Reproductive role of sterile pollen in Saurauia (Actinidiaceae), a cryptically dioecious Neotropical tree

The genus Saurauia Willd. (Actinidiaceae {= Dilleniaceae}) consists of several hundred species of tropical flowering trees of small to medium stature, some of which vary in apparent sexual allocation. The showy white flowers of several Neotropical species are sexually dimorphic. Individual trees bear thyrsiform cymes of either staminate flowers whose pistils are vestigial, or else perfect flowers with functional anthers and pistils. This morphological evidence corresponds to the rare system of sexual allocation, androdioecy. However, these species may be cryptically dioecious, as pollen from herbarium specimens with pistillate flowers representing eight species reportedly lacked nuclei or apertures, or did not stain in lactophenol or cotton blue (indicating inviability). Pollen from pistillate flowers of S. veraguensis Seem. reportedly did stain with lactophenol, but germinated poorly on conspecific stigmas. The objective of this study was to assess the reproductive role of pollen from perfect flowers of Saurauia in the field, using an undescribed species from the Costa Rican highlands.