Flower structure and development in Melanophylla (Torricelliaceae: Apiales): Lability in direction of corolla contortion and orientation of pseudomonomerous gynoecium in a campanulid eudicot

Transitions in corolla symmetry are an important aspect of angiosperm floral evolution. Contort petal aestivation is common in several groups of eudicots. In rosids, the direction of overlap between adjacent petals (handedness) of the contort corolla is often labile among flowers in a single inflorescence, but in asterids, handedness is usually stable at a supraspecific level. Taxa with contort corolla are unevenly distributed among asterids, and detailed developmental data are often lacking. We provide the first developmental study of flowers in Melanophylla (Torricelliaceae, Apiales), the only known campanulid with a contort corolla, and demonstrate that the corolla handedness is labile within a single inflorescence. Labile handedness distinguishes Melanophylla from members of lamiids with a contort corolla where handedness is stable. In Melanophylla, the handedness is determined by the arrangement of bracteoles. Petals are asymmetric from early developmental stages. The androecium is also usually contort, with handedness always opposite to that of the corolla. Anthers have broad, flat connectives, which is unusual in asterids. The gynoecium is pseudomonomerous, with the fertile carpel in a left or right-transversal position, depending on the handedness of the corolla and androecium. Symmetry patterns of all floral whorls, including the pseudomonomerous gynoecium, are strongly correlated in Melanophylla, in contrast with the unstable carpel orientation in monomerous gynoecia of Apiales studied so far. The tricarpellate pseudomonomerous gynoecia of Melanophylla and other early divergent Apiales resemble those of Dipsacales.