Proliferation of group II introns in the chloroplast genome of the green algaOedocladium carolinianum(Chlorophyceae)

Abstract

BackgroundThe chloroplast genome sustained extensive changes in architecture during the evolution of the Chlorophyceae, a morphologically and ecologically diverse class of green algae belonging to the Chlorophyta; however, the forces driving these changes are poorly understood. The five orders recognized in the Chlorophyceae form two major clades: the CS clade consisting of the Chlamydomonadales and Sphaeropleales, and the OCC clade consisting of the Oedogoniales, Chaetophorales, and Chaetopeltidales. In the OCC clade, considerable variations in chloroplast DNA (cpDNA) structure, size, gene order, and intron content have been observed. The large inverted repeat (IR), an ancestral feature characteristic of most green plants, is present inOedogonium cardiacum(Oedogoniales) but is lacking in the examined members of the Chaetophorales and Chaetopeltidales. Remarkably, theOedogonium35.5-kb IR houses genes that were putatively acquired through horizontal DNA transfer. To better understand the dynamics of chloroplast genome evolution in the Oedogoniales, we analyzed the cpDNA of a second representative of this order,Oedocladium carolinianum.MethodsTheOedocladiumcpDNA was sequenced and annotated. The evolutionary distances separatingOedocladiumandOedogoniumcpDNAs and two other pairs of chlorophycean cpDNAs were estimated using a 61-gene data set. Phylogenetic analysis of an alignment of group IIA introns from members of the OCC clade was performed. Secondary structures and insertion sites of oedogonialean group IIA introns were analyzed.ResultsThe 204,438-bpOedocladiumgenome is 7.9 kb larger than theOedogoniumgenome, but its repertoire of conserved genes is remarkably similar and gene order differs by only one reversal. Although the 23.7-kb IR is missing the putative foreign genes found inOedogonium, it contains sequences coding for a putative phage or bacterial DNA primase and a hypothetical protein. Intergenic sequences are 1.5-fold longer and dispersed repeats are more abundant, but a smaller fraction of theOedocladiumgenome is occupied by introns. Six additional group II introns are present, five of which lack ORFs and carry highly similar sequences to that of the ORF-less IIA intron shared withOedogonium. Secondary structure analysis of the group IIA introns disclosed marked differences in the exon-binding sites; however, each intron showed perfect or nearly perfect base pairing interactions with its target site.DiscussionOur results suggest that chloroplast genes rearrange more slowly in the Oedogoniales than in the Chaetophorales and raise questions as to what was the nature of the foreign coding sequences in the IR of the common ancestor of the Oedogoniales. They provide the first evidence for intragenomic proliferation of group IIA introns in the Viridiplantae, revealing that intron spread in theOedocladiumlineage likely occurred by retrohoming after sequence divergence of the exon-binding sites.