In recent years, molecular analyses of often very large data sets have given us a reasonably good picture of the evolution of flowering plants, with most higher taxa settling down to reasonably comfortable positions. The subject of today's post, however, is still something of a phylogenetic enigma.
The Dilleniaceae are a family of about 500 species found mostly in the tropics, though one genus, Hibbertia, is also diverse in temperate Australia. Members of the family are very diverse in appearance: though the majority are trees or shrubs, some are lianes or even herbs. Dilleniaceae also show a remarkable diversity in features that are relatively stable in other families, such as floral symmetry and merosity (the number of flower organs such as stamens or carpels) (Horn 2009). Despite this diversity, Dilleniaceae are constant enough in other features that they have been recognised as a unified group since at least the 1800s. More questionable is their relation to other flowering plants: they are certainly members of the Pentapetalae, but the presence in some species of seemingly 'basal' characters (such as ladder-like perforation plates in the xylem and leaves with disorganised venation) lead some authors to regard them as an evolutionary link between the more basal magnoliids and a group of pentapetalous plants with centrifugal (starting from the centre and moving outwards) stamen development, called the Dilleniidae. 'Dilleniids' are now recognised as polyphyletic, including members of both the major clades Rosidae and Asteridae, but the Dilleniaceae themselves are not well resolved beyond basal Pentapetalae. Soltis et al. (2011) recently placed the Dilleniaceae as related to the clade of Asteridae + Caryophyllales + Santalales, but other analyses have placed them closer to Rosidae + Saxifragales, or even sister to all other Pentapetalae. Just to confuse matters, phylogenetic analysis within the Dilleniaceae suggests that at least some of their 'primitive' characters are in fact derived reversals of specific subtaxa (Horn 2009).
Relationships within the Dilleniaceae are perhaps better understood. The pantropical genus Tetracera was placed by horn (2009) as the sister to all other Dilleniaceae, which are divided between a strictly Neotropical clade (the Doliocarpoideae) and a strictly Old World clade. The Old World clade is in turn divided between two biogeographically distinct subclades, one centred in southern Asia (the Dillenioideae) and the mostly Australasian genus Hibbertia. Hibbertia and the Dillenioideae overlap only in northernmost Australia, southern New Guinea, Fiji and Madagascar (where both Dillenia and Hibbertia have representatives on the eastern side of the island). The Neotropical Doliocarpoideae are mostly lianes or scandent shrubs, with the only tree being the savannah species Curatella americana. The liane form is much rarer among Old World Dilleniaceae, most of which are trees or shrubs, though the small southern Asian genus Acrotrema contains rhizomatous herbs (and may be phylogenetically within the genus Dillenia). A group of succulent Australian species with photosynthetic stems, previously recognised as the genus Pachynema, have been reclassified by Horn (2009) as a derived subgroup of Hibbertia.
Horn, J. W. 2009. Phylogenetics of Dilleniaceae using sequence data from four plastid loci (rbcL, infA, rps4, rpl16 intron). International Journal of Plant Sciences 170 (6): 794-813.
Soltis, D., E., S. A. Smith, N. Cellinese, K. J. Wurdack, D. C. Tank, S. F. Brockington, N. F. Refulio-Rodriguez, J. B. Walker, M. J. Moore, B. S. Carlsward, C. D. Bell, M. Latvis, S. Crawley, C. Black, D. Diouf, Z. Xi, C. A. Rushworth, M. A. Gitzendanner, K. J. Sytsma, Y.-L. Qiu, K. W. Hilu, C. C. Davis, M. J. Sanderson, R. S. Beaman, R. G. Olmstead, W. S. Judd, M. J. Donoghue & P. S. Soltis. 2011. Angiosperm phylogeny: 17 genes, 640 taxa. American Journal of Botany 98 (4): 704-730.