Floral morphoanatomy in Gymnocalycium (Cactaceae): Phylogenetic insights from structural analysis

The genus Gymnocalycium represents a distinct lineage within the tribe Cereeae (Cactaceae), recently assigned to the monogeneric subtribe Gymnocalyciinae. Although molecular phylogenetic studies consistently supports the monophyly of the genus, detailed floral morphoanatomical information remains scarce. This study investigates the floral morphoanatomy of four species representing four subgenera (Gymnocalycium saglionis, G. schickendantzii, G. schroederianum, and G. spegazzinii) using light and scanning electron microscopy. Our findings revealed conserved floral morphoanatomical features among the species examined, including nectar chambers with nectarostomata, nectariferous epidermis composed of trichomes or papillae, and rounded-oblate orbicules distributed along the inner anther walls. To our knowledge, this study provides the first report of orbicules in the subfamily Cactoideae. Pollen grains are consistently isopolar, radiosymmetric, and trizonocolpate, with a tectate-punctate exine, whereas interspecific differences occur mainly in exine ornamentation and pollen morphometric variables. Gymnocalycium schroederianum is distinguished by branched stigmatic lobes and nanospinulate pollen ornamentation, while the other species exhibit nanospinulate to microspinulate pollen ornamentation. Branched multicellular trichomes were also observed in the nectar chambers of G. sagionis, G. schickendantzii and G. spegazzinii, a character previously unreported in the family. Overall, the results expand the comparative knowledge of floral morphoanatomy in Gymnocalycium and reveal both conserved and variable characters among the species examined, providing a basis for future studies integrating floral morphology, reproductive biology, palynology, and phylogenetic data.