The origin and evolution of green algal and plant actins

Abstract

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.

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“…It also has chloroplasts for photosynthesis. The evolutionary position of the species is located before the divergence of land plants [44,45]. The release 1.0 of its genome sequence has approximately 9 × whole genome shotgun coverage [46].…”

Phylogeny and Molecular Evolution of the Green Algae

“…Numerous studies have focused on identifying the closest living relative to the land plants, and several lineages have been proposed based on morphological, ultrastructural and molecular data (Graham, 1984;Graham, 1993;Kranz et al, 1995;Bhattacharya & Medlin, 1998;An et al, 1999;Graham et al, 2000;McCourt et al, 2004). Multi-marker phylogenies have been sensitive to taxon and gene sampling and provided equivocal results, suggesting the Charophyceae (Karol et al, 2001;Turmel et al, 2007a;Cocquyt et al, 2010b), Zygnematophyceae (Turmel et al, 2006a;Lemieux et al, 2007;Rodríguez-Ezpeleta et al, 2007;Wodniok et al, 2011), Coleochaetophyceae (Turmel et al, 2009a;Turmel et al, 2009b;Finet et al, 2010) as the sister lineage of the embryophytes.…”

Multiple Actin Isotypes in Plants: Diverse Genes for Diverse Roles?

“…The phylogenetic analysis suggested that vegetative actins represent a more primitive group of actins that share common ancestry with fern actins. It has been suggested that the reproductive group of actins has probably evolved from the vegetative actins, perhaps reflecting the need of new functions in newly evolved structures (An et al, 1999). …”