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The first land plants burst forth from ancient algae onto the Earth about 550 million years ago. This one-off evolutionary event, known as plant terrestrialization, fundamentally changed the planet's surface and atmosphere and made possible the development of all other terrestrial life - including humans. Now, led by the University of Nebraska-Lincoln, a team of 50 scientists from 20 research institutions worldwide has mapped the genome of four strains of that ancient Zygnema algae, uncovering the genetic innovations of the earliest land plants. "This is an evolution story," said Yanbin Yin, a computational biologist from the University of Nebraska-Lincoln and co-corresponding author of the study. "It answers the fundamental question of how the earliest land plants evolved from aquatic freshwater algae." Genome sequencing is the process of determining an organism's complete genetic material (DNA), which is assembled into a computational representation. It provides a valuable resource for studying species evolution and understanding genetic diversity. A whole genome sequence is more useful if the assembly is at the level of the chromosomes, where genes are physically located.
The researchers assembled four multicellular Zygnema strains using two from an algae culture collection at the University of Texas Austin and two from the University of Göttingen in Germany. Zygnema belongs to Zygnematophyceae, a class of freshwater and semi-terrestrial algae with more than 4,000 described species that have adapted to withstand extreme stressors like UV light, extreme dryness, and freezing. A defining feature of land plants is their multicellular bodies. The genes involved in multicellularity and response to environmental stress are closely connected and provide a foundation for plants' adaptability. Using cutting-edge DNA sequencing techniques, the researchers generated a complete, chromosome-scale genome for the algae. Before this study, only the genomes of unicellular Zygnematophyceae had been sequenced, and this was the first chromosome-level genome assembly for this class of algae. Comparing the genomes with those of other plants and algae, the researchers uncovered the genetic innovations employed by Zygnema. They found "genetic programs" involved in growth and development, cell division and cell wall biosynthesis and remodeling, and genes triggered by environmental cues. This co-expression of genes suggested they worked together to sense the environment and regulate plant growth accordingly. "Our gene network analyses reveal co-expression of genes, especially those for cell wall synthesis and remodifications that were expanded and gained in the last common ancestor of land plants and Zygnematophyceae," Yin said. "We shed light on the deep evolutionary roots of the mechanisms for balancing environmental responses and multicellular cell growth." The researchers say their discovery will lead to further studies that could be significant for bioenergy, water sustainability, and carbon sequestration. "Not only do we present a valuable, high-quality resource for the entire plant scientific community, who can now explore these genome data, our analyses uncovered intricate connections between environmental responses," said Jan de Vries, co-corresponding author from the University of Göttingen. The study was published in the journal Nature Genetics. Source: University of Nebraska-Lincoln