Palynological Characters and Their Phylogenetic Signal in Rubiaceae

Abstract

In the 1990s Rubiaceae became a hot spot for systematists, mainly due to the comprehensive treatment of the family by Robbrecht in 1988. Next to the exploration of macromolecular characters to infer the phylogeny, the palynology of Rubiaceae finally received the attention it deserves. This article aims to present a state-of-the-art analysis of the systematic palynology of the family. The range of variation in pollen morphology is wide, and some of the pollen features are not known from other angiosperm taxa; e.g., a looplike or spiral pattern for the position of apertures in pantoaperturate grains. We compiled an online database at the generic level for the major pollen characters and orbicule presence in Rubiaceae. An overview of the variation is presented here and illustrated per character: dispersal unit, pollen size and shape, aperture number, position and type, sexine ornamentation, nexine pattern, and stratification of the sporoderm. The presence/absence and morphological variation of orbicules at the generic level is provided as well. The systematic usefulness of pollen morphology in Rubiaceae is discussed at the (sub)family, tribal, generic, and infraspecific levels, using up-to-date evolutionary hypotheses for the different lineages in the family. The problems and opportunities of coding pollen characters for cladistic analyses are also treated.

Morpho-anatomical and morphometric studies of the floral structures of the distylous Oldenlandia salzmannii (Rubiaceae)

“…The differences in pollen size between morphs and the number of pollen grains per anther are the principal polymorphism characteristics correlated with heterostyly (Ganders 1979;Dessein et al 2005), as there is a correlation between pollen size and number of pollen grains which results in many small grains or in a few large grains. Pollen dimorphism in respect to the size is so frequent in distylous taxa that Darwin (1877) considered it a basic characteristic of heterostyly.…”

A new species of Colletoecema (Rubiaceae) from southern Cameroon with a discussion of relationships among basal Rubioideae

“…Among basal rubioideae, only the tribe Urophylleae is characterized by exclusively 3-colporate pollen grains. The tribe ophiorrhizeae has 3-or 4-colporate grains, while Lasiantheae and coussareeae have (2-)3-4(-5)-and 2-4-colporate grains, respectively (Dessein et al 2005).…”

Presence of the protruding oncus is affected by anther dehiscence and acetolysis technique

“…The term was proposed to replace 'pollen bud' following Weber and Igersheim's (1994) objection that 'pollen bud' was an inappropriate term. However, pollen buds have usually been described as containing large vacuoles (Igersheim & Weber, 1993;Hansson & El-Ghazaly, 2000;Dessein et al, 2005), whereas depictions of protruding onci show them lacking vacuoles or, at most, possessing very small vacuolar remnants (Tilney & Van Wyk, 1997;Kuang et al, 2008). This distinction was not made by Tilney and Van Wyk (1997).…”

“…However, since that time, several authors have suggested that protruding onci and pollen buds are distinct structures based on three criteria: pollen buds are usually larger than protruding onci, they usually possess a vacuole (Igersheim & Weber, 1993;Weber & Igersheim, 1994;Hansson & El-Ghazaly, 2000) and they are severed from the grains before shedding (Philip & Mathew, 1975;Chennaveeraiah & Shivakumar, 1983;Priyadarshan & Ramachandran, 1984;Weber & Igersheim, 1994;Dessein et al, 2005). In general, large, conspicuous vacuoles are not found in protruding onci, while cytoplasmic inclusions are occasionally present.…”