Texas Tech researcher Matt Johnson found that mosses have been extremely dynamic throughout their more than 400 million-year history.
Mosses may be one of the most resilient types of plants on Earth. After all, they're the first plant life emerging in Antarctica as the glaciers melt and expose ground that's been under the ice for millennia, as one Texas Tech University researcher is already studying.
But what makes mosses so resilient? What gives them such different qualities than other plants?
In a Nature Communications paper published last week, an international research team led in part by Texas Tech's Matt Johnson used DNA-sequencing technology to reconstruct the family tree of mosses, which goes back at least 400 million years, to answer these questions.
"We wanted to study the 'tree of life' in mosses to better understand how the amazing diversity of structures in this group came to be through evolution," said Johnson, director of the E.L. Reed Herbarium and an assistant professor in the Department of Biological Sciences. "Although many would consider mosses to be 'simple' or 'primitive,' they have developed a wide range of strategies for thriving on land – all without roots, seeds or vascular tissue. We were excited to use new low-cost DNA sequencing technologies to produce a very large dataset for the phylogeny, which increases the accuracy of our results compared to previous studies."
Based on comparisons of 142 species of moss from 29 of the 30 orders of mosses in existence, the researchers were able to re-evaluate what was known of moss evolution and suggest an overall phylogenetic tree. The resulting evolutionary history of mosses also represents a more robust framework to study the evolution of major morphological innovations.
The study analyzed hundreds of genetic sequences from the nuclei of the mosses and
other cellular components, or organelles, called plastids and mitochondria. To their
knowledge, the researchers write, this is the first study to take such a holistic
approach.
The team found that mosses have been extremely dynamic throughout their evolutionary
history – much more so than seed-based plants. The results also give insight into
the rate of evolutionary changes, especially big changes that result in the creation
of different branches of the phylogenetic tree.
"There are more than 13,000 species of mosses worldwide – more than the number of species of birds or mammals," Johnson said. "Our study has representatives from each of the major groups of mosses, and provides the framework about how the major groups are related to each other. We also are the first study to use a large amount of DNA data from each of a plant's three genomes.
"Our findings confirm that a major theme in moss evolution is changes to the way a moss releases its spores."
Mosses are remarkably diverse, most notably in the structures that aid in the release of spores for reproduction. These differences are the basis for scientists' categorization of mosses into five major groups. Mosses use spores, rather than seeds, for reproduction.
Mosses are widespread and are found in all non-marine ecosystems across the globe, often playing critical ecological roles, such as in nutrient cycling. Having been around for at least 400 million years, and having maintained what seems to be a fairly simple architecture, mosses were long thought to be "sphinxes" of the past or evolutionary dead ends, but are now emerging as highly dynamic lineages.
Due to their long heritage, mosses have been faced with great changes over the course of their existence, which may have led to their fascinating diversity. From external forces such as mass extinction events, to changes in global temperatures, or internal forces such as whole genome duplications, mosses have persisted and adapted to numerous challenges.
Their diversity reflects major and unique abilities to withstand and adapt to challenging environments such as those on rock surfaces and tree trunks. Mosses also adapt to changeable conditions, whereas very few flowering plants can survive without roots and rely solely on rain – or even moisture supplied only by fog – not only to water them, but also to "feed" them, said study co-author Bernard Goffinet.
Studying the evolution of mosses has been a challenge until the advent of DNA-sequencing technology, partly because of the absence of mosses in the fossil record, but also because of their great diversity. Even with DNA technology, many ambiguities have persisted as the DNA studies contradict previous inferences.
Goffinet says DNA-based inferences are critical to resolving the relationships of dynamic lineages such as mosses.
"The focal point of the study is to address the evolution of the peristome, a structure that controls or aids in spore dispersal," he said. "This structure is to the moss evolutionary systematist what the flower is to the flowering plant systematist: it is thought to inform us on relationships. Mosses with the same peristome architecture should be more closely related than mosses with different peristomes.
"Since morphology may be misleading, DNA sequences are seen as an independent source of information from which to reconstruct or infer relationships, and thereby test hypotheses," Goffinet added. "DNA may reveal signatures of shared ancestry that have been erased from the morphological space, due to convergent or reverse evolution.
"The way you are built and the way you look is not always the best indicator of your evolutionary story."
Collaborators on this research come from the Fairy Lake Botanical Garden & Chinese Academy of Sciences; BGI-Shenzhen, a genome sequencing center in China; Centro de Ciências do Mar, University of Algarve, Portugal; Augustana College, Illinois; Duke University; University of Liège, Belgium; Swedish Museum of Natural History; Royal Botanic Garden Edinburgh; California Academy of Sciences; Nees Institute for Biodiversity of Plants, University of Bonn, Germany; Chicago Botanic Garden; and University of Connecticut.
