Systematic and morphologic survey of orbicules in allergenic angiosperms

Orbicules are sub- or pauci-micronic sporopollenin-coated particles associated with pollen grains. Recent studies have highlighted their importance in terms of systematic value or as vectors of allergens that may trigger pollinosis. While orbicules have indeed been reported for most families and genera of allergological importance, their usefulness in systematics remains little explored. Our study aims: (a) to discover the presence/absence of orbicules in species of allergological interest hitherto overlooked regarding these particles; (b) to propose standard descriptors for orbicule morphology; and (c) to link the features of orbicules to a systematic framework. We observed or measured selected orbicule traits in SEM micrographs of pollen sacs and pollen grains of 23 species of 17 genera in Asteraceae, Betulaceae, Fagaceae, Oleaceae, Poaceae, Plantaginaceae and Polygonaceae families. Categorical and continuous traits were included in a matrix and then subjected to NMDS as ordination method to assess the usefulness of orbicule traits as systematic characters. We detected orbicules both on the locule wall and on the pollen exine in almost all investigated species and described unknown nanoparticles smaller than orbicules. Based on orbicules traits, NMDS separated the samples at species level. We detected and described orbicules on the tapetum and on the pollen exine in 19 target species and observed unknown nanoparticles that might explain (a) the process of deposition of sporopollenin during orbicule development and (b) the onset of pollinosis before and after the dispersion of pollen grains into the air. Finally, within the restricted sample examined, selected orbicule traits allowed to resolve the data set at species level