The origin and evolution of green algal and plant actins

The Viridiplantae are subdivided into two groups: the Chlorophyta, which in cludes the Chlorophyceae, Trebouxiophyceae, Ulvophyceae. and Prasinophyceae ; and the Streptophyta, which includes die Charophyceae and all land plants . Within the Streptophyta, the actin genes of the angiosperms diverge nearl y simultaneously fi om each other before the separation of monocots and dic ots. Previous evolutionary analyses have provided limited insights into the gene duplications that have produced these complex gene families. We addre ss the origin and diversification of land plant actin genes by studying the phylogeny of actins within the green algae, ferns, and fern allies. Partia l genomic sequences or cDNAs encoding actin were characterized from Cosmari um botrytis (Zygnematales), Selaginella apoda (Selaginellales), Anemia phyl litidis (Polypodiales), and Psilotum triquetrum (Psilotales). Selaginella c ontains at least two actin genes. One sequence (Ac2) diverges within a grou p 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 (A c1) is the sister to all remaining land plant actin sequences, although the internal branches in this portion of the tree are ver!: short. Use of comp lete actin-coding regions in phylogenetic analyses provides support for the separation of angiosperm actins into two classes. N-terminal "signature" s equence analyses support these groupings. One class (VEG) includes actin ge nes that are often expressed in vegetative structures. The second class (RE P) includes actin genes that trace their ancestry within the vegetative act ins and contains members that are largely expressed in reproductive structu res. Analysis of intron positions within actin genes shows that sequences f rom both Selaginella and Cosmarium contain the conserved 20-3, 152-1, and 3 56-3 introns found in many members of the Streptophyta. In addition, the Co smarium actin gene contains a novel intron at position 76-1.