The Classification and Functional Significance of Staminodes in Angiosperms

All staminodes in an androecium fail to produce viable pollen grains and cannot contribute directly to male fitness. Staminodes are identified in the flowers of approximately 54% of known genera representing >32% of all angiosperm families. The functional morphology and biochemistry of staminodes differs significantly from stamens with fertile anthers. In the absence of sperm production, some staminodes evolved novel adaptations contributing to the reproductive success of their flowers. We subdivided these staminodes into eight functional types: 1) Staminodes offering visual/olfactory cues; 2) Staminodes offering edible/inedible rewards; 3) Staminodes that deceive pollinators with false rewards; 4) Staminodes facilitating or directing the movements of pollinators in flowers;5) Stami nodes that facilitate stigma movement; 6) Staminodes functioning as secondary pollen presenters; 7) Staminodes that protect other floral organs; 8) Staminodes that prevent mechanical self pollination (autogamy). As a component within a flower, the majority of staminodes function as promoters of reproductive success by interacting directly with the pollinator to increase pollination efficiency (both pollen dispersal and deposition). Therefore, it is not surprising that one staminode may have more than one function over the flower′s lifespan and is closely associated with the size, abundance, behavior and taxonomic diversity of pollinators, floral predators and thieves. To correctly evaluate the function of staminodes, multi disciplinary approach using a range of protocols, equipment and materials is suggested. This approach allows us to compare the roles different staminodes play in the reproductive success of both closely and distantly related angiosperms, then uncover their evolutionary significance in angiosperm diversification.