Date of the Monocot--Dicot Divergence Estimated from Chloroplast DNA Sequence Data

The divergence between monocots and dicots represents a major event in higher plant evolution, yet the date of its occurrence remains unknown because of the scarcity of relevant fossils. We have estimated this date by reconstructing phylogenetic trees from chloroplast DNA sequences, using two independent approaches: the rate of synonymous nucleotide substitution was calibrated from the divergence of maize, wheat, and rice, whereas the rate of nonsynonymous substitution was calibrated from the divergence of angiosperms and bryophytes. Both methods lead to an estimate of the monocot-dicot divergence at 200 million years (Myr) ago (with an uncertainty of about 40 Myr). This estimate is also supported by analyses of the nuclear genes encoding large and small subunit ribosomal RNAs. These results imply that the angiosperm lineage emerged in Jurassic-Triassic time, which considerably predates its appearance in the fossil record (approximately 120 Myr ago). We estimate the divergence between cycads and angiosperms to be approximately 340 Myr, which can be taken as an upper bound for the age of angiosperms.