Phylogeny Friday - 06 July 2007

This week's phylogeny takes a look at the green portion of the eukaryotic tree. These are the eukaryotes capable of photosynthesis. Eukaryotes first obtained the ability to perform photosynthesis when a eukaryotic cell absorbed a photosynthetic cyanobacterium. This was followed by a few subsequent horizontal transmissions of the endosymbiont between unrelated eukaryotic lineages. Although eukaryotic chloroplasts can trace their origins to a single endosymbiosis, they don't share that organelle based on simple common descent.

Green plants -- one of the photosynthetic eukaryotic lineages -- have greatly diversified since they first appeared about one billion years ago. The green algae are the most basal of the green plants. Major adaptations evolved along the lineage leading to land plants allowing for the organisms to live more of their lives on land -- freeing them of their need to be in water. They include vasculature (both for structural support and transfer of water and minerals through the plant), pollen (for transmission of male gametes outside of water), and seeds (for the protection of embryos). Finally, the most numerous group of land plants on earth today, the flowering plants, use flowers and fruit to help with fertilization and transmission of embryos, respectively.

A phylogeny of the green plants -- with some of the major evolutionary events marked on the tree -- can be seen below.

Depicted are relationships among the three lineages of plants: glaucophytes (freshwater algae; blue), rhodophytes (red algae; red), and the green plants (chlorophytes, charophytes, and land plants; green). Estimated dates for some nodes are listed in millions of years before present. The primary endosymbiotic event is estimated to have occurred at least 1.6 billion years ago. A deep split within the green lineage created the chlorophyte clade and the charophyte plus land plant clade. Note that both the charophytes and the bryophytes are grades and are not monophyletic. Major events in the evolution of land plants are demarcated with arrows. Species for which complete nuclear genome sequences are available are listed in color (photographs at right; the three angiosperm species are pictured upper left). Species positioned in large phylogenetic gaps where genome sequences would be informative (black) include the following: the basal lineage of land plants, the liverworts, charophycean algal lineages (Chara, Coleochaete) that are sisters to land plants, and the gymnosperms, which are the sister group to flowering plants (angiosperms). Also included is a multicellular chlorophytic green alga. Secondary endosymbiotic events have occurred within both the red algae (e.g., diatoms, pictured) and green plants. Pie chart shows the relative species richness of the major clades. The vast majority of species within the Plantae are angiosperms (250,000 species), with other groups having substantially fewer described species (numbers approximated): glaucophytes 13; rhodophytes 5,920; chlorophytes 3,720, charophytes 3,400; bryophytes 17,000 (liverworts 7,000, mosses 10,000, hornworts 100); lycophytes 1,225; ferns 12,000; gymnosperms 800.

Bowman JL, Floyd SK, and Sakakibara K. 2007. Green genes -- comparative genomics of the green branch of life. Cell 129:229-234 doi:10.1016/j.cell.2007.04.004

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