Palynological characters and their phylogenetic signal in Rubiaceae

Botanical Review, The, July-Sept, 2005 by Steven Dessein, Helga Ochoterena, Petra De Block, Frederic Lens, Elmar Robbrecht, Peter Schols, Erik Smets, Stefan Vinckier, Suzy Huysmans

Supratectal elements are absent in the majority of Rubiaceae. A number of tribes have pollen both with and without supratectal elements; for example, Coffeeae, Pavetteae, and Gaertnereae. Only in a few taxa--e.g., Rubieae (Huysmans et al., 2003) and all 24 genera of the PECC-complex (Huysmans et al., 1999, unpubl.)--the sexines are invariably echinate. The supratectal elements can be (micro)spines, microgemmae or microverrucae, or tiny striae. In several Australian Gardenia species, pollen grains with small gemmae intermingled with very large ones have been observed (Puttock, 1992).

Distinct margines of supratectal elements are rather exceptional. In Spermacoce natalensis Hochst., for instance, spines occur in an elliptic field around the ectocolpus but the rest of the sexine is psilate (Dessein et al., 2002a).

6. Nexine Ornamentation

The inside of acetolyzed Rubiaceae pollen is often granular. The nature of this granular layer was recently studied ontogenetically and histochemically by El-Ghazaly and Huysmans (2001). They found that it is a distinct wall layer with a particular mode and timing of development, different from both the endexine and the intine. Histochemically, this layer differs from the endexine in having fewer lipids and more proteins and is distinguished from the intine in containing more pectin and less acidic polysaccharides. The granular layer also resists acetolysis and was called the "membranous granular layer" (MGL). El-Ghazaly and Huysmans (2001) describe a MGL in four dicots (including Rondeletia odorata) and one monocot. The thinnings often observed at the inside of acetolyzed Rubiaceae pollen are located in the MGL and not in the endexine. For an unambiguous understanding, we prefer to use the circumscription "inner surface of the nexine" in what follows.

The systematic importance of nexine characters at the inside of pollen grains is repeatedly stressed in Rubiaceae (Jansen et al., 1996a; Van Campo, 1978), and we are fully convinced of their potential as phylogenetic markers (Huysmans et al., 1998a, 1999). The ornamentation of the nexine is mostly granular (Fig. 37); few genera have an entire smooth inner nexine surface (e.g., Chazaliella E. M. A. Petit ex E. M. A. Petit & Verdc.). In Coptosapelta Korth. the nexine is bumpy, like water droplets on an oily surface (Fig. 39; Verellen et al., 2004).

[FIGURES 37, 39 OMITTED]

Irregular grooves in the inner nexine surface, called "endocracks," are common in Rubiaceae (Fig. 38). They sometimes reduce the granular layer into isolated patches, as clearly seen in Isertia Schreb. (Huysmans et al., 1998a), Capirona Spruce, and Gaertnera Lam. (Fig. 40; Jansen et al., 1996a).

[FIGURES 38 & 40 OMITTED]

7. Stratification of the Pollen Wall

In general, Rubiaceae pollen corresponds to the basic pattern of pollen-wall stratification in angiosperms: rectum, columellae, foot layer, endexine, intine (Figs. 41-42, 44). The relative proportions of the respective layers are variable, even within one pollen grain. In a few genera (e.g., Coptosapelta, Durringtonia) columellae are reduced or totally lacking (Fig. 43). Atectate pollen has so far been observed only in Versteegia cauliflora (De Block & Robbrecht, 1998). Free-standing bacula in the lumina of a reticulate sexine (Fig. 34) occur in several species belonging to at least 21 genera scattered all over the family. It seems to be a typical character for many genera of the Morindeae (e.g., Prismatomeris Thawaites, Caelospermum Blume).