A new species from Ethiopia and an interesting disjunction in Tylophora (Asclepiadaceae)

A new species of Tylophora, T. corollae Bullock ex Meve & Liede (Asclepiadaceae), is described from Ethiopia. This delicate twiner is characterized by yellow, shield-shaped corona lobes with apical appendages. The non-twining shrub up to lm high, T. fleckii (Schltr.) N.E. Br., so far known to be endemic to Namibia, is for the first time recorded from NE Africa. The interesting biogeographical implications resulting from the extraordinary gap of 4500km in its distribution are discussed.


I N T R O D U C T I O N
In the context of the preparation of accounts for the Flora of Ethiopia and the Flora of Somalia by one of the authors (SL), the material of Tylophora housed in BM, EA, FT, K, and P was investigated.
Only sparse material originating from this region is available, consisting mainly of the widely distributed African elements T. heterophylla A. Rich., T. oblonga N.E. Br. and T. sylvatica Decne. However, one specimen each of two species so far unknown in the area were found among the material housed in K, raising the number of Tylophora taxa for the combined range of both Floras to five species. T. corollae Bullock (in sched., K). Type: Ethiopia, Begemder Prov., 82km S Gondar, along the road to Bahar Dar, C.12 C 05'N 37"40'E, alt. c.2250m, 16 ix 1970iso. BR, K).
Habitat, c.2250m a.s.l.; twining in shrubs on a rather steep rocky slope. Slope additionally covered by small trees and alternating with open patches covered by herbs.
Habitat. 150O-235Om a.s.l.; in sand along a water course (Somalia: Wood, in sched.); usually in boulder fields or among rocks on steep to gentle slopes high up on mountains (Namibia: Bruyns, pers. comm.).

Flowering time. December-July.
Conservation status. Collected once only in Somalia, but sporadically frequent in Namibia (Bruyns, pers. comm.) and therefore not threatened as a species.

D I S C U S S I O N
The variability of coronal morphology encountered in Tylophora is rather low, especially among the African representatives. Normally simple staminal corona lobes develop from the staminal column at the base of the anthers (cf. T. fleckii, Fig. 3). Very often, the corona lobes widen or elongate at their bases to form a pentagonal outline of the pollination apparatus (e.g. T. fleckii, T. sylvatica). In T. corollae a new character is added to the range of coronal structures in the genus. Here, the shield-shaped, rectangular corona lobules are tipped by appendages which support the backs of the anthers and equal them in length. The resulting impression is that of a little crown, to which the specific epithet chosen by A. Bullock (in sched.) obviously refers.
Among African taxa, the formation of additional corona appendages has so far been described only for Tylophora erubescens (Liede & Meve) Liede (Liede & Meve, 1994;Liede, 1996). There, however, the appendages are reflexed and originate from the abaxial regions of the corona. The same construction is found in the non-African T. indica (Burm.f.) Merrill, which ranges from the Seychelles in the west via the Indian subcontinent as far east as the Malayan region. The much elongated, erect corona appendages typical for T. indica again emerge from the back of the coronal lobules. The habit of T. indica, its indumentum, the shape and colour of the corolla and the often shortly stipitate gynostegium are very similar to what is found in T. corollae. However, differences occur in the shape of the corona lobes as well as in the guide rails and the orientation of the pollinia (erect in T. corollae, pendulous in T. indica), thus supporting the independent status of T. corollae. Indeed, T. indica closely resembles T. erubescens from Malawi and Tanzania judged by gynostegial and coronal characters.
T. fteckii has been recorded from various localities in Namibia ( Fig. 2; Bruyns, 1990). This first record from Somalia indicates a considerable disjunction in its distribution area. From the morphological point of view, the only difference between the Namibian representatives and the Somalian ones is in the annulus, which, though gentle, is clearly observable in Namibian flowers, but reduced to a very slight elevation of the corolla in Somalian ones. However, this difference is of a very minor nature and does not require formal recognition, especially in the light of the considerable variability in annulus formation in other asclepiad species (e.g. Orbea verrucosa (Masson) L.C. Leach;Leach, 1978).
The specimen of the Somali collection is without fruits. Since fruit morphology is a very conservative character in Asclepiadaceae, differentiations on this level are not to be expected.
The gap between the two partial distribution areas spans at least 4500km. Species whose distribution areas extend more or less uninterruptedly from SW to NE Africa are not especially rare (e.g. compare Verdcourt, 1969;Thulin, 1994). Verdcourt therefore argues for the existence of an 'arid corridor' between the south-western and north-eastern parts of the continent at least since the Miocene. Uninterrupted arid conditions should have prevailed since then, before the pluvial periods of the Pleistocene gave rise to profound shifts in the vegetation cover of Africa (Verdcourt, 1969;Van Zinderen Bakker, 1976). However, while parts of the Sahara were not arid, in SW Africa and Somalia even the wettest periods did not result in non-arid conditions (Verdcourt, 1969). These areas therefore constituted refuges for semidesert plant life before a somewhat shifted and reduced 'arid corridor' recently arose across the continent more or less without interruption. Other hypotheses, such as 'long-distance dispersal', seem to be rather unlikely considering the enormous size of the gap found in T. fleckii. Marked NE-SW disjunctions are quite rare at species level (cf. Thulin, 1994), and also in Asclepiadaceae this is the only one known so far. Mostly, vicarious species pairs or independent species of single genera have evolved in these distant areas. The disjunctions in African Asclepiadaceae primarily involve genera exhibiting NE-SE (e.g. Orbea, Leach, 1978) or NW-SE disjunctions (e.g. Brachystelma, Meve & Porembski, 1993;Neoschwnannia, Meve, 1995).
The morphological conformity and stability shown by collections of T. fleckii from both partial areas and the enormous size of the gap between them indicate, in our opinion, a very high age for this species.
The erect growth form exhibited by T. fleckii is a rare feature within African Tylophora, shared only by T. congolana from West and Central Africa and T. coddii Bullock (1964) from the Transvaal, South Africa. T. congolana usually grows on seasonally wet and muddy grasslands or even in swamps, whereas T. coddii is reported from rocky stands comparable to those where T. fleckii is found.