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This simple alga is land plants’ closest relative.

Barbara and Michael Melkonian

Alien genes from bacteria helped plants conquer the land

A gelatinous blob on top of a wet stone has led to the discovery that bacteria likely helped plants conquer land. In 2006, on a plant collecting trip about 50 kilometers from the University of Cologne in Germany where he worked, phycologist Michael Melkonian came upon an unusual alga, known until then only from a 19th century French natural historian’s description. Now, Melkonian and his colleagues have analyzed its genome—and that of a close relative—to track down genes important for life’s emergence from water. At least two of those genes come from soil bacteria and were likely transferred into an ancestor shared by these algae and land plants.

Sequencing these two algae, Spirogloea muscicola and Mesotaenium endlicherianum, “is a milestone for the field of early plant evolution,” says Jan de Vries, an evolutionary plant biologist at the Georg August University of Göttingen in Germany. And the link to soil bacteria is an added bonus. “That horizontal gene transfer may have contributed to the colonization of land is pretty exciting,” says Pamela Soltis, a plant evolutionary biologist at the University of Florida in Gainesville who was not involved in the work. Although it’s well accepted that bacteria exchange genes, examples of gene transfer to more complex organisms are still controversial. If this case holds up, it demonstrates how important the process can be for evolution, she adds.  

She and others have puzzled for decades about how plants made landfall. Comparing genomes of close relatives is one way to track down when landlubber traits evolved, but only since 2012 have researchers considered these algae, part of the class Zygnematophyceae, to be the closest to land plants. Most Zygnematophyceae are aquatic and have huge, complicated genomes, but not S. muscicola or M. endlicherianum. Their genomes are less than one-tenth the size of the human genome, and both live on moist surfaces, suggesting a somewhat terrestrial existence.

So Melkonian teamed up with genomicist Gane Ka-Shu Wong from the University of Alberta in Edmonton, Canada, and sequenced the algal genomes. They then compared the two genomes to those of nine land plants and other algae. The researchers found 902 genes in 22 gene families that the two semiterrestrial algae and land plants shared but that other algae lacked. Those genes represent the ones that had evolved just before these two groups branched away from each other on the plant family tree, about 580 million years ago.

The genomic tool chest for coping with life out of water was fuller than the researchers had expected. Two of the shared gene families code for genes that help plants cope with desiccation and other stresses.

To the researchers’ surprise, those genes are also in soil bacteria—and no other organism. Because the bacteria evolved before the algae, the researchers conclude these genes jumped into the common ancestor of these algae and land plants from bacteria in nearby soil, they report today in Cell. Because claims of horizontal gene transfers between bacteria and more complex organisms are often disputed as contamination, the researchers say they made sure that the sequenced algae were pure and checked that the genes next to the transferred genes were plantlike and not bacterialike, as they would be if they were contaminants. “I am very convinced that the two gene families described in the paper were acquired from bacteria,” says Jinling Huang, a plant biologist at Eastern Carolina University in Greenville, North Carolina.

Although the new paper presents the first published Zygnematophyceae genomes, Jocelyn Rose, a plant biologist at Cornell University, and colleagues have analyzed another, Penium margaritaceum. They did not look for signs of horizontal transfer, but in an 8 November preprint on it, they report evidence of some adaptations to land, such as large numbers of genes for cell walls and for coping with bright light. The two studies drop “a critical piece into the jigsaw puzzle that will ultimately reveal the evolutionary innovations that allowed the emergence of land plants,” Rose says.

Moreover, says James Doyle, an emeritus plant evolutionary biologist at the University of California, Davis, the Cell paper “really cements the relationship between Zygnematophyceae and land plants in a spectacular way.” And together, both projects show that “land adaptations predate land plants.”