Patterns of Embryological and Biochemical Evolution in the Asterids

Abstract The Monopetalae or Sympetalae have been resurrected recently as the Asteridae s.l., a large clade that includes almost all sympetalous angiosperms. Phylogenetic analyses of a large, combined data set of sequences of four genes (18S rDNA, rbcL, ndhF, atpB) have improved our understanding of the evolutionary history of this large clade of angiosperms. Most of the taxa fall within one of four major clades, the Cornales, Ericales, euasterids I (comprising Lamiales, Solanales, Boraginaceae, Gentianales, and Garryales), and euasterids II (comprising Asterales, Apiales, Dipsacales, Escalloniales, and Aquifoliales). We used the phylogenetic tree based on our recent phylogenetic analysis of four genes to elucidate the evolutionary patterns of the ovule and the diversification of iridoid biosynthesis in the asterids. Asterid-specific patterns of character distribution can be seen. Unitegmic-tenuinucellate ovules occur almost universally in the asterid clade. Cellular endosperm is ancestral in the asterids; nuclear endosperm emerged several times. Shedding of tricellular pollen does not seem to be a useful character for higher level taxonomy except for uniting Apiales and Dipsacales. Reversals from nuclear endosperm formation and tricellular pollen are not observed. Iridoids occur in many asterid clades and are probably ancestral. The ability to produce iridoids, however, has been lost frequently. Secoiridoids predominate in the euasterids II, whereas carbocyclic iridoids are most diverse in Lamiales. Although this study showed high correlation of unitegmic-tenuinucellate ovules with iridoid synthesis, several questions remain. Studies of the ontogeny and biosynthesis of these characters can now focus especially on those areas of transition determined in this analysis. Communicating Editor: James R. Manhart