A New Species of Tylophora (Apocynaceae: Asclepiadeae) from Central Somalia

Abstract

Summary. A new species of Tylophora, T arachnoidea, is described from central Somalia. An illustration is provided and the possible affinities of the species are discussed. Mats Thulin and Abdi Dahir made a single collection of this unusual asclepiad in 1989 on the slopes of a limestone gorge in the administrative region of Mudug, central Somalia. It has not been collected since. The form of the corona is very distinctive with a fleshy apical portion radiating from the staminal column and a slender tail reflexed down the column. This structure is unique within the Asclepiadoideae and, together with the erect pollinia, suggested initially that the specimen could represent a new genus in the tribe Marsdenieae. While preparing accounts of several asclepiad genera for Thulin's Flora of Somalia, I re-examined the collection, and observed a number of characters which indicate that its affinities lie closer to Tylophora R. Br. than to genera in the Marsdenieae sensu stricto. Until recently, classifications of the Asclepiadaceae have put undue emphasis on the orientation of pollinia, and placed Tylophora in the tribe Marsdenieae (Bentham & Hooker 1876) or subtribe Marsdeniinae (Schumann 1895) as the pollinia are generally erect in relation to the corpusculum. N. E. Brown (1907: 523) cast some doubt on this simplistic approach, observing that most of the South African species have distinctly pendulous pollinia, and others have medifixed pollinia that could be classed as neither erect nor pendulous. He assigned Tylophora to the tribe Cynancheae (now correctly the Asclepiadeae), where pendulous pollinia are the norm, but argued that the genus was intermediate between that tribe and the Marsdenieae. The reduced vertical dimension of the anther results in the pollinaria lying in a more or less horizontal position at the margins of the stigma head, rather than the vertical position more typical of both Asclepiadeae and Marsdenieae. This development is paralleled in the New World genera Matelea Aubl. and Gonolobus Michx., now regarded as derived members of the tribe Asclepiadeae (Swarupanandan et al. 1996; Liede 1997; Endress & Bruyns 2000). Swarupanandan et al. (1996) and Omlor (1998) demonstrated that anther morphology in Tylophora is consistent with that of the Asclepiadeae, and molecular analyses published by Civeyrel et al. (1998) using matKand Sennblad (1997) using rbcL point to a derived position of the genus within the Asclepiadeae. Studies of trnT-L, trnL and trnL-F genomes by Liede (2001), and additionally with ITS (Liede et al. 2002), indicate a