Credit: R. B. PEABODY

Mushrooms are the tips of icebergs. They are the fruit bodies of basidiomycete fungi that proliferate in soil, extracting nutrients from wood and other solid materials through networks of invasive filaments. Mushroom development usually follows the fusion of two genetically compatible colonies (or mycelia). But research carried out by Robert B. Peabody and colleagues (Fungal Genet. Biol. 29, 72–80; 2000) provides a different picture. It seems that some mushrooms are mosaics that develop from several genetically distinct populations of cells. The mosaics form either through meiosis in the mated mycelium and assortment of haploid strains within the fruit body, or by the copulation of several partners.

Peabody et al. looked at a species called Armillaria gallica in Massachusetts (a cluster of fruit bodies is shown in the picture). This is the fungus that became famous when a 10,000-kg mycelium was discovered that has remained genetically stable for more than 1,500 years (M. L. Smith et al. Nature 356, 428– 431; 1992). Peabody and co-workers analysed tissue samples from mushrooms collected over the past 20 years, and showed that at least two mycelia had combined to form all mushrooms collected before 1988. They then went on to examine single cells isolated from fruit bodies, the results revealing that some mushrooms contained up to nine genetically distinct individuals.

Although there is strong evidence for mosaicism in specimens collected before 1988, the phenomemon seems to have ended then. The reason is unknown, but it could be due to environmental influences. For example, dry years are less favourable for fungal growth, and in these circumstances the involvement of numerous mycelia might be necessary to support the emergence of fruit bodies.

The identification of mosaicism in A. gallica adds a layer of complexity to our understanding of basidiomycete development. Individuals that compete for food in their mycelial phase must intertwine and become sheathed in a common gel-like extracellular matrix to sculpt the tissues of the mosaic mushroom's stalk, cap and the gills that line the cap. Spore formation will then take place after nuclear fusion and meiosis within specialized cells in the gills called basidia.

Further work will be needed to establish the degree of cooperation between individuals that is required to form mosaics. For instance, determining the proportion of spores that reflect the genotype of each participant should reveal any competition for occupancy of the gill surfaces, and also show who fertilized whom.