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

In Pseudosabicea arborea, apart from size differences, pollen dimorphism is conspicuous in several other characters (Huysmans et al., 1998a). The sexine pattern is reticulate in long-styled flowers and perforate in short-styled flowers; apertures are larger in longistylous flowers; and the exine is twice as thick in longi-pollen ([ or -] 2 [micro]m) as in brevi-pollen ([ or -] 1 [micro]m).

C. ORBICULE CHARACTERISTICS OF RUBIACEAE

Orbicules are tiny sporopollenin particles up to 3-4 [micro]m in diameter that line the inner tangential and radial walls of the tapetal cells (Huysmans et al., 1998b, 2000). Thus orbicules coat the inner locular surface and are in close contact with the pollen grains. Orbicules may have an electronlucent core, but they are always acellular structures. They are often, but not always, present in Rubiaceae.

The first report of the presence of orbicules in Rubiaceae was by Andronova (1984). She reported orbicules in several unrelated species of the family. Because her work was published in Russian, however, little attention has been paid to this study. Consequently, Pacini and Franchi (1993) did not include any Rubiaceae in their listing of angiosperm species with secretory tapetum reported to have orbicules. In the same year, Igersheim (1993a) reported the presence of orbicules in the monospecific Caribbean genus Strumpfia. Since then, two subfamilies of Rubiaceae have been screened for the presence of orbicules: Huysmans et al. (1997) investigated 14 genera of Cinchonoideae s.str., and Vinckier et al. (2000) studied 32 genera of Ixoroideae s.1. Orbicules have also been reported in several other genera. In Table II all present data are summarized at the generic level.

Six orbicule types can be described in Rubiaceae (Huysmans et al., 1997; Vinckier et al., 2000):

I. Spiny orbicules (Figs. 45-47). Orbicules are up to 3 [micro]m in diameter and are not embedded in tapetal membrane (although exceptional semi-embedded forms do occur). The orbicular wall is smooth but beset with a few to numerous spinules; one or a few perforations may occur. A core is absent or present.

[FIGURES 45-47 OMITTED]

II. Microrugulate orbicules (Fig. 48). Orbicules are less than 2 [micro]m in diameter and are embedded in the tapetal membrane. Their shape is more or less spherical, but they seem to consist of several elongated elements.

[FIGURE 48 OMITTED]

III. Smooth orbicules (Figs. 49-51). Orbicules are less than 1.4 [micro]m and are rarely embedded in the tapetal membrane. Aggregates of two or more orbicules may occur. The orbicular wall is smooth. Subtype IIIa has rounded, oblate orbicules with small perforations in the wall; subtype IIIb has more flattened, circular orbicules with a single central indentation (doughnut shaped).

[FIGURES 49-51 OMITTED]

IV. Irregularly folded orbicules (Figs. 52-53). Orbicules are irregular in shape and not spherical because of a folded surface. Perforations may occur, but a core is absent. They are not or only slightly embedded in tapetal membrane.