Calvatia craniiformis

Description

The fruit bodies of Calvatia craniiformis grow to dimensions of 6–20 cm (2.4–7.9 in) tall by 8–20 cm (3.1–7.9 in) wide, and have a form ranging from pear-shaped, to flattened-spherical, to obovate (roughly egg-shaped), to tunicate (like an inverted cone). At the bottom is a thick, often crumpled base attached to a cord-like rhizomorph, which is often encrusted with surrounding soil. The rhizomorphs are well developed, and when cut into longitudinal section, reveal three distinct tissues: an outer cortex, a subcortical layer, and a central core. The thin and fragile exoperidium (the outer layer of "skin") is whitish-gray to gray, and initially smooth before becoming areolate (divided by cracks into discrete areas). The base extends up one-third to one-halfway into the puffball (where it becomes the columnella) tapering to a point. The gleba is initially whitish, and then yellow-green, and finally brownish-green in older specimens with mature spores.

Spores are spherical, hyaline (translucent), and measure 2.5–3.4 μm in diameter. They are thick-walled with a short pedicel (a tubelike extension), and are ornamented with tiny spines (verrucae) that are roughly equidistant from each other. Capillitial threads are long, hyaline, and branched, measuring 2.4–4 μm thick. They are septate and occasionally have pits on their walls. The exoperidium comprises thick-walled, inflated hyphae mixed with sphaerocysts (spherical cells), while the endoperidium is made of tightly interwoven, thick-walled hyphae. In the rhizomorphs, the hyphae in the central core are several times as thick as those in the surrounding subcortex.

Using light microscopy, the spores of Calvatia craniiformis are generally indistinguishable from those of C. rubroflava and C. gigantea; electron microscopy reveals that each has distinctive spore ornamentation. C. craniiformis features small, well-separated verrucae (wartlike projections) up to 0.2 μm tall with rounded tips. In comparison, C. gigantea has larger verrucae (up to 0.4 μm tall) that are more irregularly arranged.

Development

Fruit bodies arise from the tips or the lateral branches or rhizomorphs. New fruit bodies are made of an exterior of thick hyphae similar to those found in the rhizomorph core; in contrast, the hyphae of the interior originate from the puffball's core. Unlike some other puffball species, the peridium does not differentiate into a distinct exoperidium and endoperidium; rather, the outer layer develops the features of a pseudoparenchyma (a tightly organised tissue where the tightly packed cells resemble plant parenchyma) as the radial and tangential hyphae become interweaved. In time, the peridium dries and falls off in flakes to expose the underlying gleba.

Similar species

The brain-like surface folds and mature olive-brown gleba are characteristic of Calvatia craniiformis, but younger puffballs that have not yet developed these characteristics may be difficult to identify to species. Another edible puffball, C. cyathiformis (lilac puffball) grows to similar dimensions but has gleba that is purple-brown when mature. Calvatia fragilis is smaller and pink or purple mature gleba. C. bicolor is a smaller, rounder puffball that could be confused with younger specimens of C. craniiformis, but the former species has more coarsely ornamented spores, and lacks a distinct subgleba. Handkea utriformis is roughly similar in appearance to C. craniiformis, but unlike the latter it develops a cavernous opening to reveal an olive-brown gleba, and has distinct slits in its capillitial threads.

Uses

Calvatia craniiformis is an edible species. Young puffballs with a firm, white gleba have a mild odor and pleasant taste. Early 20th-century mycologist Charles McIlvaine noted over a century ago that "the slightest change to yellow makes it bitter." Versatile in cooking, the puffball absorbs flavors well.

In the United States, the Ojibwe used the powdery gleba as a hemostatic agent to staunch the flow of nosebleeds: the spore powder was inhaled through the nostrils. It is now known that this practice can lead to the pulmonary disease lycoperdonosis, which causes symptoms similar to pneumonia. It is also used as a hemostatic agent in Chinese and Japanese folk medicines.

Distribution and ecology

In Asia, it has been recorded from China, India, Indonesia, Japan, Malaysia, and South Korea. The brain puffball has been recorded from Australia. In North America, its range includes the eastern and southern United States, and Mexico. In Michigan, it is one of the few macrofungi found regularly in black locust plantations.

Brain puffballs grow singly or in groups in fields and open woods, hardwood forests, and wet areas. Although Calvatia craniiformis is generally considered a saprobic species, in controlled laboratory conditions, an ectomycorrhizae between the fungus and American sweetgum (Liquidambar styraciflua) was reported in a 1966 publication. A Chinese study showed that C. craniifromis would readily form mycorrhiza with poplar seedlings on unsterilized, but not on sterilized soil. Later research was unable to establish any similar association between C. craniiformis and Pinus ponderosa.

Fruit bodies serve as a food source for several species of flies, and have been recorded as occasional food of the pleasing fungus beetle Tritoma unicolor.

Research

Extracts of the puffball have strong antitumor activity in mouse models attributable to protein-bound polysaccharides, the compounds calvatan, craniformin, and a tautomer of rubroflavin. Calvatan is thought to act by stimulating the immune response, rather than by killing cells. Craniformin, originally reported in 1997, is an azoformamide compound. Three sterol compounds have been identified from the fungus: ergosta-4,6,8(14), 22-tetraene-3-one, ergosta-7,22-diene-3-ol, and ergosterol peroxide.

Three azo- and azoxyformamides compounds have been isolated and identified from the puffball and tested for their ability to inhibit the growth of plants: 4-methoxybenzene-1-ONN-azoxyformamide, 4-methoxybenzene-1-azoformamide, and 4-hydroxybenzene-1-azoformamide. Only the first compound was significantly inhibitory, suggesting that the presence of the azoxy moiety is required for growth inhibition.