Pollen morphology of Croton of the New World and pollen evolution of tribe Crotoneae (Euphorbiaceae)

Abstract

Croton has ca. 1200 species and a pantropical distribution, representing the second largest genus in Euphorbiaceae. In this work, we aim to characterize morphologically the pollen grains of 105 species of Croton, besides other representatives of tribe Crotoneae, to provide palynological data sustaining its current delimitation and classification and understand how pollen grains evolved in the tribe. Floral buds were obtained from specimens and then acetolyzed, measured and described under light and scanning electronic microscopy. Pollen grains analyzed are medium to large, spherical, apolar, inaperturate and have the sexine thicker than the nexine. The Croton pattern is composed of pila or spine as ornamentation subunits, whose surface may be psilate, plicate or striate. Data on type, disposition, and distribution of sexine elements in the lumen of the rosettes are important to distinguish species. The spine Croton pattern has a single origin in Croton and is derived from the pilum type. Our study corroborates the stenopalynous status of Crotoneae since palynological features are highly conserved in the tribe. On the other hand, we point to the potential of the configuration of ornamentation units associated with the diameter of pollen grains to understand pollen evolution in the tribe.