Molecular Biology and Evolution, Vol 16, 275-285, Copyright © 1999 by Society for Molecular Biology and Evolution
SS An, B Mopps, K Weber and D Bhattacharya
The Viridiplantae are subdivided into two groups: the Chlorophyta, which
includes the Chlorophyceae, Trebouxiophyceae, Ulvophyceae, and
Prasinophyceae; and the Streptophyta, which includes the Charophyceae and
all land plants. Within the Streptophyta, the actin genes of the
angiosperms diverge nearly simultaneously from each other before the
separation of monocots and dicots. Previous evolutionary analyses have
provided limited insights into the gene duplications that have produced
these complex gene families. We address the origin and diversification of
land plant actin genes by studying the phylogeny of actins within the green
algae, ferns, and fern allies. Partial genomic sequences or cDNAs encoding
actin were characterized from Cosmarium botrytis (Zygnematales),
Selaginella apoda (Selaginellales), Anemia phyllitidis (Polypodiales), and
Psilotum triquetrum (Psilotales). Selaginella contains at least two actin
genes. One sequence (Ac2) diverges within a group of fern sequences that
also includes the Psilotum Ac1 actin gene and one gymnosperm sequence
(Cycas revoluta Cyc3). This clade is positioned outside of the angiosperm
actin gene radiation. The second Selaginella sequence (Ac1) is the sister
to all remaining land plant actin sequences, although the internal branches
in this portion of the tree are very short. Use of complete actin-coding
regions in phylogenetic analyses provides support for the separation of
angiosperm actins into two classes. N-terminal "signature" sequence
analyses support these groupings. One class (VEG) includes actin genes that
are often expressed in vegetative structures. The second class (REP)
includes actin genes that trace their ancestry within the vegetative actins
and contains members that are largely expressed in reproductive structures.
Analysis of intron positions within actin genes shows that sequences from
both Selaginella and Cosmarium contain the conserved 20- 3, 152-1, and
356-3 introns found in many members of the Streptophyta. In addition, the
Cosmarium actin gene contains a novel intron at position 76-1.
ORIGINAL ARTICLE
The origin and evolution of green algal and plant actins
Department of Biological Sciences, University of Iowa, Iowa City 52242- 1324, USA.
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