Amphistemon and Thamnoldenlandia, two new genera of Rubiaceae (Spermacoceae) endemic to Madagascar

Abstract

Amphistemon and Thamnoldenlandia are described as two new genera of Rubiaceae endemic to Madagascar. The two novelties belong to the predominantly herbaceous tribe Spermacoceae. Amphistemon is unique within the tribe in having its stamens inserted at two levels in the corolla tube. The genus includes two species: the subshrub A. humbertii and the geoxylic herb A. rakotonasolianus. Thamnoldenlandia includes only one species, T. ambovombensis, which differs from most other Spermacoceae in being a medium-sized shrub with winged seeds. We present a detailed description of Amphistemon and Thamnoldenlandia, including observations of pollen and seeds. A molecular phylogenetic study based on atpB-rbcL, petD, rps16 and trnL-trnF sequences demonstrates that the two new genera belong to the Hedyotis–Oldenlandia complex of tribe Spermacoceae. Amphistemon and Thamnoldenlandia are sister taxa and share a common ancestor with the Madagascan endemic genus Astiella. A second, but not closely related, Madagascan clade includes the endemic genera Lathraeocarpa and Gomphocalyx and the Afro-Madagascan genus Phylohydrax. This indicates that the Madagascan endemic members of Spermacoceae are the result of at least two independent colonization events, most likely by long-distance dispersal from the African mainland. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 163, 447–472.