The fungus kingdom contains diverse eukaryotic organisms, including the yeast that we add in fermentation to make beer, the mold that grows on old bread, and the mushrooms that we eat. While we are familiar with many types of fungi, scientists are still trying to fill out the fungal tree of life. Mainly, researchers are unsure about the limits of fungal diversity and how different fungi relate to one another evolutionarily.

A recent paper in Nature suggests that scientists have been unaware of a large fraction of organisms in the fungus kingdom. Lead author Meredith Jones and her colleagues report the discovery of a new clade, an entirely new branch on the fungal tree of life. They named this new clade cryptomycota, which roughly translates to “hidden from the kingdom Fungi.” The new group appears to be extremely diverse; the authors estimate that the biodiversity of the cryptomycota clade might be similar to that of the entire known fungus kingdom.

Jones and her team first discovered the cryptomycota clade by aligning DNA sequences from published fungal phylogenies with DNA sequences in GenBank, the NIH database of all publicly available DNA sequences. This initial work was followed by increased genetic sampling and analysis, which told them that the cryptomycota clade is likely most closely related to the Rozella genus from the chytrid division of the fungus kingdom, which is generally composed of simple organisms.

They also learned that cryptomycota lived both in diverse habitats and geographical locations. The DNA matches came from sources in oxygen-depleted environments, soils, freshwater planktonic samples, and marine and fresh water sediments.

To study actual organisms from this new clade, the authors needed to first find fungi in water sources. They used probes that target specific DNA sequences in organisms from the cryptomycota clade on samples from three freshwater reservoirs in Dartmoor National Park, four costal marine surface water samples from Devon, UK, and a freshwater pond near where the authors worked (either the University of Exeter or the National History Museum in London). They found fungi cells in the freshwater sources, but not from the marine samples.

Using the freshwater samples, the authors recovered cryptomycota cells that were egg-shaped and 3 to 5 micrometers in diameter. They also found that 47 to 85 percent of the cells possessed a single flagellum, which led the authors to propose that the cells were in zoospore form. In the zoospore form, cells are actively seeking out food, growing, and reproducing.

If these cells follow the cycle of typical chytrid cells, the rest of the cells (the ones that don’t have a flagellum) would be in cyst form, which has a cell wall and lacks a flagellum. The odd thing is that the authors could not find cell walls on any of the cryptomycota cells.

The cell wall is an important evolutionary development for Fungi, as it allows cells to resist high osmotic pressure, enabling them to feed on dissolved organic compounds using osmosis. In the Rozella genus, cells don’t grow their own cell wall. Instead, they take cell wall material in a parasitic manner when they infect a host.

While the cryptomycota cells don’t appear to have cell walls, Jones and her group found some non-flagellate cells attached to other types of cells. Thus, cryptomycota cells could be parasitic or saprotrophic, with the fungi feeding on surrounding dead or decayed organic matter.

As this is the first time that scientists have knowingly studied cryptomycota cells, the researchers have yet to piece together a complete picture of the cell's life cycle. They also can't explain how these cells have found a way to survive in diverse environments without a cell wall. Nevertheless, the discovery of the cryptomycota clade is quite significant. The finding could drastically change the current view of the fungal tree of life, especially as the new clade has a biodiversity that might rival what is currently known of the entire fungus kingdom.

Nature, 2011. DOI: 10.1038/nature09984 (About DOIs)

Listing image by Cassius V. Stevani, Institute of Chemistry, University of São Paulo