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The evolutionary history of bryophytes is fundamental not only to answering the “hows, whys, and wheres” of bryophytes, but also in certain respects, to answering these questions for all of plant life. Over time scales of thousands, millions, and hundreds of millions of generations, evolution makes its presence known by the diversity of life it produces. Understanding how a group of species evolved can only be answered with a complete family tree of the species. The science that aims to uncover the relationships of species is known as phylogenetics. Phylogenetics usually entails the collection of a large amount of data that is either morphological or DNA or amino acid sequence. This data is then analyzed by a computer program to produce a ‘family tree’ called a phylogeny. How closely related different species are is determined by the order of branching and by the branch lengths separating them.

Fossils of plant spores 475 million years old (thought to be liverwort spores) have been found and attest to the great antiquity of land plants. The land plants are thought to have evolved from a group of green algae. Bryophytes have long been considered the simplest and earliest-diverging groups of land plants. However, even with DNA sequence data it is very difficult to determine precisely how the three lineages of bryophytes (hornworts, liverworts, and mosses) are related to each other and to the vascular plants. Recently, introns in mitochondrial DNA have provided new, strong evidence for the relationships among bryophyte groups, which has led to a growing consensus on the issue. Liverworts are thought to be the earliest-diverging branch of land plants, followed by mosses, followed by the hornworts and vascular plants which are therefore considered “sister groups” (Figure 1). An interesting morphological characteristic that may have a common origin in the hornworts and vascular plants is the indeterminate growth of the sporophyte. This would become important in the eventual evolution of a dominant sporophyte life history seen in the seed plants and the variously interpreted intermediate stages seen in ferns and their allies. The mosses, hornworts, and early vascular plants (which are now extinct) have a columella present in the sporangium that is lacking in the liverworts, which suggests that this evolved after the liverworts had diverged from the remaining stock of the land plants. Despite these examples, there is plenty of contradictory evidence from morphology, biochemistry, and DNA sequences that suggest alternative phylogenies and therefore different evolutionary histories. The phylogeny considered here is thought to be correct due to the conservative nature of intron evolution, whereas morphological characters and molecular sequences are known to be more prone to convergent evolution and therefore the creation of misleading phylogenies.

Figure 1. Diagram of the relationships of some of the bryophytes of Stanley Park. Members of all the groups listed were found in Stanley Park except for i) Marchantiaceae, which was included because it contains a relatively common garden weed, Marchantia polymorpha, and because it serves as a place holder for an important group of liverworts, the complex thalloids; ii) hornworts, relatively rare but constitute the third lineage of bryophytes. See text for a discussion of the morphological characters.

Traditionally the liverworts have been divided into three groups based on their morphology: complex thalloid, simple thalloid, and leafy liverworts. The basic relationship between these groups is outlined in Figure 1; however, there are a few caveats. Two genera of morphologically ambiguous affinity (Treubia and Haplomitrium) are now thought to be the earliest-diverging lineage of liverworts, and Blasia, formerly considered a simple thalloid liverwort, is now considered to be basal to the complex thalloid liverworts. Phylogenetic and fossil evidence suggest that complex thalloid and leafy liverworts arose from a simple thalloid-like ancestral liverwort. It is thought that extant simple thalloid liverworts are the remnants of what used to be a more diverse and dominant life form. The extant simple thalloids have retained the greatest number of primitive characters, the complex thalloids have generally evolved more complex internal organization to be better suited to drier conditions, and the species-rich leafy liverworts have evolved, as their name suggests, ‘leaves’.

DNA sequence data complements traditional morphological data and has helped lead to a renewal in the study of moss phylogenetics (Figure 1). The earliest diverging mosses lack peristome teeth, and comprise of Sphagnum and several other genera, although the precise relationships between these groups and the remaining peristomous mosses is unknown. The earliest peristomate mosses have teeth made of complete (but dead) cells and are known as nematodontous mosses, but few other morphological characteristics unite these mosses. Within the nematodontous mosses, a group with teeth made up cell fragments evolved, the arthrodontous mosses. The arthrodontous, or joint-toothed, mosses have peristome teeth which move in relation to the relative humidity of the environment. The growth habit of mosses can be divided into two major groups, the acrocarpous mosses in which the male and female structures terminate the main branches and the pleurocarpous mosses in which the reproductive structures are borne on side branches. Pleurocarpy has evolved only once in the mosses and this occurred within the arthrodontous group. The pleurocarpous mosses comprise approximately one half of the moss species known in the world.

References and Further Reading
An asterisk refers to a source used in the construction of the phylogeny presented in Figure 1.

