Abstract
SummaryIn the Asteraceae, the two most taxonomically significant aspects of the sesquiterpene lactone chemistry are the patterns of skeletal and substitutional diversity. Sesquiterpene lactones can be subdivided into different classes based on their carbon skeletons. Superimposed on each skeleton is a set of substituents which collectively define a specific compound. Both skeletal classes and substitutional features display taxonomically useful patterns of distribution.A cladistic technique is demonstrated for converting these chemical features into taxonomic characters. Each character is derived via a biosynthetically based evaluation of structural diversity and determination of structural homology. Each set of homologous skeletal and substitutional characters from a taxon is arranged in a biogenetic transformation series. Outgroup comparison is used to determine the polarity of each character transformation series. The distribution of the resulting novel (apomorphic) skeletal and substitutional characters within this taxon are then used to generate a cladogram depicting the chemical synapomorphies. The chemistries of the taxa, Tetragonotheca L. and Iva L. (Asteraceae: Heliantheae), provide the two examples for the four‐step cladistic analysis. The methods employed in this analysis of sesquiterpene lactone distribution may also be applicable to other classes of complex natural products.
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