野牡丹异型雄蕊的功能分化

Division of labor of heteromorphic stamens in Melastoma malabathricum

野牡丹科植物具有形态、大小和颜色显著不同的两种异型雄蕊。关于其异型雄蕊是否具有功能分化还一直存在争论。本文以野牡丹科植物野牡丹(Melastoma malabathricum)为实验材料,比较了两种异型雄蕊在传粉过程中的功能作用。结果表明,两种异型雄蕊在形态、花粉量和人工控制实验条件下的结籽数,以及主要传粉昆虫木蜂(Xylocopa sp.)访花时的行为等方面都有显著差异,说明两种雄蕊在传粉过程中存在一定的功能分化:外轮紫色雄蕊中的花粉为后代提供雄配子,而内轮黄色雄蕊中的花粉则为传粉昆虫提供食物。但两种雄蕊在花粉活性、花粉组织化学成分和结实率方面差异均不显著,表明两者在生理上并没有分化。实验结果还表明,除花前套袋不结实外,自交、异交和自然对照都具有较高的结实率,说明野牡丹不存在无融合生殖和主动自交及自交不亲和现象,为兼性异交。

Heteranthery in flowers has often been explained by Darwin＇s ‘division-of-labor＇ hypothesis. In order to test the hypothesis, we examined pollination pattern and the roles of two dimorphism stamens in pollination in Melastoma malabathricum; this species contains two sets of stamens differing markedly in shape, size and color. We found differences in morphological characters, numbers of pollen grains, seed sets following experimental treatments, with stamens removed and pollinator behaviour between the two types of stamen. The anther of purple stamens contained more pollen grains than that of yellow stamens. Carpenter bees （genus species） alighted only on the yellow stamens, and grasped their five anthers. Flowers that had their yellow stamens removed set significantly fewer seeds than intact flowers. There was no difference in the seed set of flowers with purple stamens removed and intact flowers. These results suggest that the yellow anthers function to attract pollinators and satisfy the insects＇ demand for pollen, and the purple anthers satisfy plant＇s need for safe gamete dispersal. However, there were no differences in pollen viability, histochemistry, or fruit set between anther type, suggesting the lack of physiological differentiation. Our experimental manipulations also demonstrate the absence of self-pollination and agamospermy in Melastorna malabathricum, and that the breeding system is facultative xenogamy.