Revisiting the Life Cycle of Dung Fungi, Including Sordaria fimicola.

Dung fungi, such as Sordaria fimicola, generally reproduce sexually with ascospores discharged from mammalian dung after passage through herbivores. Their life cycle is thought to be obligate to dung, and thus their ascospores in Quaternary sediments have been interpreted as evidence of past mammalian herbivore activity. Reports of dung fungi as endophytes would seem to challenge the view that they are obligate to dung. However, endophyte status is controversial because surface-sterilization protocols could fail to kill dung fungus ascospores stuck to the plant surface. Thus, we first tested the ability of representative isolates of three common genera of dung fungi to affect plant growth and fecundity given that significant effects on plant fitness could not result from ascospores merely stuck to the plant surface. Isolates of S. fimicola, Preussia sp., and Sporormiella sp. reduced growth and fecundity of two of three populations of Bromus tectorum, the host from which they had been isolated. In further work with S. fimicola we showed that inoculations of roots of B. tectorum led to some colonization of aboveground tissues. The same isolate of S. fimicola reproduced sexually on inoculated host plant tissues as well as in dung after passage through sheep, thus demonstrating a facultative rather than an obligate life cycle. Finally, plants inoculated with S. fimicola were not preferred by sheep; preference had been expected if the fungus were obligate to dung. Overall, these findings make us question the assumption that these fungi are obligate to dung.

Exclusionary interactions among diverse fungi infecting developing seeds of Centaurea stoebe.

Developing seeds are expected to be strongly defended against microbial attack. In keeping with this, only 26% of seeds of Centaurea stoebe from its native and invaded ranges in Eurasia and North America were infected with fungi, and 92.2% of those were infected with a single fungus per seed. Even when developing seeds in flower heads were inoculated under conducive conditions for infection with 14 of these seed-infecting fungi, re-isolation of inoculants was only 16% overall, and again limited to the particular inoculant. Environmental fungi (i.e. those not isolated from seed of C. stoebe) were present in control flower heads under conditions conducive to infection but they were never re-isolated from fully developed seeds in any experiments. When two or three seed isolates were co-inoculated to compete in flower heads, only one inoculant, and always the same one, was re-isolated from all matured seeds, regardless of maternal plant genotype. PCR-based detection methods confirmed that these fungal interactions were exclusionary rather than suppressive. In these strongly defended, developing seeds, we had expected the plant to control not only the overall level of infection but also the outcome of co-inoculations. Consequences for the next plant generation of this exclusionary competition among seed-infecting fungi included effects on seedling emergence, growth and fecundity.

A novel plant-fungal mutualism associated with fire.

Bromus tectorum, or cheatgrass, is native to Eurasia and widely invasive in western North America. By late spring, this annual plant has dispersed its seed and died; its aboveground biomass then becomes fine fuel that burns as frequently as once every 3-5 y in its invaded range. Cheatgrass has proven to be better adapted to fire there than many competing plants, but the contribution of its fungal symbionts to this adaptation had not previously been studied. In sampling cheatgrass endophytes, many fire-associated fungi were found, including Morchella in three western states (New Mexico, Idaho, and Washington). In greenhouse experiments, a New Mexico isolate of Morchella increased both the biomass and fecundity of its local cheatgrass population, thus simultaneously increasing both the probability of fire and survival of that event, via more fuel and a greater, belowground seed bank, respectively. Re-isolation efforts proved that Morchella could infect cheatgrass roots in a non-mycorrhizal manner and then grow up into aboveground tissues. The same Morchella isolate also increased survival of seed exposed to heat typical of that which develops in the seed bank during a cheatgrass fire. Phylogenetic analysis of Eurasian and North American Morchella revealed that this fire-associated mutualism was evolutionarily novel, in that cheatgrass isolates belonged to two phylogenetically distinct species, or phylotypes, designated Mel-6 and Mel-12 whose evolutionary origin appears to be within western North America. Mutualisms with fire-associated fungi may be contributing to the cheatgrass invasion of western North America.