ABSTRACT
Contort petal aestivation is common in several groups of eudicots. Two mirror-shaped types of contort aestivation can be recognized. In some eudicots (mostly asterids), handedness of the contort corolla is fixed at the level of species. In other species (mostly rosids), its handedness is not fixed, and both left- and right-contort corolla is then present in different flowers, often in the same inflorescence. Given so high phylogenetic signal in characters related to handedness of the contort corolla, it is important to understand mechanisms of its developmental regulation. In species with unfixed handedness of the contort corolla, the direction of contortion depends on the direction of the spiral of sepal initiation or aestivation within the whorl of calyx. These observations suggest that certain transference of positional information from calyx to corolla takes place in species with unfixed handedness of the contort corolla. The direction of the calyx spiral is likely determined by the arrangement of bracteoles. But how does the direction of the calyx spiral technically determine the handedness of corolla? Two possible mechanisms could be hypothesized: (1) The signal is transferred through mechanical forces in the developing flower. The petals appear simultaneously and the corolla is at first polysymmetric. During subsequent growth of petals, their shape is constrained by mechanical forces governed by the sepals so that a contortion of certain handedness appears; (2) The signal is transferred on an earlier stage, during prepatterning of petals on floral meristem, so that the sites of petal initiation are ab initio more or less asymmetric. In the present chapter, this problem is discussed based on observation of early development of flowers with unfixed handedness of the contort corolla in Melanophylla (Torricelliaceae).
