This revision emphasizes surface colors of mature
stromata, of subsurface granules, and of stromatal pigments in KOH. Petch (1924) was
perhaps the first to recognize the potential value of soluble pigments to Hypoxylon
taxonomy. Whalley and Greenhalgh (1973), in a numerical analysis of British Hypoxylon
species, recognized that colored and noncolored species clustered together and, moreover,
that colored species with papillate ostioles clustered with colored species with
umbilicate ostioles, i.e., pigmentation and associated characters were more reliable
indicators of affinity than ostiolar morphology. Whalley and Whalley (1977) extracted
pigments of Hypoxylon species with ethyl acetate and examined them by
thin-layer chromatography. Most species showed constant and characteristic pigment band
patterns that were more or less useful in identifications. Interestingly, pigment patterns
did not seem useful in resolving the H.
rubiginosum complex in Europe. Additional collections of this species
complex from elsewhere would undoubtedly have yielded significant and useful pigment data.
stygium and H.
michelianum did not yield pigment in ethyl acetate and the authors
speculated that these species either lack pigment altogether or lose discernible pigment
during maturation (Whalley and Whalley, 1977). It is noteworthy that these species yield
pigment in KOH and, unsurprisingly, shows that pigments extracted depend in part on the
Unfortunately, there are few detailed analyses of the chemical structure of stromatal pigments. Edwards et al. (1991) chemically characterized a green pigment found in mycelium of H. fragiforme and H. howeianum when grown on malt extract medium. It was named hypoxyxylerone. Interestingly, the KOH-soluble pigment extracted from mature stromata of these fungi is orange. A similar or identical compound from H. fragiforme, mitorubrin, has been found and similar compounds identified by Whalley and Edwards in H. howeianum (Whalley and Edwards, 1995). Whether or not one pigment color can be easily transformed to another is unknown. The relationship of colors to secondary metabolites is largely unknown. Current information on the structure of pigments and secondary metabolites is reviewed by Whalley and Edwards (1995). Although secondary metabolites might eventually prove to be widely useful in Hypoxylon taxonomy, the subject is beyond the scope of this revision. Whatever the eventual chemical status of stromatal pigments we are convinced stromatal colors and pigments extracted by KOH are useful and apparently stable taxonomic characters. It is possible, however, that pigmentation changes as stromata mature. Only pigment data from stromata bearing mature perithecia should thus be assessed for the purposes of this revision.