Distyly represents a unique floral adaptation found in certain plant species. It is a specialized reproductive strategy that ensures successful reproduction through distinct flower forms. It involves structural variations within flowers, influencing their pollination mechanisms. Distyly shows how plants have evolved intricate systems to optimize reproductive success in diverse environments.
Understanding Distyly
Distyly is a type of heterostyly with two distinct flower morphs within a single species. These morphs are known as “pin” flowers and “thrum” flowers. Their defining characteristic is the reciprocal arrangement of reproductive organs: the stigma (pollen-receiving part) and anthers (pollen-producing part).
In “pin” flowers, the pistil, which includes the style and stigma, is long, extending prominently from the flower, while the stamens are short and positioned lower within the floral tube. Conversely, “thrum” flowers possess a short pistil, with the stigma located deep within the flower, and long stamens that extend outwards. This creates a mirrored, reciprocal arrangement, where the anther height of one morph matches the stigma height of the other morph, and vice versa.
The Evolutionary Advantage of Distyly
Distyly primarily evolved as a mechanism to promote cross-pollination, also known as outcrossing, and to prevent self-pollination within the same plant. This outcrossing strategy provides significant genetic benefits, such as increased genetic diversity within a plant population. Greater genetic diversity can lead to enhanced adaptability, allowing a species to better respond to changing environmental conditions or new disease pressures.
The reciprocal arrangement of stigmas and anthers physically facilitates pollen transfer between different floral morphs via pollinators. This precise pollen transfer mechanism, known as reciprocal herkogamy, reduces the likelihood of pollen from a flower landing on its own stigma, which would lead to self-pollination. By promoting the exchange of genetic material between different individuals, distyly helps to reduce inbreeding depression, a phenomenon where repeated self-fertilization can lead to a decrease in fitness due to the expression of harmful recessive alleles.
Mechanisms of Pollination in Distylous Plants
Pollination in distylous plants relies on the interaction between pollinators and the two floral morphs. When a pollinator, such as an insect, visits a “pin” flower, pollen from the short stamens is deposited on a lower part of the pollinator’s body, like its proboscis or the underside of its head. If the same pollinator then visits a “thrum” flower, the pollen from the pin flower contacts the short stigma of the thrum flower, which is positioned at a similar height to the pin flower’s anthers.
Conversely, when a pollinator visits a “thrum” flower, pollen from its long stamens is deposited on a higher part of the pollinator’s body. Upon visiting a “pin” flower, this pollen transfers to the long stigma of the “pin” flower, aligning with the height of the “thrum” flower’s anthers. This ensures that pollen is effectively transferred between morphs, promoting cross-pollination.
Beyond the physical arrangement, many distylous plants also exhibit a “heteromorphic incompatibility” system, or physiological incompatibility. This means that even if pollen from a “pin” flower lands on the stigma of another “pin” flower, or “thrum” pollen lands on a “thrum” stigma, fertilization does not occur. This physiological barrier acts as a chemical safeguard, preventing self-fertilization and reinforcing the plant’s reliance on cross-pollination between different morphs.
Common Examples of Distylous Plants
Distyly is observed in approximately 15% of flowering plant species and has evolved independently in at least 28 angiosperm families. The classic example of a distylous plant is the primrose, belonging to the genus Primula. These plants exhibit “pin” and “thrum” flower forms.
Other common examples include flax (Linum usitatissimum), certain Lythrum species, and buckwheat. Jasmine (Jasminum species) and various species of Cryptantha display distyly. The genus Villarsia, predominantly found in Australia, includes ten distylous species out of fifteen, highlighting the widespread nature of this reproductive strategy.