Pollen morphology of NW European representatives confirms monophyly of Rubieae (Rubiaceae)

This study focuses on the six genera of Rubieac that occur in NW Europe: Asperula, Crucianella, Cruciata, Galium, Rubia, and Sherardia. The pollen morphology of 29 species was studied using light microscope and scanning electron microscope observations. Several features demonstrate the advanced nature of the tribe: simple apertures, up to 13 ectocolpi, supratectal microspines, a characteristic and unique endopattern, and absence of orbicules. Asperula, Cruciata, Galium, and Rubia cannot be distinguished by pollen morphology only. The monospecific genus Sherardia can be recognized easily by the high numbers of apertures (10-13) that are slit-like and the very fine perforations in the tectum. Some Crucianella species have relatively larger perforations in the tectum. None of the species investigated produces orbicules, which makes Rubieae, next to Gardenieae, the second rubiaceous tribe entirely without orbicules. Our palynological data are interpreted in the broader perspective of all herbaceous Rubiaceae. The Rubieae are unique among Rubiaceae in the combination of the following pollen features: several colpate apertures, a perforate and microechinate tectum, a relatively small size, the absence of endoapertures, a coarse nexine area beneath the ectocolpi, and the absence of orbicules. The predictive value of pollen morphology is therefore extremely high at the tribal level and supports the monophyly of Rubieae. (C) 2003 Elsevier B.V. All rights reserved.