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

Abstract

Directional asymmetry is a systematic difference between the left and right sides for structures with bilateral symmetry or a systematic differentiation among repeated parts for complex symmetry. This study explores factors that produce directional asymmetry in the flower of Iris pumila, a structure with complex symmetry that makes it possible to investigate multiple such factors simultaneously. The shapes and sizes of three types of floral organs, the falls, standards, and style branches, were quantified using the methods of geometric morphometrics. For each flower, this study recorded the compass orientations of floral organs as well as their anatomical orientations relative to the two spathes subtending each flower. To characterize directional asymmetry at the whole‐flower level, differences in the average sizes and shapes according to compass orientation and relative orientation were computed, and the left–right asymmetry was also evaluated for each individual organ. No size or shape differences within flowers were found in relation to anatomical position; this may relate to the terminal position of flowers in Iris pumila, suggesting that there may be no adaxial–abaxial polarity, which is very prominent in many other taxa. There was clear directional asymmetry of shape in relation to compass orientation, presumably driven by a consistent environmental gradient such as solar irradiance. There was also clear directional asymmetry between left and right halves of every floral organ, most likely related to the arrangement of organs in the bud. These findings indicate that different factors are acting to produce directional asymmetry at different levels. In conventional analyses not recording flower orientations, these effects would be impossible to disentangle from each other and would probably be included as part of fluctuating asymmetry.

Phenotypic Selection on Flower Traits in Food-Deceptive Plant Iris pumila L.: The Role of Pollinators

“…The experiment was conducted on I. pumila clonal plants originating from Deliblato Sands and grown in clay pots for more than twenty years under common-garden conditions in an experimental garden of the Institute for Biological Research 'Siniša Stanković' in Belgrade, Serbia [29,35]. During the blooming phase in spring 2017, flowers were collected daily.…”

“…Another distinguishing feature of the genus Iris, including the food-deceptive species Iris pumila, is the complex actinomorphic flower [31,32]. The Iris floral ground plan (and more generally in Iridaceae) includes four trimerous whorls made up of functionally distinct floral organs [33][34][35]. The two outer whorls are formed of morphologically distinct petaloid tepals (fall and standard, respectively), followed by a whorl of three stamens and a tricarpellar gynoecium, with petaloid style branches in the center of the flower (Figure 1).…”

“…The proximal part of the fall with the overarching style branch and stamen form a pollination tunnel (Figure 1). Consequently, each I. pumila flower has three separate labiate pollination units (tunnels), which are perceived by pollinators as a single bilaterally symmetrical flower [34,35]. flowers [11,14,15].…”

Shape asymmetry — what's new?

“…2B), where the parts are attached to each other in a fixed arrangement (such as the left and right halves of our face, where the plane of symmetry runs through the middle of the face) [30,31]. In some instances, investigators have a choice between these types of symmetry [27]: in flowers, for instance, a study might investigate the object symmetry of the whole flower [32,33], or it might focus on the variation among the different petals separately and use the method for matching symmetry [24,34]. In addition, the parts of a structure with complex symmetry may each be symmetric in themselves, for instance if the parts of a complex structure each have bilateral object symmetry [24,27,[34][35][36].…”

“…In most such studies, compass orientation is not recorded and therefore this component of asymmetry is normally included in the estimate of fluctuating asymmetry, along with any asymmetry from phenotypic plasticity in response to smaller-scale heterogeneity (e.g., shading) and the asymmetry due to developmental instability. Because only this single species has been examined to date [24,34], it is impossible to gauge how widespread this phenomenon may be. Likewise, quantifying how much fluctuating asymmetry originates from phenotypic plasticity in response to environmental heterogeneity is very difficult.…”