Pollen has higher water content when dispersed in a tricellular state than in a bicellular state

ABSTRACT Pollen is generally dispersed in a sexually immature and somewhat dehydrated, metabolically quiescent state. Yet, in some species, pollen at anthesis is well-hydrated and metabolically active, and in 30 % of angiosperms pollen is dispersed after having formed its sperm cells. Pollen water content and sexual maturity may be correlated, either because both are subject to trade-offs between dispersal viability and post-pollination performance, or because the traits display developmental linkages. We inferred relative water content of sexually immature (“bicellular”) and sexually mature (“tricellular”) pollen of 30 species of angiosperms using a hydration index (HI) that ranges from zero to one, based on how near fresh pollen volume is to its minimal (dehydrated) or maximal (hydrated) volume. Tricellular pollen had 30 % higher HI than bicellular pollen (P < 0.005), after controlling for initial pollen size (larger pollen had higher HI; P < 0.05). A literature survey of 344 species indicated that the tricellular and hydrated states were strongly associated, although all four trait state combinations were present (P < 0.0001). Our results suggest that a common mechanism for the repeated origins of tricellular pollen has been via the loss of controlled pollen dehydration, which enables either accelerated or extended pollen development.

摘要 花粉通常以性发育未成熟、适度脱水且代谢静止的状态完成传播。然而部分物种的花粉在开花期处于充分水合且代谢活跃的状态，且30%的被子植物的花粉会在形成精子细胞后才进行传播。花粉含水量与性成熟度可能存在关联，其原因或为二者均需在传播活力与授粉后表现之间进行权衡，或为两类性状存在发育层面的关联。本研究基于新鲜花粉体积与其最小（脱水状态）或最大（水合状态）体积的接近程度，采用取值范围为0至1的水合指数（hydration index, HI），对30种被子植物的性发育未成熟二细胞型（bicellular）花粉与性发育成熟三细胞型（tricellular）花粉的相对含水量进行了推断。在控制初始花粉体积（体积更大的花粉水合指数更高；P < 0.05）的条件下，三细胞型花粉的水合指数较二细胞型花粉高出30%（P < 0.005）。针对344个物种的文献调研显示，尽管四类性状状态组合均有存在，但三细胞型花粉与水合状态存在显著关联（P < 0.0001）。本研究结果表明，三细胞型花粉多次独立起源的常见机制，是丧失了受控的花粉脱水过程，该过程可加速或延长花粉的发育进程。