Fluctuating asymmetry of floral organ traits in natural populations of Iris pumila from contrasting light habitats
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Fluctuating asymmetry (FA), the subtle, random difference between the left and right side of bilateral structures, has often been used as a tool for evaluating developmental instability (DI) in natural populations subjected to environmental stresses. A general assumption underlying these studies is that the level of DI should increase with stress intensity. We examined the level of floral FA in six natural populations of Iris pumila experiencing sun-exposed (more stressful) and shaded (less stressful) environmental conditions. We used two single-trait indices (size-dependent FA1 and size-scaled FA8a) and one multi-trait index (FA17) to assess the FA levels in three floral traits: fall width, standard width and style branch width. Although floral FA was present in all FAs and appeared to be greater in plants from full sunlight than in those beneath vegetation canopy, only the FA17 index detected a significant FA-stress association. Although the FA1 index had no statistical power to reveal differences in the floral FA at any of the hierarchical levels studied, between alternative light habitats, among populations from comparable environmental conditions, among individual clones within each population or between different floral organs of a single flower, the FA8a index detected significant between-trait variation in the degree of floral FA within the same Iris individuals.