Bryophytes have certainly evolved!

In a study published in Nature Communications, researchers at the Botanical Institute of the University of Liege show that the first plants which colonized the land have never stopped evolving.  The current species are more recent than had previously been thought. Bryophytes underwent two bursts of diversification after the Cretaceous.

Bryophytes appeared some 475 million years ago and were the first plants to colonize the land. In the same way as sharks, which have also existed for such a long time, current bryophytes are very similar in appearance to their ancestors. Such is the extent of this similarity that they were believed to have barely evolved during the hundreds of millions of years they existed before the appearance of humans. They are sometimes described as actual “living fossils”. There are three groups of bryophytes: mosses, liverworts and hornworts. In total, these groups represent around 16,000 species of plants, three times more than those that comprise the taxonomy of mammals! The team led by Alain Vanderpoorten, who is a research associate at the F.R.S.-FNRS in the Unit of Biology of Evolution and Conservation of the Botany Institute of the University of Liege, has been closely studying the diversification of mosses. “We have been studying the phylogeny of different groups of mosses as well as the diversification and dynamics of moss populations in Europe”, explains Benjamin Laenen, who has conducted doctoral studies within the above-mentioned team. The researchers are, for example, attempting to define the impact of glaciation on the evolution of bryophytes and the role played by Atlantic islands in the colonization of the land by these plants.

An almost complete phylogenetic tree

In his thesis, Benjamin Laenen has focussed on the diversification of bryophytes over the course of geological eras. Part of the results of his research has been recently published in the journal Nature Communications (1). “This was a collaborative work carried out with Professor Shaw’s laboratory at Duke University in the United States”, explains the researcher. “They sequenced 8 different genes for nearly 90% of liverwort genera and we used this set of data to construct the phylogeny of the group”. This research has also led to an enhancement of the “tree of life” that hundreds of scientists and amateurs have constructed together in the context of the project entitled the “Tree of Life Web”. While this is a great achievement from a phylogenetic point of view, it is not an end in itself from the point of view of the researchers from Liege. “We have used phylogeny as a tool that can help us to better analyze the evolutionary process of these plants”, continues Benjamin Laenen. This is because of the fact that, contrary to popular belief, these plants are not living fossils and have in fact evolved over the last 475 million years! “For a long time it was thought that plants had a limited evolutionary potential but this has proved false”, the researcher reveals. “We have demonstrated that the mosses and liverworts of today are much more recent than was previously believed. Many of them appeared after the Cretaceous, during a first diversification outburst”. This “bryophytes boom” coincides with the same phenomenon observed in ferns, another group of plants that were said to be “primitive”.

When forests of flowering plants offered new habitats

One of the theories that make it possible to explain this diversification outburst after the Cretaceous is based on the appearance of the first forests of flowering plants during this period. “These forests facilitated the creation of new habitats for already existing plants and could have marked the beginning of the broad diversification of bryophytes”, explains Benjamin Laenen. Analysis of the phylogenetic tree created in the context of this study also shows a second diversification outburst of bryophytes around 30 million years ago. “These results prove that mosses and liverworts continue to evolve even though they resemble very primitive plants in terms of their morphology”, continues the scientist. It should not be assumed that because it is difficult to tell the difference between a moss today and those of 200 million years ago that these plants have not evolved or that their evolution is slower. The different species of bryophytes continue to propagate and the current species are recent. “Our results make it possible to refute the idea that bryophytes are threatened by more recent plants. This is certainly not the case, these plants evolve together”, explains Benjamin Laenen. His research has made it possible to study the impact of this type of sexual system (hermaphrodite/separate sexes) on mosses and their diversification. This research work will certainly soon be the subject of another publication…

The ecological role of bryophytes

Why bother studying bryophytes? “This is a question I am often asked”, answers the amused researcher. “Firstly, this is because they are not well known and they were the pioneering plants on Earth. They therefore developed in very different conditions to those we know today and adapted over time”, continues Benjamin Laenen. “But also because, for example, the vegetation of the Great North is mainly made up of sphagnum which constitute the largest carbon sink in this part of the globe”, he indicates. “In addition, bryophytes actively participate in the regulation and purification of water, the protection of soils against erosion or against biomass production by peatlands in particular”. They therefore play a crucial environmental role. Often used as a bioindicator of pollution, bryophytes are very sensitive and are threatened by the increasing pollution due to industrialization. While not necessarily emblematic, there are still good reasons why bryophytes should attract the attention of scientists and anyone else interested in the history and future of our planet…

(1) Laenen B, Shaw B, Schneider H, Goffinet B, Paradis E, Désamoré A, Heinrichs J, Villarreal JC, Gradstein SR, McDaniel SF, Long DG, Forrest LL, Hollingsworth ML, Crandall-Stotler B, Davis EC, Engel J, Von Konrat M, Cooper ED, Patiño J, Cox CJ, Vanderpoorten A, Shaw AJ..Extant diversity of bryophytes emerged from successive post-Mesozoic diversification bursts. Nat Commun. 2014 Oct 27;5:5134. doi: 10.1038/ncomms6134.