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The Genus Megacollybia  

[ Basidiomycetes > Agaricales > Tricholomataceae . . . ]

by Michael Kuo

Until recently, mycologists assumed the world had one species of Megacollybia, the "Megacollybia platyphylla" (AKA Tricholomopsis platyphylla) featured in field guides--a medium-sized, deadwood-associated mushroom with a white spore print, a streaked-looking brown to gray-brown cap, and a stem base often attached to white cords. However, a recent study (Hughes and collaborators, 2007) based on DNA results for specimens from across the globe found support for at least eight or nine separate species, with Megacollybia platyphylla limited to Europe. In North America the authors found support for four species. However, specimen sampling included very few from western North America, and almost none from the Midwest; it remains to be seen whether further North American species might turn up with additional study--in fact the authors suggest the possibility of "further cryptic species" within at least one of our North American taxa.

Figuring out what the four (so far) North American species are is not the easiest task if one doesn't have a pocket DNA sequencer (one doesn't). Physical features--both macro- and microscopic--are quite similar among the species, and the researchers were hard put to

    separate the major (and sometimes minor) clades into recognizable morphotaxa. For the most part, this is very difficult, for once geographic location has been dismissed as a character, little else is unique or startling.

Many contemporary researchers face a similar dilemma; DNA-defined species do not always express themselves with observable differences in physical features. But Hughes and collaborators found some tentative morphological separators; I have combined these with ecological and geographic data to construct the key below.


Tentative Key to 5 Megacollybia Taxa in North America  


 Frequently collected look-alikes:
 

  • Mature cap only 2-6 cm across; stem not attached to white cords, only 2-5 mm thick; gills distant, with cross-veins; spores amyloid.

  • Not growing on wood; cap broadly conic; gills attached to the stem by a notch; stem fairly stocky (to 2 cm or more wide), not attached to white cords; clamp connections absent.
  • Tricholoma virgatum
    (and other gray spp. of Tricholoma)



    1.Found from the Rocky Mountains westward, on the wood of conifers; cap fairly pale, with well separated streaks; gills becoming pale yellow; basidia 36-47 µ long.

    1.Found east of the Rocky Mountains, on the wood of hardwoods; cap variously colored and streaked; gills not becoming pale yellow; basidia usually somewhat shorter than above.
    2


    2.Known from eastern Texas; stem base often abruptly expanded; basidia small (26-36 µ long).
    Megacollybia texensis

    2.Known from elsewhere in eastern North America; stem base not usually abruptly expanded; basidia usually somewhat longer than above.
    3


    3.Known from Arkansas and Tennessee; terminal cells of pileipellis (over the disc) large and often inflated, measuring 40-113 x 9-47 µ, often with a nipple-like tip; cheilocystidia large (up to 65 x 18 µ) and interspersed with filamentous hyphal tips.
    Megacollybia subfurfuracea

    3.Known from various locations east of the Rockies, including Arkansas and Tennessee; terminal cells of pileipellis and cheilocystidia not as above.
    4


    4.Cap pale to medium brown, often with olive shades mixed in; stem fairly long and slender; rhizomorphs often inconspicuous or even absent; cheilocystidia usually not projecting.

    4.Cap gray-brown to dark brown or gray, usually without olive shades; stem fairly stout; rhizomorphs usually numerous; cheilocystidia usually projecting.



    References

    Hughes, K. W., R. H. Petersen, J. L. Mata, N. V. Psurtseva, A. E. Kovalenko, O. V. Morozova, E. B. Lickey, J. C. Blanco, D. P. Lewis, E. Nagasawa, R. E. Halling, S. Takehashi, M. C. Aime, T. Bau & T. Henkel (2007). Megacollybia (Agaricales). Reports of the Tottori Mycological Institute 45: 1-57.

    Smith, A. H. (1960). Tricholomopsis (Agaricales) in the Western Hemisphere. Brittonia 12: 41-70.



    Cite this page as:

    Kuo, M. (2010, May). The genus Megacollybia. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/megacollybia.html

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