The role of heterotopic shifts in floral nectary evolution of Anacardiaceae

The flowers of Anacardiaceae are known to produce nectar, in most cases secreted by a nectary disk, which may have an intra- or more rarely extrastaminal position. Exceptions include species that produce nectar via secretory trichomes on the petals and/or base of the stamens. To evaluate whether differences in nectary morphology and position result from independent ontogenetic trajectories, we investigated the development of floral nectaries differing in morphology and position in three phylogenetically unrelated species of Anacardiaceae: Anacardium humile A.St.-Hil. (corolline nectariferous trichomes), Schinus mole L. (intrastaminal nectary disk) and Mangifera indica L. (extrastaminal nectaries), using light and scanning electron microscopy. Additionally, we examined the evolution of floral nectaries using the Maximum Likelihood method to infer transitions between nectar gain and loss within the family and to assess potential correlations with pollination mode. In Schinus, the nectary disk develops from an intercalary growth of the receptacle, leading to the formation of a continuous disk in an intrastaminal position. In contrast, the nectaries of Mangifera develop from five individual primordia located at the base of the filaments, indicating a positional shift due to heterotopic meristem displacement. In Anacardium, each nectariferous trichome originates from successive divisions of a single protodermal cell, and the suppression of the disk is linked to functional replacement by the secretory trichomes. Ancestral state reconstructions indicate that the intrastaminal disk represents the most likely ancestral condition, but repeated positional shifts and developmental divergence reveal this structure as a homoplastic feature within the family, reflecting the interplay between developmental flexibility and evolutionary innovation.