Floral morpho-anatomy and reproductive ecology of Spondias macrocarpa Engl. (Anacardiaceae), a vulnerable neotropical andromonoecious tree

Abstract

The genus Spondias has a wide variety of sexual systems, with complete monoecious to polygamodioecious species. Spondias macrocarpa (Anacardiaceae) is an endemic species from Brazil, previously described as hermaphroditic, and considered vulnerable due to massive deforestation. However, to our knowledge, no further details on the breeding system or morphological studies are available. Herewith, we examine the floral morpho-anatomy and breeding system of this species to better understand the sexual function of different floral morphologies and the role of pollinators in its reproduction. We used light and scanning electron microscopy to study the morphology and anatomy of the flowers. Additionally, we carried out experiments on pollen viability, stigmatic receptivity, and controlled pollinations. We found both bisexual and male flowers in the same inflorescence, with the staminate flowers presenting cryptic unisexuality. Investment in bisexual flowers was higher than in male ones, although more than 70% of the inflorescence was made up of male flowers. Most male flowers are present in the proximal region of the inflorescence, and there is no temporal separation between male and female functions in bisexual flowers. We show that S. macrocarpa is andromonoecious with cryptic-monoecy and a self-incompatibility system. Under natural conditions, S. macrocarpa is probably a pollinator generalist, with small bees being its main floral visitors. High levels of pollen robbing and low levels of crossing by floral visitors may result in the observed low fruit set. Further studies on the pre- and post-pollination barriers and on the reproductive ecology of extant natural populations should elucidate the reason for such a low fruit set.