Pollen diversity of <i>Adenia</i>, <i>Crossostemma</i> and <i>Schlechterina</i> (Passifloraceae <i>sensu stricto</i>): types of apertures and implications for evolutionary studies

The genera Adenia Forssk., Crossostemma Planch. ex Benth. and Schlechterina Harms are members of the tribe Passifloreae DC., one of two tribes belonging to Passifloraceae sensu stricto. The genus Adenia is considered the sister group of all Passifloraceae sensu stricto, or as the sister to the other genera of the tribe Passifloreae. Crossostemma and Schlechterina are sister taxa. The objective of the present study was to characterise the pollen morphology of these three genera and to determine whether Adenia is palynologically distinct from the monotypic genera Crossostemma and Schlechterina. Pollen grains of eight species (six of Adenia, one of Crossostemma and one of Schlechterina) were acetolysed, measured, described and illustrated using light and scanning electron microscopy. The pollen grains of the analysed species are medium-sized isopolar monads that are subprolate, oblate-spheroidal or prolate-spheroidal with a subcircular to quadrangular amb and a small polar area; the pollen of Crossostemma and Schlechterina are 3-colporate, while that of Adenia is 4-colporate or rarely 3–4-colporate (in one studied species), and the sexine is reticulate, heterobrochate or homobrochate (Crossostemma), with simplicolumellate walls. The pollen grains of Adenia differ from those of Crossostema and Schlechterina mainly in the number of apertures. Multivariate analysis of the genera studied here, performed with published pollen data for Paropsia Noronha ex Thouars and Barteria Hook. f., revealed the formation of two large groups, one comprising Barteria and another comprising the other genera. Pollen morphology reveals that Paropsia has greater affinity with the genera of the tribe Passifloreae. The results lead to the hypothesis that genera belonging to the tribe Passifloreae possess pollen with fewer apertures than the pollen of taxa of the tribe Paropsieae.