbuscular mycorrhizal fungi at the soil surface

In experimental microcosms, three Glomus spp. were subjected to heating to over 200 °C at the soil surface and 70° at 5 cm to determine the effect of fire on survival of arbuscular mycorrhizal fungi. Heating reduced the quantity of propagules surviving at the soil surface and the effect declined with depth. While all propagules are likely to be affected by heat, we argue that the hyphal network is most severely disturbed and probably responsible for declines in density of fungi observed in the field following fire.

Sporocarpic Endogonales and Glomales in the scats of Rattus and Perameles

Fifteen species of fungi in the Glomales and Endogonales (Zygomycetes) were recovered from faecal pellets of species of Rattus and the bandicoot Perameles nasuta in the Sydney region and from Rattus villosissimus in the Simpson Desert. Fungi were recovered in all seasons of the year. Most attempts to locate the fungal sporocarps at the soil surface failed. Many of the fungi formed mycorrhizas when inoculated onto seedlings in sterile soil.

Utilisation of carbon substrates by orchid and ericoid mycorrhizal fungi from Australian dry sclerophyll forests

The utilisation of a range of cell-wall-related and aromatic carbon substrates by multiple genotypes of three ericoid mycorrhizal fungal taxa was compared with two orchid mycorrhizal fungal taxa. Both groups of fungi catabolised most common substrates, though significant inter- and intraspecific variability was observed in the use of a few carbon substrates. Orchid mycorrhizal fungi had limited access to tannic acid as a carbon source and did not use phenylalanine, while the ericoid mycorrhizal fungi used both. Utilisation of tryptophan was limited to single genotypes of each of the orchid mycorrhizal fungi, and to only two of the three ericoid mycorrhizal fungi examined. Although broadly similar, some significant differences apparently exist in carbon catabolism of ericoid and orchid mycorrhizal fungi from the same habitat. Functional and ecological implications of these observations are discussed.

The growth response of some Chytridiomycota to temperatures commonly observed in the soil

Chytridiomycota were isolated into pure culture from cool temperate and warm semi-arid soils of eastern Australia. In pure culture these fungi responded variably to the range of temperatures commonly recorded in their environment. All members of the Blastocladiales, Spizellomycetales and Chytridiales grew in culture at temperatures up to 30 degrees C. Some isolates from the Blastocladiales and Spizellomycetales continued to grow at or above 37 degrees. Some isolates of the Chytridiales grew up to but not beyond 35 degrees. All isolates in the Chytridiales were able to resume growth at 20 degrees after brief exposure to temperatures higher than the maximum growth temperature, but were killed by exposure to higher temperatures for 7 d. Because in the natural soil habitat temperature may exceed the maximum for growth it may be a limiting factor that determines the distribution of chytrids in the soil.

Access to organic and insoluble sources of phosphorus varies among soil Chytridiomycota

The sources of minerals accessed by fungi in the Chytridiomycota (chytrid) in soil are largely unknown. The ability of ten species of soil chytrids to use various sources of phosphorus was examined in vitro. While all grew on orthophosphate, fifty per cent of isolates grew on phytic acid, and one isolate grew on DNA as the sole source of phosphorus. All isolates solubilised and utilised CaHPO4. Most isolates utilised hydroxyapatite when NH4+ was the nitrogen source. When ammonium was omitted, 50% of isolates solubilised hydroxyapatite. Many soil chytrids may utilise phosphomonoesters as the sole source of phosphorus, and access to DNA appears limited. We suggest that the capacity to use different sources of phosphorus may influence the diversity of chytrids found in Australian soils.

Involvement of exodermal passage cells in mycorrhizal infection of some orchids

Roots with velamen of three orchids were examined for mycorrhizal infection: an epiphytic orchid Stanhopea tigrina , a lithophyte Dendrobium kingianum and a terrestrial Epidendrum radicans . Approximately 50% of all exodermal cells were passage cells that remained non-lignified. Long cells of the exodermis were lignified, but not suberized. A similar proportion of passage cells was infected in young and old root sections. Most coils were located in the three cell layers of cortex beneath the exodermis. A larger proportion of coils was collapsed in the older part of the roots of D. kingianum and E. radicans .

Freeze tolerance of soil chytrids from temperate climates in Australia.

Very little is known about the capacity of soil chytrids to withstand freezing in the field. Tolerance to freezing was tested in 21 chytrids isolated from cropping and undisturbed soils in temperate Australia. Samples of thalli grown on peptone-yeast-glucose (PYG) agar were incubated for seven days at -15 degrees C. Recovery of growth after thawing and transferring to fresh medium at 20 degrees C indicated survival. All isolates in the Blastocladiales and Spizellomycetales survived freezing in all tests. All isolates in the Chytridiales also survived freezing in some tests. None of the isolates in the Rhizophydiales survived freezing in any of the tests. However, some isolates in the Rhizophydiales recovered growth after freezing if they were grown on PYG agar supplemented with either 1% sodium chloride or 1% glycerol prior to freezing. After freezing, the morphology of the thalli of all isolates was observed under LM. In those isolates that recovered growth after transfer to fresh media, mature zoosporangia were observed in the monocentric isolates and resistant sporangia or resting spores in the polycentric isolates. Encysted zoospores in some monocentric isolates also survived freezing. In some of the experiments the freezing and thawing process caused visible structural damage to the thalli. The production of zoospores after freezing and thawing was also used as an indicator of freeze tolerance. The chytrids in this study responded differently to freezing. These data add significantly to our limited knowledge of freeze tolerance in chytrids but leave many questions unanswered.