Ancestral chloroplast genome in Mesostigma viride reveals an early branch of green plant evolution

Sequence comparisons suggest that all living green plants belong to one of two major phyla(1-3): Streptophyta(4) (land plants and their closest green algal relatives, the charophytes); and Chlorophyta(5) (the rest of green al gae). Because no green algae are known that pre-date the Streptophyta/Chlor ophyta split, and also because the earliest diverging green algae show cons iderable morphological variation, the nature of the unicellular flagellate ancestor of the two green plant phyla is unknown(1,6,7). Here we report tha t the flagellate Mesostigma viride belongs to the earliest diverging green plant lineage discovered to date. We have sequenced the entire chloroplast DNA (118,360 base pairs) of this green alga and have conducted phylogenetic analyses of sequences derived from this genome. Mesostigma represents a li neage that emerged before the divergence of the Streptophyta and Chlorophyt a, a position that is supported by several features of its chloroplast DNA. The structure and gene organization of this genome indicate that chloropla st DNA architecture has been extremely well conserved in the line leading t o land plants.