From Huang et al. (in review)

Bertholletia excelsa clade (100% BS; Figs. 1A, 7)—This clade includes only Bertholletia excelsa, which is distributed throughout Amazonia and parts of the Guianas (see Fig. 21 in Prance & Mori, 1990). In the present study, B. excelsa is sister to the Lecythis chartacea clade. It differs from species of that clade by having two calyx lobes (16, Fig. 6B; character 16), seeds without an aril (47, Fig. 6G; character 47), and a type of secondary indehiscence in which the seed is smaller than the opercular opening (Character 41, Fig. 6E, 6F). Bertholletia excelsa provides good examples of petals pressed against the androecium (Fig. 6A; character 33) and, as seen in the field or in color images, of the yellow color on the androecial hood at the entrance into the flower. Bertholletia is the only genus of the family with a boney seed testa and the complete absence of an aril (Tsou & Mori, 2002). It does, however, share the following features with species of the L. poiteaui and L. chartacea clades: similar androecia with swept in appendages (Fig. 6D), 4-locular ovaries (Fig. 6C), and long, slender, oblique or geniculate styles (Fig. 6B).

Mori and Prance (1990) hypothesized that B. excelsa might be related to Lecythis lurida (Miers) S. A. Mori. This hypothesis was based on the following shared characters of the two species (Mori & Prance, 1990): the presence of cuticular papillae on the abaxial leaf blade surface (character 5) (see Fig. 96 in Mori & Prance, 1990), hood appendages swept or curved inward without forming a complete coil (character 31; Figs. 6D), and mature fruits that fall to the ground with the seeds remaining inside (character 40), and a unique opercular dehiscence (character 41) (Fig. 6E–6F) in Bertholletia and indehiscence, but of a different type (Fig. 5I, 5M–N), in L. lurida. The relationship of B. excelsa with L. lurida and related species of the L. poiteaui clade is not supported by this study; instead our trees suggest that B. excelsa is sister to the L. chartacea clade.

There are no other species of Lecythidaceae with fruits morphologically similar to those of B. excelsa. The fruits of B. excelsa have thicker and woodier pericarps and are, in fact, dehiscent but the opercular opening is smaller in diameter than that of the seeds, and the operculum falls into the fruit when it dehisces (Tsou & Mori, 2002) (Fig. 6E–F). It has been hypothesized that this type of dehiscence is related to selection for dispersal by rodents, especially agoutis (Ducke, 1948; Prance & Mori, 1978). In neotropical Lecythidaceae, shifts to different dispersal agents and accompanying morphological changes have occurred a number of times (Tsou & Mori, 2002). For example, in Allantoma there has been a shift from wind-dispersal facilitated by a unilateral seed wing in most terra firme species to the water-dispersed A. lineata with only a vestigial seed wing (Huang et al., 2008), and from the terra firme dehiscent-fruited, arillate seeded, animal-dispersed L. chartacea to the riverine, indehiscent-fruited, non arillate seeded, water-dispersed L. rorida O. Berg (Kubitzki & Ziburski, 1994). Thus, species of neotropical Lecythidaceae often belong to the same genus even though the morphological adaptations for seed dispersal by different dispersal agents are quite different.

Other morphological chracters that would suggest relationships between B. excelsa and some species of Lecythis may be similarly misleading and are homoplasious on our trees. For example many species of Amazonian Lecythidaceae have thick cuticles and papillae that arise from them, most likely to reduce water loss from the leaves—thus, the presence or absence of papillae should not be given much weight in predicting evolutionary relationships in this family. Even the unique two-lobed calyx (Fig. 7B) of B. excelsa is not an absolute indicator of evolutionary relationships because nearly all zygomorphic-flowered neotropical Lecythidaceae (including B. excelsa) have six calyx-lobe primordial in early floral development (see Fig. 78 in Tsou & Mori, 2007).