:Xylariaceae: Hypoxylon

STROMATAL PIGMENTS

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. Hypoxylon 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 solvent employed.

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.