I got an email this week from a colleague about the state of the relationships among the three groups of bryophytes: Mosses, Liverworts, and Hornworts.
The first questions to consider - Are they three separate lineages? OR One monophyletic lineage?
If they are three separate lineages, what order should they be placed in relative to tracheophytes (plants that have tracheids, a special type of xylem)?
Well it all depends on which data are used.
Data from sperm ultrastructure (Garbary et al. 1993) and DNA data from entire chloroplast genomes (Nishiyama et al. 2003) points to the three groups being part of a monophyletic lineage, as in the diagram on the right.
However all the other studies that I can think of support these three lineages as independent and as a grade diverging prior to the evolution of the tracheophytes.
This phylogenetic relationship was proposed based on morphology, physiology, and biochemistry data (Mishler & Churchill 1984). Then subsequently supported by molecular sequence data (Mishler et al. 1994).
Another alternative hypothesis was supported by sporophyte morphological data (Garbary & Renzaglia 1998) and cox3 mitochondrial sequence data (Malek et al. 1996).
So, I think that this could be presented to students as an active scientific example of different data giving conflicting signals. Often science is much messier than we explain to students. They could be presented with multiple alternative hypotheses for these relationships and have to discuss the different scenarios or perhaps the different types of data used for each.
However, if I only wanted to present one phylogenetic relationship to my students I would go with this one, below. This relationship is supported by Qui et al. (2006), which uses molecular sequence data from the chloroplast, mitochondria and nucleus on over 100 taxa. Others may disagree, but this is the phylogeny that I would recommend using as our most current hypothesis for teaching students about relationships among the bryophytes.
Garbary, D. J., K. S. Renzaglia & J. G. Duckett. 1993. The phylogeny of land plants: A cladistic analysis based on male gametogenesis. Plant Systematics and Evolution 188: 237-269.
Garbary, D. J. & K. S. Renzaglia. 1998. Bryophyte phylogeny and the evolution of land plants: Evidence from development and ultrastructure. Pp. 45-63 in J. W. Bates, N. W. Ashton & J. G. Duckett (Editors), Bryology for the Twenty-first Century. Maney Publishing, Leeds.
Kenrick, P. & P. R. Crane. 1997. The Origin and Early Diversification of Land Plants. Smithsonian Institution Press, Washington, D.C.
Malek, O., K. Lättig, R. Hiesel, A. Brennicke & V. Knoop. 1996. RNA editing in bryophytes and a molecular phylogeny of land plants. The European Molecular Biology Organization Journal 15: 1403-1411.
Mishler, B. D. & S. P. Churchill. 1984. A cladistic approach to the phylogeny of "bryophytes." Brittonia 36:406-424.
Mishler, B. D., L. A. Lewis, M. A. Buchheim, K. S. Renzaglia, D. L. Garbary, C. F. Delwiche, F. W. Zechman, T. S. Kantz & R. L. Chapman. 1994. Phylogenetic relationships of the “green algae” and “bryophytes”. Annals of the Missouri Botanical Garden 81: 451-483.
Nishiyama, T., P. G. Wolf, M. Kughita, R. B. Sinclair, M. Sugita, C. Sugiura, T. Wakasugi, K. Yamada, K. Yoshinaga, K. Yamaguchi, K. Euda & M. Hasebe. 2004. Chloroplast phylogeny indicates that bryophytes are monophyletic. Molecular Biology and Evolution 21: 1813-1819.
Qiu, Y.-L., L. Li, B. Wang, Z. Chen, V. Knoop, M. Groth-Malonek, O. Dombrovska, J. Lee, L. Kent, J. Rest, G. F. Estabrook, T. A. Hendry, D. W. Taylor, C. M. Testa, M. Ambros, B. Crandall-Stotler, R. J. Duff, M. Stech, W. Frey, D. Quandt & C. C. Davis. 2006. The deepest divergences in land plants inferred from phylogenomic evidence. Proceedings of the National Academy of Sciences of the USA 103: 15511-15516.