Directional asymmetry and direction-giving factors: Lessons from flowers with complex symmetry

Area of Science:

• Developmental Biology
• Plant Morphology
• Evolutionary Biology

Background:

• Directional asymmetry (DA) describes systematic differences between left and right sides or repeated parts.
• Investigating DA in complex structures like flowers can reveal multiple influencing factors simultaneously.
• Iris pumila flowers offer a model system for studying DA due to their complex symmetry.

Purpose of the Study:

• To explore the factors contributing to directional asymmetry in the flower of Iris pumila.
• To differentiate between environmental and developmental influences on floral asymmetry.

Main Methods:

• Geometric morphometrics were used to quantify the shapes and sizes of floral organs (falls, standards, style branches).
• Compass and anatomical orientations of floral organs were recorded for each flower.
• Analysis involved computing size and shape differences based on orientation and evaluating left-right asymmetry per organ.

Main Results:

• No significant size or shape differences were found in relation to anatomical position, possibly due to the flower's terminal position.
• Clear directional asymmetry of shape was observed concerning compass orientation, likely driven by environmental gradients (e.g., solar irradiance).
• Significant directional asymmetry was detected between the left and right halves of floral organs, attributed to bud arrangement.

Conclusions:

• Different factors operate at different levels to generate directional asymmetry in Iris pumila flowers.
• Environmental factors influence whole-flower shape asymmetry, while developmental factors (bud arrangement) affect organ-level asymmetry.
• Failure to account for orientation in analyses can lead to misclassification of DA as fluctuating asymmetry.