Dialypetalanthus includes one species of large trees (to 30 m tall) found in wet forests in the southern half of the Amazon basin.This species was long included in its own monotypic family, Dialypetalanthaceae. That family was classified differently by various 20th century authors, in various orders; for example, Cronquist included it in Rosales, and it has also been placed in Myrtales, Cornales, and Gentianales (this last in a circumscription that exlcuded Rubiaceae from Gentianales). Dialypetalanthus has also been classified in its own family and included with Rubiaceae in a different circumscription of Rubiales. Vegetatively it and in most reproductive characters Dialypetalanthus  agrees with Rubiaceae in its opposite leaves, well developed stipules that are fused around the stem, terminal cymose inflorescences, 4-merous flowers, bilocular inferior ovaries, and septicidal capsules with numerous winged seeds.

However Dialypetalanthus is anomalous in Rubiaceae in its pubescence with some stellate or dendritic trichomes, petals that are secondarily free and imbricated with usually (though not always) one external and apparently one internal, 16-25 stamens that are fused at their bases into a ring and held erect in a tube-like arrangement, quite large poricidal anthers, and the absence of a nectary disk on top of the ovary. These last three features are characteristic of the buzz-pollination syndrome and seem to indicate that flower visitors are rewarded with pollen, but field observations are missing. Dialypetalanthus is also unusual in its stipules, which characteristically split medially on each interpetiolar portion to produce two intrapetiolar stipules at each node, though there is variation in the pattern of splitting and some stipules become interpetiolar and bilobed, or nearly free. Piesschaert et al. (1997) studied herbarium material of this species and found the seeds to have a longitudinally spiralled form that produces a rotation movement as the seed falls, and they postulated this form aids dispersal; this too has not yet been studied in the field. 

The morphology and anatomy of Dialypetalanthus were detailed by Piesschaert et al. (1997), who concluded it is probably related to Rubiaceae. They also noted that the corolla lobes or petals fall off around the time of anthesis, and suggested that the large yellow anthers may function as the main pollinator attractant. They found the petals to be free in the flower, but the anthers to fall off as a single group. Ontogeny of the gynoecium was additionally detailed by Figueiredo et al. (2017), who found it similar to some other Rubiaceae genera. Anatomy and ontogeny of the whole flower was also detailed in depth by Vrijdaghs et al. (2022). They found that the earliest developmental stages of the flowers of Dialypetalanthus concur with the development of flowers in other Rubiaceae through the development of a central floral cup. The seemingly free petals actually are lobes of the corolla that are positioned on the rim of the floral cup. The large number of stamens follows from a switch in meristem form from individual stamen primordia to a circular intercalary meristem (i.e., primary androecium primordium) at the inside of the floral cup, from which secondarily up to ten stamens originate. These flowers are notably unusual in form within Rubiaceae, although petals that are apparently free petals are also found in Mastixiodendron and Mussaendopsis, which belong to the same tribe as Dialypetalanthus.

Molecular analyses place this species well nested within the Rubiaceae, in the Tribe Dialypetalantheae (Bremer & Eriksson, 2009; as Condamineeae). Kainulainen et al. (2010; as Condamineeae) found it related within this tribe to Wittmackanthus and Bothriospora. The main anomalous features for Rubiaceae of Dialypetalanthus are floral, with doubling and separation of structures, but ontogenetic study now documents the development of these features from a basic form similar to other Rubiaceae (Vrijdaghs et al., 2022). 

In general aspect Dialypetalanthus is similar to Capirona and some species of Calycophyllum. Calycophyllum differs in its stipules that are triangular and interpetiolar; Capirona differs in its larger corollas with petals fused into a tubular corolla and 5 stamens. Dialypetalanthus plants with their intrapetiolar stipules and stellate trichomes are sometimes confused with Malpighiaceae, but that family has superior ovaries. The flowers of Dialypetalanthus resemble those of some Myrtaceae in their well developed numerous showy anthers, as noted by various authors, but Dialypetalanthus lacks gland dots in the leaves and Myrtaceae lack stipules. Similar intrapetiolar stipules are found in Elaeagia, which has small beaked capsules with angled seeds, petals fused in a tubular corolla, and 5 stamens.

Author: C.M. Taylor & Elmar Robbrecht 
The content of this web page was last revised on 8 July 2024.
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.

• Piesschaert, F., E. Robbrecht & E. Smets. 1997. Dialypetalanthus fuscescens Kuhlm. (Dialypetalanthaceae): the problematic taxonomic position of an Amazonian endemic. Ann. Missouri Bot. Gard. 84(2): 201–223.
• Bremer, B. & T. Eriksson. 2009. Time tree of Rubiaceae: Phylogeny and dating the family, subfamilies, and tribes. Int. J. Pl. Sci. 170(6): 766–793.
• Kainulainen, K., C. H. Persson, T. Eriksson & B. Bremer. 2010. Molecular systematics and morphological character evolution of the Condamineeae (Rubiaceae). Amer. J. Bot. 97(12): 1961–1981.
• Figueiredo, R. da C., R. C. Vieira, J. E. A. Mariath, M.C. de C. Moço & K. L. G. De Toni. 2017. Development of carpels and ovules in Dialypetalanthus fuscescens Kuhlm (Rubiaceae): an enigmatic taxon. Acta Bot. Brasil. 31(1): 128–133.
• Vrijdaghs, A., P. De Block, K. L. G. De Toni, E. Smets & E. Robbrecht. 2022. Floral ontogeny links Dialypetalanthus (Condamineeae) with the floral developmental morphology of other Rubiaceae. Pl. Ecol. Evol. 155(3): 379-393.