Buck, W.R., and Goffinet, B. 2000. Morphology and classification of mosses. In Bryophyte biology. Edited by A.J. Shaw and B. Goffinet. Cambridge University Press, Cambridge. pp. 71-123.

Cox, C.J., Goffinet, B., Shaw, A.J., and Boles, S.B. 2004. Phylogenetic relationships among the mosses based on heterogeneous Bayesian analysis of multiple genes from multiple genomic compartments. Syst. Bot. 29: 234-250.

Crandall-Stotler, B.J., Forrest, L.L., and Stotler, R.E. 2005. Evolutionary trends in the simple thalloid liverworts (Marchantiophyta, Jungermanniopsida subclass Metzgeriidae). Taxon 54: 299-316.

Davis, E.C. 2004. A molecular phylogeny of leafy liverworts (Jungermanniidae: Marchantiophyta). In Molecular systematics of bryophytes. Edited by B. Goffinet, V. Hollowell, and R. Magill. Missouri Botanical Garden Press, St. Louis, Missouri. pp. 61-86.

Dombrovska, O., and Qiu, Y.-L. 2004. Distribution of introns in the mitochondrial gene nad1 in land plants: phylogenetic and molecular evolutionary implications. Molec. Phylo. Evol. 32: 246-263.

Forrest, L.L., and Crandall-Stotler, B.J. 2004. A phylogeny of the simple thalloid liverworts (Jungermanniopsida, Metzgeriidae) as inferred from five chloroplast genes. In Molecular systematics of bryophytes. Edited by B. Goffinet, V. Hollowell, and R. Magill. Missouri Botanical Garden Press, St. Louis, Missouri. pp. 119-140.

Goffinet, B. 2000. Origin and phylogenetic relationships of bryophytes. In Bryophyte biology. Edited by A.J. Shaw and B. Goffinet. Cambridge University Press, Cambridge. pp. 124-149.

*Goffinet, B., and Buck, W.R. 2004. Systematics of Bryophyta (mosses): from molecules to a revised classification. In Molecular systematics of bryophytes. Edited by B. Goffinet, V. Hollowell, and R. Magill. Missouri Botanical Garden Press, St. Louis, Missouri. pp. 205-239.

Groth-Malonek, M., and Knoop, V. 2005. Bryophytes and other basal land plants: the mitochondrial perspective. Taxon 54: 293-297.

Groth-Malonek, M., Pruchner, D., Grewe, F., and Knoop, V. 2005. Ancestor of Trans-splicing mitochondrial introns support serial sister group relationships of hornworts and mosses with vascular plants. Molec. Biol. Evol. 22: 117-125.

He-Nygrén, X., Ahonen, I., Juslén, A.,Glenny, D., and Piippo, S. 2004. Phylogeny of liverworts – beyond a leaf and thallus. In Molecular systematics of bryophytes. Edited by B. Goffinet, V. Hollowell, and R. Magill. Missouri Botanical Garden Press, St. Louis, Missouri. pp. 87-118.

*He-Nygrén, X., Juslén, A., Ahonen, I., Glenny, D., and Piippo, S. 2006. Illuminating the evolutionary history of liverworts (Marchantiophyta) – towards a natural classification. Cladistics 22: 1-31.

Judd, W.S., Campbell, C.S., Kellog, E.A., Stevens, P.F., and Donoghue, M.J. 2002. Plant systematics: a phylogenetic approach, 2nd ed. Sinauer, Sunderland, Massachusetts.

Kelch, D.G., Driskell, A., and Mishler, B.D. 2004. Inferring phylogeny using genomic characters: a case study using land plant plastomes. In Molecular systematics of bryophytes. Edited by B. Goffinet, V. Hollowell, and R. Magill. Missouri Botanical Garden Press, St. Louis, Missouri. pp. 3-12.

*Knoop, V. 2004. The mitochondrial DNA of land plants: peculiarities in phylogenetic perspective. Curr. Genet. 46: 123-139.

Samigullin, T.K., Yacentyuk, S.P., Degtyaryeva, G.V., Valiehoroman, K.M., Bobrova, V.K., Capesius, I., Martin, W.F., Troitsky, A.V., Filin, V.R., and Antonov, A.S. 2002. Paraphyly of brophytes and close relationship of hornworts and vascular plants inferred from analysis of chloroplast rDNA ITS (cpITS) sequences. Arctoa 11: 31-43.

Shaw, J., and Renzaglia, K. 2004. Phylogeny and diversification of bryophytes. Am. J. Bot. 91: 1557-1581.

By Will Iles

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