Angiosperm origin and early stages of seed plant evolution deduced from rRNA sequence comparisons |
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Authors: | A V Troitsky Yu F Melekhovets G M Rakhimova V K Bobrova K M Valiejo-Roman A S Antonov |
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Affiliation: | (1) A.N. Belozersky Laboratory of Molecular Biology and Bioorganic Chemistry, Moscow State University, 119899 Moscow, USSR |
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Abstract: | Summary 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,
andGnetum andEphedra 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. |
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Keywords: | Chloroplast 4 5S rRNA Cytosolic and chloroplast 5S rRNAs 5 8S rRNA 18S rRNA Nucleotide sequences Phylogenetic trees Angiosperms Gymnosperms Monocotyledons Dicotyledons |
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