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How does heterostyly in primroses encourage cross-pollination?
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pollination
Common carder bumblebee (Bombus pascuorum) pollinating a honeysuckle (Lonicera species) flower.
Heterostyly is a structural adaptation in primroses that discourages self-pollination by creating variation in the length of the style and the position of the stamens.1
In most primrose populations, individuals exhibit one of two floral forms:
- Pin flowers: These possess a long style and short stamens, positioning the stigma at the flower's opening.1
- Thrum flowers: These have a short style and long stamens, which form a "thrumhead" at the flower's opening.1
This arrangement ensures that when a bee visits a flower, it collects pollen on a specific part of its body. Because the corresponding reproductive structures in the opposite flower type are positioned at the same height, the insect is highly likely to deposit that pollen onto the stigma of the other type of flower. This mechanism, regulated by a genetic system that maintains a 50:50 ratio between the two forms, effectively promotes cross-pollination and limits self-fertilization.12