MICROMOLECULAR PATTERNS AND THE EVOLUTION AND MAJOR CLASSIFICATION OF ANGIOSPERMS

Abstract

SummaryIntegration of morphological and micromolecular data sets into a single system of classification is a major challenge facing the plant systematist of our time. Here an attempt is made to this end with regard to the upper levels of angiosperm classification. In particular, natural products which derive from the shikimate pathway are considered. Their distribution and biosynthesis demonstrates the existence of micromolecular links between the magnoliid, caryophyllid and monocotyledonous families, thus paralleling morphological characters. Chemical evidence is also in favour of the earlier idea that the largest gap within angiosperms is not that between dicotyledons and monocotyledons, but one that separates magnoliid, caryophyllid and monocotyledonous families (the “magnolialean block”) from the rest of the dicotyledons (the “rosiflorean block”). However, in contrast to some previous authors, here the monocotyledons are found to encompass a homogeneous, morphologically clearly delimited assemblage. Anomalous occurrences of micromolecules, such as ellagitannins in the Nymphaeales, are discussed and the importance of chemically discordant characters for the recognition of convergences is emphasized.A biosynthetic extrapolation indicates that the early angiosperms were characterized by a full‐fledged expansion of the shikimate pathway and a broad oxidation‐reduction potential. These properties permitted the production of the neolignans, oxidative coupling products of allyl‐ and propenylphenols, and of 1,3‐diarylpropanes, highly reduced flavonoid types. From here, stepwise suppression of the shikimate pathway must have led to the utilization of intermediates of this pathway characterized by higher oxidation levels, and finally to a total blocking of the shikimate pathway for the synthesis of micromolecules. It is hypothesized that the broad oxidation‐reduction potential of the angiosperms and the full expansion of the shikimate pathway present at the beginning of their evolutionary history provided a much greater capacity of micromolecular variation than was present in other plant groups and thus contributed significantly to their evolutionary success.