Count Your Mush­rooms

This is the sec­ond in­stall­ment of this year's Fun­gus Week, our more-or-less an­nual cel­e­bra­tion of this ex­cit­ing realm of life.

by Elio

Fig. 1. Species and gen­era of fungi iden­ti­fied from 454-se­quenc­ing data in 100 Nor­way spruce logs. The in­ner part of the wheel rep­re­sents the gen­era and the outer part the species with at least 90% prob­a­bil­ity of cor­rect iden­ti­fi­ca­tion. Source

Maybe you have to be a mush­room en­thu­si­ast or a fun­gal ecol­o­gist to give this a thought, but count­ing the num­ber of mush­rooms in a tract of for­est will not tell you the size of the fun­gal bio­mass therein. The mush­rooms you see are only the fruit bod­ies. The whole fun­gal or­gan­ism con­sists of an ex­ten­sive growth and ac­cu­mu­la­tion of in­vis­i­ble hy­phae, the mycelium. Mea­sur­ing fungi by count­ing mush­rooms is like weigh­ing an or­chard by count­ing the ap­ples on ap­ple trees, only here not all "trees" pro­duce fruit. To the con­ster­na­tion of wild mush­room col­lec­tors, the co­pi­ous amounts of mycelial fil­a­ments ex­ist­ing in the soil and de­cay­ing wood may or may not pro­duce mush­rooms. What de­ter­mines which mycelia will fruit, and how pro­lif­i­cally? In ear­lier times, this co­nun­drum seemed dif­fi­cult to un­ravel, but now, with high through­put se­quenc­ing avail­able, this has be­come amenable to in­ves­ti­ga­tion.

A group of Nor­we­gian and Finnish re­searchers car­ried out an in­ten­sive study to cor­re­late the num­ber of fruit bod­ies emerg­ing from de­cay­ing tree logs with the abun­dance of the mycelia in the wood. The gen­eral con­clu­sion was that for most fun­gal species, the more mycelial mass at a site, the greater the num­ber of vis­i­ble fruit bod­ies. This may not seem sur­pris­ing, but the de­tails, based on care­ful mea­sure­ments, mat­ter. For ex­am­ple, fewer fruit bod­ies were pro­duced by those species whose fruit­ing is more en­er­get­i­cally costly, such as the ones that dis­play a cap stick­ing out from the sur­face of a tree (called pileated in the trade) as com­pared to those whose fruit bod­ies lie flat along the sur­face (known as re­supinate). The quan­ti­ties of both mycelial DNA and vis­i­ble fruit bod­ies in­creased lin­early with the in­creas­ing de­cay of the wood un­til the de­cay be­came quite ad­vanced. From that point on, the amount of mycelial DNA con­tin­ued to in­crease, whereas the fruit body count de­creased. In other words, the mycelium goes on de­vel­op­ing as the tree de­cays but this does not re­sult in the con­comi­tant for­ma­tion of fruit bod­ies. These fungi find grow­ing eas­ier than dif­fer­en­ti­at­ing un­der these con­di­tions.
 

Fig. 2. Left: The mush­room species most com­monly found in this study,Fomi­top­sis pini­cola. Source. Right: The sec­ond most com­mon mush­room species in this study, Het­er­oba­sid­ion sp. Shown here is a rep­re­sen­ta­tive of this genus, H. an­no­sum. Source

These mea­sure­ments in­volved a great deal of work. The in­ves­ti­ga­tors used 100 large (20–42 cm in di­am­e­ter and some 20 me­ters in length) Nor­way spruce logs whose stage of de­com­po­si­tion was duly eval­u­ated. The work in­volved qPCR, high through­put se­quenc­ing, and tons of sta­tis­tics. They found 198 species from the DNA data and 137 from the fruit body count. The mean num­ber of species per log was ca. 16 and 9, re­spec­tively. Al­most all the species en­coun­tered were bracket fungi. (Call­ing them mush­rooms is a mat­ter of us­age. Some peo­ple don't. I do.) All of the more abun­dant ones were woody or leath­ery in con­sis­tency, thus ined­i­ble. I pre­sume that was a source of dis­ap­point­ment to the in­ves­ti­ga­tors, be­ing that ed­i­ble mush­rooms such as shi­itake and maitake do grow on trees. The pie chart shows that a few mush­room species dom­i­nated while most species were rep­re­sented by rel­a­tively few finds. In their words: "…. species that are able to ob­tain a dom­i­nat­ing po­si­tion in the mycelial com­mu­nity pos­sess a high fruit­ing rate, pro­duce abun­dant fruit bod­ies, and have a high preva­lence both as fruit bod­ies and as DNA, sug­gest­ing a pos­i­tive feed­back-loop."

Un­til the ad­vent of read­ily avail­able DNA tech­niques, the study of fun­gal com­mu­ni­ties de­pended largely on iden­ti­fy­ing and enu­mer­at­ing fruit bod­ies. Now that the quan­tity of sub­ter­ranean or tree-dwelling mycelia can be read­ily de­ter­mined, a truer pic­ture of fun­gal abun­dance emerges, thus re­veal­ing ac­tual eco­log­i­cal re­la­tion­ships. For ex­am­ple, based on what the eye tells you, in­clu­sion of some fun­gal species in red lists of threat­ened or­gan­isms may turn out to have been pes­simistic. Some species are just stingy in fruit­ing. Their mycelia may be do­ing quite well and not be en­dan­gered.

Be­ing able to mea­sure the ac­tual mass of the or­gan­isms can ex­pand our knowl­edge of the pres­ence and ac­tiv­i­ties of fungi in the en­vi­ron­ment. As the au­thors say: "An in­ter­est­ing av­enue for fu­ture re­search would be to ex­am­ine what makes some species wait even decades un­til they form fruit bod­ies, and what trig­gers fruit body pro­duc­tion." Even those mush­room hunters who re­turn from the woods with a nearly empty bas­ket can take heart; the mycelium is ly­ing await­ing.

 

Ref­er­ence

Ovaskainen O, Schigel D, Ali-Kovero H, Au­vi­nen P, Paulin L, Nordén B, Nordén J (2013). Com­bin­ing high-through­put se­quenc­ing with fruit body sur­veys re­veals con­trast­ing life-his­tory strate­gies in fungi. The ISME jour­nal, 7 (9), 1696–1709. PMID 23575372