Systematic aspects of flavonoid biosynthesis and evolution

The use of flavonoids in evaluating contemporary taxonomic systems has relied almost exclusively on the distributional patterns of these compounds among plant groups thought to be “archaic” or “advanced” based on morphological and other non-chemical characters. The many positive correlations between flavonoid distributions and existing taxonomic frameworks have also fostered hypotheses of flavonoid evolution, thereby encouraging their use in plant phylogeny. However, as more surveys have been completed, various inconsistencies have been revealed since compounds thought to be structurally (= biosynthetically) “advanced” have been found in “primitive” or archaic plants. The resulting “devaluation” of the phylogenetic significance of flavonoids above the generic level may be mitigated to some extent by exploiting techniques that draw upon genetics, biosynthetic and statistical methods, as well as a more liberal interpretation of the phylogenetic import of flavonoid distribution in contemporary plants. For example, while individual steps in flavonoid biosynthesis are known to involve one or perhaps two genes per step, recent studies of genetic mutants show that integration of the overall flavonoid biosynthesis in a taxon is often under control by a series of genes, each of which may have qualitative/quantitative effects on several individual steps simultaneously. The production of genetic mutants whose flavonoid complements mimic those of natural taxa suggests the possibility of (1) documenting evolution of infra- and interspecific taxa chemically when morphology lacks discriminatory ability, and (2) shedding light on systematic controversies at higher taxonomic levels (i.e., betalains in the Caryophyllales). The presence of isozymes in the individual enzymatic steps of flavonoid biosynthesis suggests their potential systematic use at the infraspecific level, as has been done with other enzymes (e.g., phosphatases, sulfatases, hydrogenases), or even as high as the subclass level (e.g., dehydroquinate hydrolase). It may even be possible to evaluate the presence of the same flavonoid compound in two species by electrophoretic examination to determine if the enzymes for this final step in its formation are structurally/functionally identical, or only functionally the same (i.e., parallel evolution). Most recently flavonoids have been found in preserved fossil angiosperm leaves, some 22 × 106 K/A years old, so that direct chemotaxonomic comparisons between fossil and extant taxa are now possible and may allow some specific speculation about angiosperm evolution and phytogeography. These new, potentially useful types of data suggest that it is premature to exclude flavonoid data from phylogenetic consideration because of their ubiquity and sometimes contradictory occurrences. Considering the need for more surveys and some consistent form of statistical analysis and quantification of flavonoid distributions (a self-imposed methodological restriction rather than an informational one), overzealous criticism of the use of flavonoid data can be counterproductive, possibly discouraging attempts at creative treatments of flavonoid information beyond certain taxonomic levels.