Hymenostilbe nutans

Hymenostilbe nutans

Sunday, February 16, 2014

What is Cordyceps sinensis?

Cordyceps sinensis is the old name for a fungus that is now correctly called Ophiocordyceps sinensis. It was named originally by Berkeley in 1843 (although it had been known in Europe for about 100 years previously) and was placed in the genus Sphaeria as Sphaeria sinensis. The material had been collected from a market in Canton (Guangdong) and was passed on to the Linnaean Society who in turn passed the material to Berkeley. When Berkeley named the fungus he recognised its origin as China – hence sinensis. This is a pity because it was not possible to determine where it came from once it ended up in the Canton market thousands of miles from where we now the species does grow. The type material is still contained in the herbarium at Kew Gardens.

Since its original naming the genus has been changed a few times. Sphaeria is a genus that is not now accepted. It rapidly became a catch-all genus. With such confusion it became easier to put species originally named in Sphaeria into other genera that were well described and well-known. Consequently, Saccardo transferred Berkeley's fungus from Sphaeria to Cordyceps in 1878 and there it happily remained for ca. 130 years.

Why couldn't it stay in Cordyceps? As with Sphaeria, Cordyceps was becoming unwieldy although it was rapidly recognised as a pathogen of insects, other invertebrates (especially spiders) and of other fungi. It became convenient to lump many species into this one genus. A few attempts were made to sub-divide the genus (notably by Yosio Kobayasi in Japan from the 1930s to 1980s) but no real effort was made to break the genus up. One exception was Tom Petch who named several species to a new genus which he called Ophiocordyceps. The type species of Cordyceps produces extremely long thread-like ascospores that break into typically 128 part-spores[1]. By contrast, the type species of Petch's Ophiocordyceps had ascospores that were long (although not thread-like) but significantly did not divide into part-spores on release. They remained whole. This genus was not widely accepted by contemporaries of Petch (namely Yosio Kobayasi and Edwin Mains) and faded into obscurity.

Fast forward to the present. Importantly, the last 20+ years has seen a revolution in how we look at taxonomy - whether it be that of a humble bacterium or of a blue whale. Whereas, in the past, relationships were based on shared characteristics the advent of molecular phylogenetics has brought a powerful tool to bear upon taxonomy. Molecular methods can therefore show that there is only a 2% difference between human DNA and our nearest relatives the chimpanzee and bonobo. Within the human species there is only a 0.1% difference in the DNA of all humans.

The same techniques apply to fungi. The type species of Cordyceps is Cordyceps militaris. It was the first species named for what is now recognised as Cordyceps. It therefore stands as the basis by which all other 'Cordyceps' must be compared. A major study was published in 2007 (I was one of the authors) that looked at a wide and representative range of species that had previously been considered to be Cordyceps as well as related species of other insect fungi[2]. Our study showed that Cordyceps fell into four distinct groups that were spread across three different families. Effectively, three new genera had to be found for the species that did not group closely with Cordyceps militaris. Two new genera were named in the study – Elaphocordyceps and Metacordyceps. One group included Cordyceps sinensis and one of the species that Petch had used to create OphiocordycepsCordyceps unilateralis. As a result, Ophiocordyceps was resurrected and the Chinese Cordyceps became the Chinese Ophiocordyceps. By current thinking (but who is to say this will not change in the future) Petch was right whilst Kobayasi and Mains were wrong.

So, as noted at the beginning Cordyceps sinensis is more correctly called Ophiocordyceps sinensis. It will take time for this name to be used widely although it is catching on. Even I still refer to Cordyceps sinensis when talking generally.

References:

[1] Hywel-Jones, N.L. (2002). Multiples of eight in Cordyceps ascospores. Mycological Research 106: 2-3.
[2] Sung, G-H., Hywel-Jones, N.L. Sung, J-M, Luangsa-ard, J.J., Shrestha, B. & Spatafora, J.W. (2007). Phylogenetic classification of Cordyceps and the clavicipitaceous fungi. Studies in Mycology 57: 5-59.