Illustration: Duke University Botany Dept.
Early branch of green plant evolution discovered
February 14, 2000: Green plants encompass both land plants and green algae. According to DNA sequence comparisons, all living green plants can be subdivided into one of two phyla: the Streptophyta and the Chlorophyta. The streptophytes encompass land plants and closely related green algae known as charophytes, while the chlorophytes include the rest of the green algae. Until now, no green algae were known that predated the evolutionary divergence of green plants into two distinct evolutionary lineages, and since the earliest diverging green algae show considerable morphological variation, the nature of the unicellular ancestor of green plants was unknown. However, analysis of the chloroplast DNA (cpDNA) of the unicellular green flagellate Mesostigma viride by researchers at the Université Laval in Québec City, Canada, has provided some significant clues to the puzzle. The findings of Claude Lemieux, Christian Otis and Monique Turmel are presented in the February 10, 2000 issue of Nature.
It is generally accepted that chloroplasts are derived from cyanobacterium which took up residence within non-photosynthetic eukaryotic cells, establishing an endosymbiotic relationship whereby the eukaryotic cell provided a new niche for the bacterium, and the bacterium provided photosynthetic energy for the eukaryotic cell. Over time, the cyanobacterium evolved into a chloroplast within the host cell, giving rise to the first red algae. Glaucocystophytes (eukaryotic organisms that lack chloroplasts but instead harbor endosymbiotic and modified cyanobacteria) are thought to represent an intermediary stage in the evolution of algae possessing chloroplasts. By comparing the protein sequences of 53 genes common to M. viride, three land plants, three members of the Chlorophyta, the glaucocystophyte Cyanophora paradoxa, the red alga Porphyra purpurea and the cyanobacterium Synechocystis sp. PCC6803, the Université Laval researchers were able to determine where M. viride fits into the phylogeny of green plants.
There are several different types of models available for sequence analyses; these models generate phylogenetic tree structures that reflect the evolutionary relationships between different taxonomical groups. Branches within the tree indicate points at which the evolutionary path diverged, or split, into two or more different lineages. The vertical axis reflects time; therefore, the greater the vertical distance between two taxonomic groups within the tree, the greater the evolutionary distance between them, and the higher an organism is within the tree, the earlier in the evolutionary path it emerged. Lemieux, Otis and Turmel used several different models to compare the sequences of the 53 analyzed proteins, and generated three phylogenetic trees, T1, T2 and T3. T1 indicated that M. viride emerged about 800 million years ago before the evolutionary split of green plants into streptophytes and chlorophytes, while T2 and T3 indicated that it emerged after the split. T1 was highly supported by every analysis, and withstood elimination of possible sources of systematic bias. T2 and T3, on the other hand, were not retrieved by every model, and were poorly supported when they were retrieved.
Further evidence in support of the proposal that M. viride represents a lineage that emerged before the division of streptophytes from chlorophytes is derived from analysis of the structure of the chloroplast genome of M. viride. Several gene clusters exist within one region of the cpDNA that are common to both Mesostigma and non-green algae such as Cyanophora, but are missing from the chloroplast genomes of both streptophytes and chlorophytes. On the other hand, another region of the Mesostigma cpDNA contains gene clusters that are not found in non-green algae, but are present in green plants. This finding may provide insight into how the chloroplast genome diverged during the evolution of green plants. Morphological characteristics of M. viride, which include a biflagellate and asymmetric structure, a light-sensitive eyespot of pigment granules, square scales and a multilayered flagellar root structure, also support the proposed evolutionary pathway. Square scales are found in both Streptophyta and Chlorophyta, while eyespots and multilayered structures are found only in chlorophytes and streptophytes, respectively.
These recent findings support the view that the most ancestral
green flagellate was biflagellate and asymmetric, and possessed
an eyespot, square scales and multilayered structures. The newly
discovered evolutionary lineage provides yet more information
that will aid taxonomists in the reclassification of all
organisms according to phylogenetic relationships.
(1) Lemieux, C., Otis, C. and Turmel, M. Nature 403,649–652 (2000).