The Pollen-Reward Strategy: The Paradoxical Realities of Trading Gametes for Sex

Pollen-rewarding plants face a fundamental tension: Pollen serves simultaneously as the male gametophyte and pollinator reward. Despite this apparent paradox, pollen is the second most common floral reward and the sole reward for an estimated 20,000 angiosperm species. We review the ecology and evolution of the pollen-reward strategy, synthesizing literature encompassing pollen chemistry, floral morphology, and sex allocation. We find that this reward strategy is unevenly distributed among plant families, that the resource costs of pollen rewards remain poorly characterized, and that this reward strategy may influence plant sexual-system evolution. We offer hypotheses regarding the evolution and maintenance of the pollen-reward strategy and examine the ecological and evolutionary consequences associated with its expression. We suggest that floral rewards are not isolated traits, but key elements of plant reproductive strategies, and that this perspective allows comparison among strategies and can advance our understanding of the evolutionary ecology of plant reproduction.