Pollen morphology of the subtribe Pleurothallidinae Lindl. (Orchidaceae)

Pollinia of 21 pleurothallid genera were examined with SEM. To detect the polarity in sculpture evolution, pollinia of three species (Restrepia elegans, Myoxanthus ceratothallis, Octomeria grandiflora) were studied with TEM. Compared with other taxa of the subfamily Epidendroideae, the Pleurothallidinae reveal a strong tendency towards sporoderm reduction. Besides a primitive surface there are 5 derived types which have evolved along two major morphological lines. psilate-reticulate-octomerioid and psilate-granulate-lepanthoid/hamulate/gemmate. Caudicular regions generally show an undifferentiated exinous layer. Even 'primitive' representatives of the subtribe have a reduced stratification compared with the classic angiospermous tectate-columellate pollen wall. Starting with a partially baculate sporoderm, which already lacks the nexine (Restrepia) we find a highly reduced pollen wall with a compact sporopollenin-layer and a stratified intine in Octomeria and Myoxanthus. Pollen walls of the inner pollinium consist only of a hi-layered intine. In Octomeria inner tetrads are separated by a partially perforated cell wall comparable to the callose wall of the pollen mother cell. Palynological data and their systematic implications are summarized in a diagram. Except for Pleurothallis and Masdevallia, which seem to be paraphyletic, surface types are consistent with the genera. However, some of them have apparently evolved independently several times. Reduction patterns coincide with major trends of orchidaceous sporoderm evolution. Some of the surface types, however, have been undocumented in the family so far. They show the subtribe as a highly derived natural taxon, which is still actively evolving. Adaptations to very small pollinators (dipterans) and short dispersal distances of the pollen, along with the high humidity of the natural habitat, have apparently triggered the reduction processes of the pleurothallid sporoderm.