Biogenetic Relationships of the Lichen Substances in the Framework of Systematics

Abstract

Biogenetic relationships of secondary products in lichens can be used for cladistic analyses of evolutionary relationships among taxa. Character states are derived from biogenetic hypotheses for the origin of the steps required to generate the final products. These biogenetic characters are then used instead of the final products themselves to propose possible evolutionary histories. The Cladonia chlorophaea group provides an example of how preliminary biogenetic hypotheses can lead to a cladogram for the 14 chemotypes presently known in the complex. Secondary products of lichens are immensely valuable in constructing taxonomies. Although li- chenologists may disagree upon the interpretation of chemical variation for classification, no one today would undertake a serious systematic study of a chemically variable group without consideration of the chemical characters. In order for these data to have maximum value for systematics, we need a biogenetic theory for the substances. There have been relatively few biosynthetic studies on lichens, but evidence from pathways to similar products in nonlichen fungi and higher plants allows us to con- struct preliminary biogenetic hypotheses. These hy- potheses may shed light on the evolutionary origin of the compounds and, by extension, upon the evo- lutionary relationships among the species producing them. These ideas have indeed already influenced the classification of the lichen products themselves, and some lichen systematists have found such bio- genetically based classifications valuable in com- paring the relationships of taxa. Recent studies on secondary products of higher plants have shown that biogenetic theory can be readily incorporated into cladistic analyses. Two good examples involve flavonoids (Richardson, 1983) and sesquiterpene lactones (Seaman & Funk 1983). The present paper will briefly summarize biogenetic theory for lichen products and apply these ideas to an analysis of chemical variation in the Cladonia chlorophaea complex. Early classification of lichen products (e.g., Zopf 1907; Asahina & Shibata 1954) were constructed