Phylogenetic Relationships of Theobroma and Herrania (Sterculiaceae) Based on Sequences of the Nuclear Gene Vicilin

Phylogenetic relationships were examined within the genera Theobroma and Herrania using 18 species representing all recognized sections of both genera and three outgroups. DNA sequences for vicilin, a gene encoding a seed storage protein, were analyzed using parsimony. Both Herrania and Theobroma ap- peared monophyletic on the most parsimonious trees, but the monophyly of Theobroma was only weakly supported. The monophyly of all currently recognized sections of both genera was supported with the exception of Theobroma sect. Glossopetalum within which was nested the monotypic sect. Andropetalum. The analysis sheds light on the morphological diversification of these closely related genera and serves to dem- onstrate the potential phylogenetic utility of vicilin. The genus Theobroma L. consists of 22 species of trees growing in the understory of Neotropical low- land rainforests (Cuatrecasas 1964). The best-stud- ied species is Theobroma cacao L., the source of cocoa and chocolate (see Young 1994 for a general review). Theobroma and the genus Herrania Goudot (17 spe- cies) have been considered close relatives, placed in the family Sterculiaceae, tribe Byttnerieae (e.g., Ben- tham and Hooker 1862) or Theobromeae (e.g., Hutchinson 1967; Takhtajan 1997). The tribe Bytt- nerieae is traditionally defined by an unusual floral morphology with five concave or cup-shaped pet- als, antipetalous stamens, and antisepalous stami- nodes. In Theobroma and Herrania, the petals are cup-shaped, concealing the anthers, and the upper part of each petal may be extended to form a "lig- ule" or "lamina." The staminodes may be petaloid and showy in both genera. Although these floral characters are found in other genera of the tribe, the fleshy and indehiscent fruit of Theobroma and Herrania is unique in the tribe. In contrast, all other Byttnerieae have dry and fully or partially dehis- cent fruit. Because of this combination of characters, a close affinity between Theobroma and Herrania has been recognized; the hypothesis of a sister-group relationship between Theobroma and Herrania has been corroborated recently by sequences of the chloroplast gene ndhF (Whitlock et al. 1996, unpubl. data). Herrania and Theobroma are easy to differentiate by growth form: Herrania species are unbranching treelets with palmately compound leaves, whereas Theobroma species are branching trees with simple leaves. Nonetheless, several workers have advocat- ed combining these taxa (see Schultes 1958 and ref- erences therein) based in part on putative interge- neric hybrids (Addison and Tavares 1951). Further- more, a phenetic study of rDNA polymorphisms (Figueira et al. 1994) suggested that Herrania was nested within Theobroma. Thus, the monophyly of each genus as usually circumscribed warrants fur- ther evaluation. Cuatrecasas (1964) recognized six sections of Theobroma based on tree architecture, fruit, petal, and androecial characters. Sections Glossopetalum and Andropetalum are characterized by having broad and reflexed staminodes (Cuatrecasas 1964). They also share 7-nerved petals and woody, brown- tomentose fruit. Section Telmatocarpus has 5-nerved petals that lack petal ligules. Section Oreanthes has 3-nerved petals and a coriaceous pericarp. Section Rhytidocarpus is distinguished by woody fruit with prominent reticulate ribs and 1-nerved petals. The- obroma cacao is distinctive within the genus and is placed alone in the type section (Cuatrecasas 1964). It has several unusual characters such as a thick, fleshy pericarp, at least partly white flowers, and whorls of lateral branches in fives (all other species in the genus have branches in fours). Theobroma ca- cao is also unique in having high amounts of theo- bromine and caffeine in mature seeds but an ab- sence of tetramethyl urate (Hammerstone et al. 1994), characters that may have played a role in the species' domestication. Schultes (1958) divided Herrania into two sections based on floral morphology. Section Herrania in-