Phylogenetic and morphologic survey. of orbicules in angiosperms

Orbicules are granules of sporopollenin or polymorphic sporopollenin-elements produced by the tapetum, associated with pollen grains and tapetum cells. In angiosperms, they are common in the ANA grade and monocots, but are absent in late-branching clades such as the majority of Asteraceae and the majority of Lamiaceae, revealing an evolutionary trend from presence to absence along the phylogenetic tree. However, for several orders of angiosperms, no data on orbicules is available. We investigated orbicules in 34 species from 29 different genera and 14 orders, for which limited or no data was available (Acorales, Arecales, Buxales, Celastrales, Crossosomatales, Dilleniales, Escalloniales, Fagales, Garryales, Gunnerales, Paracryphiales, Santalales, Trochodendrales, Zygophyllales). We photographed pollen sacs and pollen grains with a SEM, then described selected orbicule traits and measured their size. We then calculated: (1) the phylogenetic signals for (a) orbicule size in a selected group of species and (b) presence/absence of orbicules in all angiosperm groups until now recorded in the literature; (2) the ancestral state of orbicule size; (3) origin, loss and transition rate for presence/absence of orbicules in angiosperms. Orbicule traits are supported both by a strong phylogenetic signal and by a significant evolutionary trait value. The evolutionary rate for the presence of orbicules confirms that it is a plesiomorphic trait showing adaptive radiation with a significant increase in origins and diversification, while the phylogenetic signal for size of the orbicules indicates evolutionary convergence. Furthermore, we detected an evolutionary trend towards a reduction in the size of orbicules, that might have led to their absence in most late-branching clades.