Evolution of Iridoids in Sympetalae

Abstract

Phytochemists know from experience where to look, in dicotyledons, for shikimate derived phenolics and for acetate derived aliphatics. The former are accumulated in the primitive, mostly woody, Magnoliidae and the latter appear in the advanced, predominantly herbaceous, Asteridae (sensu Cronquist 1968) or, as formerly designated, Sympetalae (von Wettstein 1935). So conspicuous indeed are aliphatics in the Sympetalae that Dahlgren (1977) was able to rearrange their chief orders into two blocks of superorders: the “iridoid” and the “non-iridoid” (= “polyacetylene-sesquiterpene lactone”) bearing blocks. Although the basic contributions (Jensen et al. 1975, Dahlgren et al. 1976, Dahlgren 1977) which led up to this proposal consider biosynthesis of the iridoids at length, and thus they lay more stress on chemical features than is normally done by botanists, these have admittedly been introduced only to assist at points of divergence (Dahlgren 1977). Micromolecular markers will only cease to have such an auxiliary role and equal morphology in plant classification when inserted in a unified, comprehensive framework. The following is an attempt to show, by means of the iridoids, that this is an attainable goal.