Flowers from Two to Five

Gunnera, the so-called giant rhubarb (sadly, not edible), one of the most surreal-looking of all plants. Photo from Achamore House.

This Monday's Taxon of the Week post deal with a clade that was formally named only recently, but which had been informally established some time earlier. The name "Gunneridae" was introduced by Cantino et al. (2007) for the flowering plant clade previously referred to as "core eudicots" in publications such as Soltis et al. (2003) and APG II (2003). As yet, the name doesn't appear to have appeared in print outside its original publication - time will tell whether or not it catches on.

The Gunneridae has been mainly supported as a clade by molecular sequence analyses, though other potential characters uniting its members are a number of gene duplications and the production of ellagic acid (offhand, it is intriguing how many molecular relationships within plants that were not predicted by straight morphological data have also been supported by biochemical data). The name "Gunneridae" refers to one of the basalmost divisions within this clade, the small order Gunnerales. The remaining members of the Gunneridae form a clade that Cantino et al. (2007) dubbed Pentapetalae, for reasons that I'll go into in a moment. The Pentapetalae include the vast majority of dicotyledonous flowering plants - from apples to apple cucumbers, from stonecrops to sage - with most falling into the three clades known as Asteridae, Rosidae and Caryophyllales.


Myrothamnus flabellifolius, the African resurrection plant. It starts off looking like this... (Photo from Farm Kyffhäuser)

...and ends up looking like this! (Photo from Flora of Zimbabwe)

The clade Gunnerales contains only two genera, Gunnera and Myrothamnus. Both are wind-pollinated plants with a mostly Southern Hemisphere distribution, but this is about where their similarities end. Myrothamnus are two species (one in sub-Saharan Africa, one in Madagascar) of small arid-living shrubs known as resurrection plants for their ability to dry out then seemingly spring back to life when the rains fall. Other types of plant around the world are also known by this name and share this ability, but Myrothamnus is unique in being the only resurrection plant with a woody stem (Moore et al., 2007). Gunnera is a genus of herbaceous plants found scattered through South America, Africa and south-east Asia, with outliers in Tasmania, New Zealand and Hawaii. Some Gunnera are very small - the New Zealand Gunnera albocarpa has leaves one or two centimetres long - but the genus is best-known for the gigantic South American species known as "giant rhubarbs", which describes their appearance exactly. Some species can have leaves over two metres in length, on two metre stalks (Wikipedia has examples of exact measurements). Gunnera is also the only genus of flowering plants to form a symbiotic association with nitrogen-fixing cyanobacteria (Bergman et al., 1992). Mucus-secreting glands on the stem near the base of the leaves become colonised by the cyanobacterium Nostoc punctiforme. When they first colonise the gland, the Nostoc grow as hormogonia - short filaments that lack differentiated heterocysts (the larger nitrogen-fixing cells). These multiply until they form a film over the opening of the gland. Cyanobacterial cells then migrate deeper into the gland (in complete contradiction to the attraction to light that these photosynthetic organisms show when free-living), where they somehow penetrate and enter the cells of the host Gunnera itself. Once inside the Gunnera cells, they multiply and differentiate, and Nostoc colonies within Gunnera actually contain a higher proportion of heterocysts than any free-living colonies.


Fossilised flower of a Cretaceous rosid, one of the main lineages of Pentapetalae. Fossil by John M. Miller.

The remaining core eudicots, as mentioned before, form the Pentapetalae. In high school, you may have learned that flowering plants are divided between monocots, with parallel leaf-veins and flower parts in multiples of three (trimerous flowers), and dicots, with netted leaf-veins and flower parts in multiples of five (pentamerous flowers). Like so many things we learnt in high school, this was partially correct and partially a load of twaddle. While monocots are indeed a coherent group, the "dicots" are not only paraphyletic with regard to monocots (though the majority of dicots still fall within the clade Eudicotyledoneae, the eudicots), but also include taxa with parallel leaf-veins (the Aristolochiaceae) and taxa which don't have pentamerous flowers. The basal state for flowering plants as a whole appears to be trimerous flowers, which are found in most flowering plants outside the eudicot clade (Soltis et al., 2003). The basal state for the eudicots is equivocal - that for the clade formed by eudicots with the exception of the order Ranunculales appears to be dimerous flowers (parts in multiples of two), but the Ranunculales include both dimerous and trimerous taxa. Pentamerous flowers appear to have arisen three times within eudicots - the Ranunculaceae (the buttercup family), the small family Sabiaceae and the Pentapetalae. As this covers the significant majority of "dicots" that most people are likely to ever come across, it explains how your teachers were able to get away with fudging things like that.

REFERENCES

APG II (Angiosperm Phylogeny Group). 2003. An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II. Botanical Journal of the Linnean Society 141: 399-436.

Bergman, B., C. Johansson & E. Söderbäck. 1992. The Nostoc-Gunnera symbiosis. New Phytologist 122 (3): 379-400.

Cantino, P. D., J. A. Doyle, S. W. Graham, W. S. Judd, R. G. Olmstead, D. E. Soltis, P. S. Soltis & M. J. Donoghue. 2007. Towards a phylogenetic nomenclature of Tracheophyta. Taxon 56 (3): 822-846 (abridged printed version) or E1-E44 (longer electronic-only version).

Moore, J. P., G. G. Lindsey, J. M. Farrant & W. F. Brandt. 2007. An overview of the biology of the desiccation-tolerant resurrection plant Myrothamnus flabellifolia. Annals of Botany 99 (2): 211-217.

Soltis, D. E., A. E. Senters, M. J. Zanis, S. Kim, J. D. Thompson, P. S. Soltis, L. P. Ronse De Craene, P. K. Endress & J. S. Farris. 2003. Gunnerales are sister to other core eudicots: implications for the evolution of pentamery. American Journal of Botany 90: 461-470.