Searching for the taxonomic position of the African genus Colletoecema (Rubiaceae): morphology and anatomy compared to an rps16-intron analysis of the Rubioideae

The morphology and anatomy of the African monospecific genus Colletoecema E . Petit (Rubiaceae) is documented and illustrated (including wood anatomy, pollen morphology, gynoecial and exotestal structure). Morphological and an atomical comparison shows that Colletoecema differs in many aspects from th e Psychotrieae (e.g., wood anatomy, seed structure). Consequently, the genu s' provisional position in the Psychotrieae cannot be maintained. Morindeae , the second tribe Colletoecema has been associated with, has a different g ynoecial structure. The oily endosperm and large embryo of Colletoecema is found in several genera that often were or still are included in the Psycho trieae or Morindeae (Trichostachys, Saldinia, Ronabea, and Lasianthus). How ever, they differ in several other aspects, which makes a close relationshi p doubtful. Colletoecema exhibits a unique combination of characters includ ing axillary inflorescences, banded parenchyma in the wood, ovules attached around the middle of the septum, bilocular pyrenes, seeds with oily endosp erm and large embryo, and verrucate exotestal thickenings, not encountered elsewhere in the Rubioideae. A cladistic analysis based on nucleotide seque nce variation in the rps16 intron (chloroplast DNA) confirms that Colletoec ema is related neither to Psychotrieae nor to Morindeae. Colletoecema appea rs as a sister genus of Ophiorrhiza in a weakly supported clade among the b asalmost clades of the subfamily Rubioideae, but there is very little morph ological support for this relationship.