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

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B. HETEROSTYLY AND POLLEN DIMORPHISM

Barrett (1992: 1) defined heterostyly as "a genetic polymorphism in which plant populations are composed of two (distyly) or three (tristyly) morphs that differ reciprocally in the heights of stigmas and anthers in flowers." Heterostyly is very common in Rubiaceae (cf. Anderson, 1973; Bir Bahadur, 1968b; Bremekamp, 1952, 1966), especially in genera of Psychotrieae and Spermacoceae s.1. (Robbrecht, 1988a). Given that congeners of heterostylous species are often homostylously ancestral, it must have evolved repeatedly in the family (Anderson, 1973). Pollen size dimorphism is often correlated with heterostyly. As a rule, pollen of the brevistylous morph is slightly larger than pollen in flowers with long styles (Bir Bahadur, 1963, 1966, 1968a, 1968b, 1970; Bir Bahadur & Rama Swamy, 1993; Bremekamp, 1963; Jung-Mendacolli & Melhem, 1995; Naiki & Nagamasu, 2003; Pailler & Thompson, 1997). However, the opposite situation is reported in Pentas schimperiana Vatke (Dessein et al., 2000), Pseudosabicea arborea (K. Schum.) N. Halle (Huysmans et al., 1998a), and Hindsia longiflora (Cham.) Benth. (Di Maio, 1996). In these species the larger pollen grains are observed in the longistylous flowers. However, one must bear in mind that size differences are usually slight and become obscured if different populations are compared (Puff, 1988).

Two other cases of pollen dimorphism correlated with heterostyly are frequently reported: variation in aperture number and sexine ornamentation. Baker (1956), for example, reported "spiniferous" pollen grains in short-styled flowers of Rudgea and "smooth" pollen grains in long-styled flowers. Similar cases are reported for several other genera scattered all over the family; for example, for the Danais-Payera-Schismatoclada complex (Buchner & Puff, 1993). In several species of Coccocypseleae, a tribe characterized by a double reticulum, it has been observed that the infrareticulum is better developed in the brevistylous flowers than in the longistylous flowers. Pire and Cabral (1992) made a similar observation for the genus Galianthe Griseb. (Spermacoceae s.str.). In most cases, however, the difference between the exine of the brevistylous and longistylous morphs is less pronounced: The dimorphism is often limited to a coarser reticulation in the thrum grains (Bir Bahadur, 1968b). The pollen morphs of Sabicea capitellata Benth. and S. orientalis Wernham differ morphologically only in the ratio of 3- and 4-aperturate grains. Longistylous flowers of Sabicea capitellata have considerably fewer 4-aperturate grains than do brevistylous flowers; only in the brevistylous morph do 5-aperturate grains occasionally occur. In Sabicea orientalis the opposite was observed: Brevi-pollen was mostly 3-aperturate, sometimes 4-aperturate, whereas the ratio of 3-/4-aperturate grains in longipollen is approximately 1 (Huysmans et al., 1998a). A more extreme example of difference in aperture number correlated with heterostyly was reported by Robbrecht (1977) in Hymenocoleus. Brevistylous individuals apparently produce nonaperturate pollen grains, whereas the longistylous flowers contain 3(-4-5)-porate grains. A SEM investigation of this genus should be undertaken, however.