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

The number of apertures may vary considerably within a genus, which is illustrated for Spermacoce L. (Dessein et al., 2002a), in which a weak positive correlation has been found between pollen size and number of apertures. If the pantoaperturate species are excluded from the test, the correlation is strong. Also, within Danais Comm. ex Vent. there seems to be a correlation between pollen grain size and the number of colpori. Taxa with invariably 3- or 3(-4)-colporate grains tend to have smaller grain sizes, and species with 4(-5)-colpori have larger grains (Buchner & Puff, 1993).

b. Position

In most Rubiaceae, pollen is angulaperturate--i.e., the apertures are situated at the angles of the outline in polar view (Fig. 12)--with the apertures situated only at the equator (zonoaperturate). Planaperturate pollen--i.e., apertures situated at the sides when seen in polar view (Fig. 13)--is observed in a few species only; for example, Rondeletia odorata Jacq. (El-Ghazaly et al., 2001) and Spermacoce stenophylla (Dessein et al., 2005b). Pantoaperturate grains with the apertures evenly spread over the pollen surface (e.g., in Geophila repens, Spermacoce phyteuma Schweinf. ex Hiern, and S. annua Verdc.) occur sporadically in the family (Fig. 18). Two remarkable, previously unreported, organization patterns were recently observed in Spermacoce (Dessein et al., 2002a). The first type has several short colpori arranged in a looplike pattern (Fig. 16), similar to the line on a tennis ball (S. filifolia (Schumach. & Thonn.) J.-P. Lebrun & Stork, S. octodon J.-P. Lebrun & Stork, and S. tenuissima Hiern). In the second type (Fig. 17), the colpori are arranged in a spiral pattern (S. terminaliflora Good and S. thymoidea (Hiern) Verdc.). To our knowledge, these two types of aperture position have not previously been observed in angiosperms. These types can be considered intermediate forms between zono- and genuine pantoaperturate grains. In Coffea L., spiraperturate and syncolporate grains may occur, probably because of hybridization events (Chinnappa & Warner, 1981, 1982; Stoffelen et al., 1997).

[FIGURE 18 OMITTED]

c. Type

The aperture type of Rubiaceae pollen is a major character for identification and often useful for inferring phylogeny. Compound apertures are the rule; that is, two or three (non)congruent apertures on top of each other, situated in different layers of the wall. A full set of apertures thus consists of an ectoaperture, which is a thinning or hole in the sexine, a mesoaperture situated in the foot layer, and an endoaperture in the endexine and/or membranous granular layer MGL (see below for details). The first comprehensive analysis of the structure of Rubiaceous apertures was made by Lobreau-Callen (1978). Much confusion is caused by the terminology of pollen classes such as colporate or pororate, because they are used mostly on the basis of external SEM observations only. When a grain has undifferentiated ectocolpi (without mesoapertures) in external equatorial view, it is usually named "colpate." This is the case, for example, in Chiococca phaenostemon Schlecht. (Huysmans et al., 1999). However, on broken grains or in LM, endocolpi are visible with diverging ends perpendicular to the ectocolpi. According to the definition in Punt et al. (1994), Chiococca P. Br. is thus colporate. Consequently, the thinnings at the inside of the pollen wall merit closer examination (see under endoapertures).