Rhodocalyx (Apocynaceae), a New Synonym of Prestonia

Abstract

The genus Rhodocalyx is reduced to synonymy under Prestonia. A new combination, P erecta, is proposed here. Prestonia R. Brown (Apocynaceae, Apocynoideae) is a neotropical genus of about 55 species. These lianas (rarely suffrutescent herbs) are characterized by having the corolla tube usually with a corona within (sometimes the free corona lobes are replaced by callus ridges), with a conspicuous annular corona (entire to five-lobed), clear latex, sepals with a single episepalous colleter (sometimes deeply lacerated), and truncate seeds (Woodson, 1936a; Morales, 1997a, 1997b). The genus was divided into four sections by Woodson (1936a), according to the presence or absence of the free corona lobes in the corolla tube, shape and length of the sepals, and the nature of the corolla indument. As explained by Woodson (1936a) and Morales (1997a), section Tomentosae, characterized by pubescent corollas, is taxonomically the most difficult section in the genus. During studies toward a clarification of some nomenclatural problems in Prestonia (Morales, 1997a), it became evident that a re-evaluation of the characters traditionally used to delimit the genus, as well as an analysis of its relationships with some closely related genera, was needed in order to make the generic delimitation of the genus more natural and consistent. Recently, Morales (1997b) evaluated Prestonia sect. Coalitae and transferred three species to Echites, because they lacked both an annular corona and corona lobes within the corolla, the principal characters of Prestonia, thus maintaining both genera as natural groups. According to the concepts of Woodson (1933, 1936a) and Pich6n (1950), Prestonia is easily distinguished by having the corolla tube with a conspicuous annular corona, usually with five free epistaminal corona lobes within (but these sometimes absent or reduced to callus ridges), and a solitary episepalous colleter. The two genera most closely related to Prestonia are Laubertia and Rhodocalyx, both of which also have a thickened annular corona in the mouth. Laubertia A. DC. is easily distinguished by the glandular sepals lacking colleters within; careful study of this genus shows that Laubertia could simply be a Prestonia that lost its calycine colleters, but here it is considered to stand as a good genus. This character is very important to separate some genera of Apocynaceae (e.g., Rhabdadenia, Elytropus, Cycladenia) and therefore, despite the presence of a conspicuous faucal annulus, the eglandular sepals easily separate Laubertia. Rhodocalyx Miller Argoviensis (1860) has traditionally been separated from Prestonia by its erect habit, terminal inflorescences, and corolla tube with a conspicuous annular corona in the mouth but lacking free corona lobes (Woodson, 1933; Pich6n, 1950; Ezcurra, 1992). The need to evaluate these characters when considering the reduction of Rhodocalyx to synonymy of Prestonia was previously mentioned by Ezcurra (1992), who revealed the close affinity of these genera and the similarity of R. rotundifolius Miiller Argoviensis to P riedelii (Miiller Argoviensis) Markgraf. Recently, Stranghetti and Sumiko (1996) re-evaluated Rhodocalyx in a comparative morphological study with two species of Mandevilla and two species of Prestonia that are vegetatively and superficially (fide Stranghetti & Sumiko, 1996) somewhat similar to Rhodocalyx, which reflects basically the same work as Stranghetti (1992). Stranghetti and Sumiko concluded that Rhodocalyx must be conserved as a monotypic genus. However, in the process of preparing a synopsis of Prestonia and Mandevilla, and after the examination and evaluation of the critical morphological characters (habit, indument, leaves, inflorescences, sepals, corollas, and gynoecia) in most of the species of both genera, I have found important evidence which indicates that Rhodocalyx should be included in the synonymy of Prestonia. Explanation is in order. I disagree with the relationships of Rhodocalyx with respect to Mandevilla suggested by Stranghetti and Sumiko (1996). As was shown previously, the most important characters separating Prestonia and the closely related Laubertia from the rest of the neotropical genera of Apocynaceae are the shape of the style head, attachment of the anthers to the NovoN 9: 89-91. 1999. This content downloaded from 157.55.39.104 on Wed, 28 Dec 2016 18:24:58 UTC All use subject to http://about.jstor.org/terms