| Retiniphyllum consists of shrubs and small trees that grow mostly in white-sand soils of the Guayana and Brazilian Shields, with few species extending into the Amazon Region. The delimitation of the genus has been clear throughout history because of its two collateral and pendulous ovules per locule, a condition unique in the Rubiaceae. However, for the same reason, its placement within the family has been controversial. The monographic treatment of Retiniphyllum is presented, in which twenty two species and two varieties are recognized, including one newly described species. A key for the identification of the species is provided. Each species is described, typified, and mapped, supplemented by phenological and ecological observations. The position of the genus in the subfamily Ixoroideae is discussed based on a cladistic analysis of the trnL-F intergenic spacer and the rps16 intron. The tribe Retiniphylleae is monophyletic and monotypic, and is sister to two clades that include the Ixoroideae sensu Bremekamp. The three sections proposed by Müller Argoviensis are shown to be paraphyletic, and no sections are here recognized in the genus. Botryarrhena and Scyphiphora, tentatively included in the Retiniphylleae by previous authors, are shown to be related to other tribes of the Ixoroideae. The phylogenetic relationships of Retiniphyllum are studied based on morphological and ITS sequence data. In the phylogenies obtained, Retiniphyllum secundiflorum was sister to the rest of the species, which were found in three main clades: one with species characterized by racemose inflorescences, long-tubular, white corollas, and supposedly butterfly-pollinated; another by species with racemose or umbellate inflorescences, long-tubular, red corollas, and hummingbird pollinated; and another formed by species with spicate inflorescences, red or white corollas, and hummingbird- or bee-pollinated, respectively. The phylogenies obtained indicated that the hummingbird-pollination syndrome evolved at least two times in the genus, while the bee- and supposed butterfly-pollination syndromes arose once each, independently. The historical biogeographical study showed that the ancestral area of distribution for Retiniphyllum was the Southern Guayana Region, and from there many dispersals and a few vicariant events are necessary to explain the current distribution of most species. |
