Evolution of chemical defense traits in the Leguminosae: mapping of distribution patterns of secondary metabolites on a molecular phylogeny inferred from nucleotide sequences of the rbcL gene

Abstract

The phytochemistry of the Leguminosae, a large plant family with more than 750 genera (in 40 tribes) and 18,000 species, is comparably well documented. Several types of alkaloids, non-protein amino acids, amines, flavonoids, isoflavonoids, coumarins, phenylpropanoids, anthraquinones, di-, sesqui- and triterpenes, cyanogenic glycosides, protease inhibitors and lectins have been described in this family, most of which function as defence chemicals and/or as signal compounds. Nucleotide sequences of rbcL of over 450 legumes have been determined in our laboratory that provide a phylogenetic framework to map the distribution of the main secondary metabolites. We have selected more than 150 genera which cover most tribes of the Leguminosae. In several instances, a particular group of secondary metabolites is confined to a systematically related group of species or genera. For example, the distribution of quinolizidine alkaloids is restricted to the genistoids s.l. whereas many of the other legumes produce non-protein amino acids. The distribution of quinolizidine alkaloids and non-protein amino acids is thus almost mutually exclusive. In some instances all members of a monophyletic clade share a chemical defence character (this would favour its use as a taxonomic marker), in other instances not. In many cases a loss of a particular defence trait in a member within a monophyletic taxon is substituted by a different one, which can be chemical (e.g. pyrrolizidine alkaloids instead of quinolizidine alkaloids in Crotalaria) or even mechanical (e.g., several brooms have only traces of alkaloids but produce spikes and thorns instead) by nature. Since a good taxonomic marker should be consistent for all members of a monophyletic clade, the main question is rather, what were the selective forces to activate the corresponding genes in one taxon and to turn them off in another. Since secondary metabolites play a vital role as defence or signal compounds, their occurrence apparently reflects adaptations and particular life strategies imbedded in a phylogenetic framework. A comparison of the distribution of secondary metabolites in plants with their phylogenetic relationships, thus offers information on the underlying evolutionary and ecological processes. It also offers insight into life strategies and taxonomic relationships.