Simple green.
By sequencing the genome of this moss, Physcomitrella patens (shown in a close-up shot), evolutionary biologists are learning how plants moved onto land.

David Cove

One Small Step for Plants

As the bright green fuzz on streamside rocks or the living carpet on forest floors, mosses revel in their relative simplicity, lacking the roots, seeds, and flowers typical of most land plants. Yet the first analysis of a moss genome reveals that mosses are surprisingly complex, with 35,000 potential genes--10,000 more than the first land plant sequenced--and a host of unique adaptations not found in other green landlubbers. And because of where mosses fall on the plant family tree, those genes are revealing how plants made their way onto land.

Mosses, along with hornworts and liverworts, are primitive plants called bryophytes. They diverged from the ancestors of flowering plants more than 450 million years ago. Plants got their start in water, and the move onto land by algae-like ancestors was quite challenging, requiring the evolution of the ability to deal with fluctuations in temperature and access to water, as well as to more intense sunlight. Mosses took steps to cope but never evolved certain features, such as vascular tissues that could transport water and seeds that could survive dry spells, that eventually appeared in flowering plants. By sequencing the genome of the extensively studied moss Physcomitrella patens and comparing it to the sequenced genomes of rice, the flowering plant Arabidopsis, and single-cell algae, an international team has been able to look at what the ancestral land-plant genome looked like.

Their first surprise, says developmental biologist Ralph Quatrano of Washington University in St. Louis, Missouri, was the abundance of genes. Some of these, such as the genes that help mosses come back to life after being dried out, are shared with other land plants and so evolved even earlier in plant evolution. Additional water-stress genes suggest that P. patens has evolved independent ways to deal with water shortages as well, says Quatrano. The moss also has extra genes for DNA repair to cope with damage inflicted by sunlight.

It seems likely that the ancestor of P. patens underwent a whole-genome duplication early in its history, Quatrano notes, possibly freeing up some genes to take on new functions. About 20% of the moss's genes are new to researchers and likely specific to moss, Quatrano and his colleagues report online 13 December in Science.

William "Ned" Friedman, an evolutionary biologist at the University of Colorado, Boulder, is excited about the potential of the moss genome to reveal clues about how plants accomplished the transition to land. However, as evolutionary biologist Pamela Soltis of the University of Florida, Gainesville, points out, genome researchers have barely touched the plant family tree. Fish and humans represent about the same evolutionary distance as rice and moss. That means a lot more sequencing needs to be done before the leap from water to land can be fully understood, she says.

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