Astraeus hygrometricus: The Hygroscopic Earthstar
Astraeus hygrometricus is a species of fungus that belongs to the family Diplocystaceae. It is commonly known as the hygroscopic earthstar, the barometer earthstar, or the false earthstar. It is an ectomycorrhizal fungus that grows in association with various trees, especially in sandy soils. It has a cosmopolitan distribution, but it is mainly found in temperate and tropical regions.
Morphology and Life Cycle
Astraeus hygrometricus produces fruit bodies that resemble puffballs when young and unopened. As they mature, they split open in a star-like manner, exposing the spore sac in the center. The fruit body consists of two layers: the outer layer (exoperidium) and the inner layer (endoperidium). The outer layer is brown and leathery, and it splits into 6 to 15 rays that can curl up or down depending on the humidity. The rays are hygroscopic, meaning they can absorb or release water. This allows the fungus to regulate the spore dispersal according to the environmental conditions. The inner layer is pale brown and smooth, and it contains the spore sac. The spore sac has an irregular slit or tear at the top, through which the spores are released. The spores are reddish-brown, spherical, and minutely warted. They measure 7.5â11 micrometers in diameter.
The fungus forms a mutualistic relationship with the roots of various trees, such as oaks, pines, eucalyptus, and acacias. The fungal mycelium helps the trees absorb nutrients from the soil, while the trees provide carbohydrates to the fungus. The fungus can also form sclerotia, which are hard masses of mycelium that can survive unfavorable conditions.
Taxonomy and Phylogeny
Astraeus hygrometricus was first described by Christiaan Hendrik Persoon in 1801 as Geastrum hygrometricum. In 1885, Andrew P. Morgan proposed that differences in microscopic characteristics warranted the creation of a new genus Astraeus distinct from Geastrum. However, this opinion was not universally accepted by later authorities. Several Asian populations formerly thought to be A. hygrometricus were renamed in the 2000s once phylogenetic analyses revealed they were unique Astraeus species, including A. asiaticus and A. odoratus. Similarly, in 2013, North American populations were divided into A. pteridis, A. morganii, and A. smithii on the basis of molecular phylogentics. This research suggests that the type specimen of Astraeus hygrometricus originates in a population restricted to Europe between Southern France and Turkey, with A. telleriae found nearby in Spain and Greece.
The genus Astraeus belongs to the family Diplocystaceae, which is part of the order Boletales in the class Agaricomycetes. The family also includes Diplocystis and Scleroderma.
Bioactive Compounds
Research has revealed the presence of several bioactive chemical compounds in Astraeus fruit bodies. These include:
- Astrahygrolide A: a novel sesquiterpenoid with anti-inflammatory activity.
- Astrahygrolide B: another novel sesquiterpenoid with anti-inflammatory activity.
- Astrahygrolide C: a novel sesquiterpenoid with cytotoxic activity against human cancer cell lines.
- Astrahygrolide D: a novel sesquiterpenoid with cytotoxic activity against human cancer cell lines.
- Astrahygrolide E: a novel sesquiterpenoid with cytotoxic activity against human cancer cell lines.
- Astrahygrolide F: a novel sesquiterpenoid with cytotoxic activity against human cancer cell lines.
- Astrahygrolide G: a novel sesquiterpenoid with cytotoxic activity against human cancer cell lines.
- Astrahygrolide H: a novel sesquiterpenoid with cyt