Chemotaxonomy of Xylariaceae



Since the works of Greenhalgh & Whalley (1970),  Whalley & Greenhalgh (1971), Steglich et al. (1974), Whalley & Whalley (1977), O'Hagan et al. (1992), Adeboya et al. (1995), Whalley and Edwards (1995), Hashimoto & Asakawa (1998), some members of the Xylariaceae were known for their ability to yield specific compounds, isolated from stromata or from cultures. The taxonomic value of these metabolites at the genus, species-group or species level was already recognized by these authors as a promising way to elucidate affinities within this family and discriminate closely related taxa.

The results of recent investigations on members of Xylariaceae carried out by Mühlbauer et al. (2002), Quang et al. (2002, 2003 a, 2003 b, 2004), Stadler et al. (2001a, 2001b, 2004a, 2004b) and Hellwig et al. (2005) all illustrate the high diversity of secondary metabolites and their taxonomic value. High Performance Liquid Chromatography (HPLC) combined with very accurate techniques of separation performed by Stadler et al. (2001a, 2001b) allowed for secondary metabolite profiling in most of genera in Xylariaceae, with emphasis on Daldinia and Hypoxylon which are most inventive genera as to secondary metabolites.

It is interesting to notice that in most cases, HPLC results confirmed recent delimitations of xylariaceous genera or species based on morphological anamorphic and teleomorphic data (Petrini & Müller, 1986; Ju & Rogers, 1996; Ju et al., 1997) and, in addition, shed some light on expected or sometimes unexpected affinities between distinct genera or species. HPLC is also much interesting in allowing for secondary metabolites profiling of type material, for it can be performed at micro- scale on samples of 10-50 µg, even on very old specimens such as the type of Rhopalostroma africanum collected in 1814 or that of Hypoxylon cohaerens collected prior to 1797! (Stadler et al., 2004a; Quang et al., 2005). The identification of more recent collections with the type material  therefore becomes possible, even when the type is depauperate and lacks some distinctive morphological characters. The limits of this method appear when the herbarium material has been invaded by moulds producing their own metabolites, or when preservative treatments involved artefacts able to mask the genuine metabolites.

Unlike the former genera which are associated with Nodulisporium-like anamorphs, xylariaceous genera with Geniculosporium-like anamorphs (primarily Xylaria, Nemania, Rosellinia) lack stromatal pigments and characteristic metabolites, except in one case for N. serpens (Stadler et al. 2001b). However, they often produce more or less specific compounds in culture (Adeboya et al. 1995; O'Hagan et al. 1992), but for the moment, chemotaxonomy of those genera is much less understood. Efficient chemotaxonomic methods will hopefully appear for genera like Xylaria, for which the morphological approach often proves unrewarding.