Although angiosperms outnumber other land plants nine to one, there’s still a vast “green” world beyond those blossoming trees, herbs, shrubs, and grasses. Indeed, land plants are but a twig on the tree of green eukaryotic life (below), one that extends from a major branch called the streptophytes.
The other branch, Chlorophyta, includes many of the green algae. Sprouting off toward the bottom of the chlorophyte branch are tiny organisms called Micromonas, thought to most closely represent this tree’s earliest ancestor. Two of their genomes are newly described in today's issue of Science.
No bigger than a bacterium, these minuscule marine eukaryotes have surprisingly sophisticated genomes, says Alexandra Worden, a marine microbial ecologist at the Monterey Bay Aquarium Research Institute in Moss Landing, California. Her team has deciphered the genomes of two strains of Micromonas (lower left), which proved different enough to qualify as independent species. She chose these organisms because they thrive from the tropics to the poles and likely play an important role in the ocean’s food chain and carbon cycle. They are so small that they are hard to characterize and understand without the genes in hand, and she’s very eager to learn how they will respond to a changing environment and how they fit into the marine food chain. (See video.)
Overall, the Micromonas genome is about 21 million bases long, with 10,000 genes, 2000 more than its much more streamlined relative, Ostreococcus, which has already been sequenced, twice. About 20% of the genes found in Micromonas but not in Ostreococcus are genes generally thought to have evolved only in land plants, not earlier, her team reports. For example, the team finds that Micromonas has a gene called YABBY, which is missing from other green algae and even moss, and is thought to be related to the development of leafy plants. Given that leaves don’t exist in these algae, she thinks YABBY must have played another role early in green eukaryotic evolution.
One challenge is that many genes unique to Micromonas “are genes we don’t know the function of,” she points out. “That’s disappointing. If we can figure out their functions, that’s really going to give us new insights into what these organisms have to deal with [in their environments] that we are not thinking about.”
Diagram: Adapted by P. Huey/Science
Image: A. Z. Worden, T. Deerinck, M. Terada, J. Obiyashi, and M. Ellisman (MBARI and NCMIR).