Comparative study on the evolution of chloroplast ribosomal 5S RNA of a living fossil plant, Cycas revoluta Thumb

Angiosperm origin and early stages of seed plant evolution deduced from rRNA sequence comparisons

Complete or partial nucleotide sequences of five different rRNA species, coded by nuclear (18S, 5.8S, and 5S) or chloroplast genomes (5S, 4.5S) from a number of seed plants were determined. Based on the sequence data, the phylogenetic dendrograms were built by two methods, maximum parsimony and compatibility. The topologies of the trees for different rRNA species are not fully congruent, but they share some common features. It may be concluded that both gymnosperms and angiosperms are monophyletic groups. The data obtained suggest that the divergence of all the main groups of extant gymnosperms occurred after the branching off of the angiosperm lineage. As the time of divergence of at least some of these gymnosperm taxa is traceable back to the early Carboniferous, it may be concluded that the genealogical splitting of gymnosperm and angiosperm lineages occurred before this event, at least 360 million years ago, i.e., much earlier than the first angiosperm fossils were dated. Ancestral forms of angiosperms ought to be searched for among Progymnospermopsida. Genealogical relationships among gymnosperm taxa cannot be deduced unambiguously on the basis of rRNA data. The only inference may be that the taxon Gnetopsida is an artificial one, and Gnetum and Ephedra belong to quite different lineages of gymnosperms. As to the phylogenetic position of the two Angiospermae classes, extant monocotyledons seem to be a paraphyletic group located near the root of the angiosperm branch; it emerged at the earliest stages of angiosperm evolution. We may conclude that either monocotyledonous characters arose independently more than once in different groups of ancient Magnoliales or that monocotyledonous forms rather than dicotyledonous Magnoliales were the earliest angiosperms. Judging by the rRNA trees, Magnoliales are the most ancient group among dicotyledons. The most ancient lineage among monocotyledons leads to modern Liliaceae.