A unicellular green alga that belongs to the class Prasinophyceae. Phylogenetic trees inferred from the nucleotide sequences of the RNAs of the small ribosomal subunit put the prasinophytes at the base of the Chlorophyta. The entire genome of the chloroplasts of M. viride has been sequenced and found to consist of 118,360 base pairs. This chDNA contains 135 ORFs, more genes than reported for any land plant or algal chDNA. Mesostigma chloroplasts have a gene organization very similar to that of land plant chloroplasts. This species therefore seems to be a living fossil (q.v.) that shows close similarities to the organisms that lived about 800 million years ago and that were the ancestors of both green algae and land plants. See Classification, Protoctista, Chlorophyta; Chronology, 2000, Lemieux, Otis, and Turmel; Genome Sizes and Gene Numbers; chlorophyte, chloroplast DNA (chDNA).
Subjects: Genetics and Genomics.