Abstract:

Background & Aims: Pollen grains in natural plant populations exhibit remarkable diversity in morphological features including size, shape, surface ornamentation, aperture number, and coloration. Despite this interspecific variability, pollen morphology typically remains relatively conserved within a single species, a characteristic of significant value for both taxonomic classification and studies of reproductive success. Intriguingly, pollen dimorphism—the production of two or more distinct pollen types within an individual plant—has been documented in certain taxa, challenging the traditional assumption of pollen uniformity. While preliminary investigations have explored the phylogenetic implications and adaptive advantages of this phenomenon, systematic syntheses addressing its evolutionary drivers and ecological functions remain limited. A comprehensive analysis of pollen dimorphism is thus critical to unraveling its role in shaping floral diversity, optimizing pollination efficiency, and mediating coevolutionary dynamics between angiosperms and their pollinators. 

Progress: This study proposes a comprehensive classification system for pollen dimorphism, categorizing the phenomenon into four distinct types based on morphological and physiological characteristics: size dimorphism, shape dimorphism, color dimorphism, and vitality dimorphism. Notably, certain plant species demonstrate concurrent expression of two or more dimorphic pollen types. Through extensive literature analysis, we have reconstructed a phylogenetic framework encompassing 116 genera across 51 angiosperm families that exhibit pollen dimorphism. The evolutionary emergence of this trait shows significant correlation with specialized reproductive strategies, including but not limited to heteranthery, cryptic dioecy, and enantiostyly (mirror-image flowers). These floral adaptations, frequently associated with pollen dimorphism, appear to have co-evolved with specific pollination syndromes and mating system optimizations, suggesting an adaptive mechanism for reproductive assurance and resource allocation efficiency in flowering plants. 

Prospects: Current research on pollen dimorphism remains in its nascent stages, with critical knowledge gaps persisting in understanding its genetic architecture, ontogenetic regulation, ecological significance, and evolutionary trajectories. Addressing these fundamental questions requires multidisciplinary approaches integrating molecular genetics, developmental biology, and ecological genomics. This line of investigation holds significant implications for elucidating the evolutionary innovation of pollen morphology, identifying the evolutionary drivers of floral diversification, and deciphering the intricate plant-pollinator coevolutionary dynamics. Systematic exploration of pollen dimorphism may ultimately provide novel insights into the evolutionary optimization of plant reproductive strategies under selective pressures.

Key words: pollen dimorphism, breeding system, pollen morphological variation, reproductive characteristics, adaptive evolution