Asterophora - mushrooms on mushrooms
There are many fungi, mostly moulds, that grow on mushrooms but there are very few species of mushrooms that grow on other mushrooms. Asterophora is a genus with mushrooms as fruiting bodies in which all species grow on other mushrooms, generally those in the genera Russula and Lactarius. The classic Asterophora picture is probably plate 5 in part 8 of Oscar Brefeld's Untersuchungen aus dem Gesammtgebiete der Mykologie, published in 1889. It has been reproduced numerous times and here it is:
The large central figure in this plate shows a number of specimens of Asterophora lycoperdoides growing on a dark brown mushroom of the species Russula nigricans. In Brefeld's book that central figure showed all the mushrooms life-size, with the left hand one of the largest two Asterophora caps being 25 millimetres in diameter. Brefeld referred to this species as Nyctalis asterophora.
In the painting you can see that the upper surfaces of some of the caps are breaking up. In fact the cap surfaces will eventually break down into a brown, powdery mass. Each granule is a chlamydospore. The spores that are produced on the basidia that line the gills of mushrooms are sexual spores and their production involves the mixing of genes from two parents. Many fungi also produce one or more types of asexual spores, where there has been no mixing of genes from two parents. Spores produced on basidia are more precisely called basidiospores to distinguish them clearly from the various types of asexual spores. Chlamydospores are single-celled and during their development the protoplasm in a cell contracts and a thick wall, typically pigmented, forms. Chlamydospores develop in a variety of places on fungi, depending on the species involved.
The chlamydospores of Asterophora lycoperdoides are markedly knobbly, generally between 10 and 20 micrometres in length, but sometimes longer, and the above plate shows a few. In the lower right hand corner of that plate you can see some germinating. On the right are three of the chlamydospore drawings shown enlarged.
These chlamydospores can form in the terminal cells of hyphae or in cells away from a hyphal apex so that chains of chlamydospores can be created. When mature the connections between neighbouring chlamydospores or between chlamydospores and adjacent, non-chlamydospore cells weakens and finally the individual chlamydospores become free. Generally the chlamydospores start forming early and in abundance during the development of the mushrooms of Asterophora lycoperdoides whereas gills develop later and slowly. Mushroom development often comes to a stop with the cap surfaces a mass of powder but with gills still very rudimentary or even non-existent. In such circumstances there would be little or no spore production on the gills. Indeed, even when gills are present, basidia are rarely found in Asterophora lycoperdoides and are found only on the large caps. It appears that in these chlamydospore development has been slower or later in development than usual. The chlamydospores are easily blown away by the wind and they, rather than basidiospores, are the dominant form of spread in this species.
In 1831 Julius Vinzenz von Krombholz, a medical professor at the University of Prague, published the first report of propagation experiments with such chlamydospores. He sowed some on the cap of a young, healthy Russula adusta and in three weeks obtained the mushrooms of Asterophora lycoperdoides. Krombholz had collected several specimens of Russula adusta. The one on which he sowed the chlamydospores he had first washed carefully with spring water. Of the other Russula adusta specimens he washed some and left others as they were and then put the lot (including the one sown with chlamydospores) under a bell jar in his work room in moderate light. By the ninth day Krombholz noted that most of the Russulas unsown with chlamydospores had gone rotten. The inoculated Russula had immature Asterophoras. Krombholz, in a translation by the English mycologist AHR Buller, observed that:
The inoculated Asterophora-bearing fungi lived longer and broke down with difficulty; their substance remained firm, and they conducted their juices to the parasite, the neighbourhood of which was continually moist.
Brefeld repeated these experiments half a century later and carried out other experiments (with both chlamydospores and basidiospores) on Asterophora life cycles which he wrote about and illustrated in the book noted at the beginning of this page. Brefeld's careful and thorough experiments cleared up the confused ideas about Asterophora that had been published by some mycologists.
The first published description and illustration of Asterophora lycoperdoides had been by Pier Antonio Micheli in his book Nova Plantarum Genera, which appeared in Florence in 1729. He placed the fungus in the genus Fungoidaster. Micheli observed the knobbly chlamydospores with a microscope, for he noted that this fungus had "star-like seeds" (semine stellato in his words) that formed on the upper surface. He recorded it as growing on half-rotten mushrooms of the species now known by the name Lactarius piperatus. Here is his illustration (figure 1 in plate 82) which shows some fuzzy ball-like heads atop narrow stems. All are shown growing from a section of the upper surface of a mushroom cap, with part of the cap's margin curved back and showing the outer ends of the gills that are below the cap. This is a fairly simple picture but one that conveys the powdery nature of the cap surface. I have taken this illustration from the copy of Micheli's work in the digital library (http://bibdigital.rjb.csic.es/ing/index.php) of the Real Jardín Botánico in Madrid. From the stated page size of Micheli's book I estimate the rightmost stem to have had a length of between 25 and 30 millimetres in the book.
Another well-known species in the genus is Asterophora parasitica (Nyctalis parasitica to Brefeld) and it is shown in plate 6 of Brefeld's book, with a layout similar to that of plate 5. In this species the gills are more often well-developed but, again, basidia are rarely found. The smooth, ellipsoid chlamydospores, between 10 and 20 micrometres in length, form within the gill tissue. Below are parts of Brefeld's plate 6. Leftmost is the central figure, with an added scale bar. To its right is a drawing of part of a gill, showing some embedded chlamydospores as well as three clusters of basidiospores. Rightmost are parts of hyphae within which chlamydospores have been forming.
An illustration (but no description) of this species was first published in 1792 by Jean Baptiste François Bulliard (though he is commonly known as Pierre Bulliard) in plate 574 of his multi-volume, multi-year Herbier de la France. The first description was published in 1801 by Christiaan Hendrik Persoon, who also referred his readers to Bulliard's plate 574. Persoon noted that the species grew on decaying mushrooms and he noted the appearance. There was no mention of any microscopic features, such as chlamydospores.
The only species of Asterophora known from Australia is Asterophora mirabilis and the first published description of this species appeared in 1995. It has been found only at several sites in the cool temperate rainforests of southern Victoria and Tasmania, growing on either Russula or Lactarius mushrooms. The species has knobbly chlamydospores, giving it some similarities with Asterophora lycoperdoides, but in appearance it is closer to Asterophora parasitica. Moreover, the chalmydospores appear mostly in the gill tissue or in the cap flesh, with very few found on the cap surface.