COMPARATIVE STRUCTURE AND DEVELOPMENT OF POLLEN AND TAPETUM IN MALPIGHIALES, WITH A FOCUS ON THE PARIETAL CLADE

Malpighiales are one of the least resolved of the major angiosperm orders; more comparative data are needed to elucidate deep relationships and character evolution within the order. This article reevaluates pollen and tapetal characters, with a focus on one major subclade, the parietal clade. Pollen structure and, where possible, development are examined in representatives of the parietal clade from Passifloraceae (Passiflora, Turnera), Salicaceae (Populus, Salix), and Violaceae (Viola), and in Euphorbia (Euphorbiaceae), Hypericum (Hypericaceae), and Linum and Reinwardtia (Linaceae) from other Malpighiales. Pollen and tapetal characters in the parietal clade and selected outgroups are tabulated, using both original observations and an extensive literature search. Optimization of pollen and tapetal characters onto a recent molecular phylogeny indicates that lalongate endoapertures are a potential synapomorphy for the parietal clade. Passifloraceae (including the former Turneraceae) have some potential apomorphies: larger pollen size, greater diversity in endoaperture shape, and a plasmodial tapetum. The occurrence of a thick bilayered or channeled apertural intine may support a sister relationship between Passifloraceae and Violaceae, together with pollen with more than three colporate apertures. Very small pollen size indicates that Goupiaceae are potentially more closely related to the Lacistemataceae-Salicaceae clade than to Passifloraceae-Violaceae. Highly apomorphic, thin-walled, inaperturate pollen occurs in the wind-pollinated Populus. Outside the parietal clade, Malpighiales pollen is highly diverse, including large, intectate, baculate pollen in Linaceae with an exine composed of sporopollenin granules that could be an example of paedomorphosis, and unusual aperture structure in Euphorbia, with similarities to opercula. This pollen diversity may be a consequence of the considerable ecological diversity of Malpighiales and their rapid radiation in the mid-Cretaceous, and it indicates the need for further work on pollen and tapetal characters in this clade.