Morphology and evolution of the corolla and corona in the Apocynaceae s.l.
published: Dec 14, 2005
Postgenital fusion of the upper corolla tube — the part above the insertion of the filaments — is widespread among the Apocynaceae s. l. In the subfamilies Rauvolfioideae, Apocynoideae, Periplocoideae, and Secamonoideae it occurs regularly, although there are exceptions. In postgenitally fused tubes the marginal meristems at the base of the petal initials do not expand laterally to form an annular meristem, or if they do, growth of this meristem is inhibited and the petals elongate separately, adjoining each other. Postgenital fusion starts rather early in ontogeny and the fusion zone elongates along with the growth of the tube.Corolline coronas are initiated along the fusion line from the base of the upper corolla tube to the petal sinus. The working hypothesis in the present paper is that the originally free petal margins are a prerequisite for the formation of corolline coronas in the interpetal (staminal) sectors of the tube. This would also explain the reversal from the ancestral congenital sympetaly to the choripetalous initiation of the tube and the following postgenital fusion: it is a consequence of selection pressure favouring guiding structures in the staminal sectors of the upper corolla tube.The problems of homology of the coronal structures associated with the corolla will be discussed extensively in this paper. The results show that there is only one consistent type of corolline corona present in Rauvolfioideae, Apocynoideae, and Periplocoideae. The staminal corona is ancestral to Secamonoideae and Asclepiadoideae and can be considered a key innovation. The replacement of the corolline corona, however, has occurred gradually, since in the basal taxa (Secamonoideae, Fockeeae, Marsdenieae) postgenital fusion is found and a number of species combine both a corolline and a staminal corona in their flowers.The systematic placement of the Periplocoideae within the Apocynoideae, as favoured by recent molecular phylogenies, is supported by the results on the morphology of the corolla and corona presented here.