Berberidopsis beckleri, floral development and evolution of the core eudicots

The floral development of Berberidopsis beckleri is compared with its sister species B. corallina to clarify the origins of the pentamerous pentacyclic flower of the core eudicots. Both species are unusual amongst core eudicots in having flowers arranged on short axillary shoots with spiral phyllotaxis and an undifferentiated perianth.

Late floral and gynoecial differentiation of Berberidopsis beckleri. — Late floral and gynoecial differentiation of *Berberidopsis beckleri*. (A) Longitudinal section of young flower. Note the progressive change in size of bracts and tepals. (B) Partially sectioned flower showing the massive protruding intercarpellary ridges in the ovary. (C) Flower with partly dissected ovary showing early ovule differentiation on the intercarpellary protrusions. (D) Transverse section of young ovary showing five parietal placental ridges with two series of ovules each. (E) Longitudinal section of ovary showing three of the five placentas with ovules. (F) Transverse section of nearly mature ovary with four placental ridges and anatropous ovules. (G) Adaxial view of the upper part of immature anther with long apical connective. (H) View of five-lobed stigma at anthesis completely covered with exudate. (I) Lateral view of lobed, extrastaminal disc nectary; stamens removed. Ri, intercarpellary protuberance; Pl, placenta. Scale bars = 100 µm, except (G–I) = 200 µm.

Ronse de Craene suggests that floral evolution in core eudicots proceeded with a progressive reduction of perianth parts linked with a transition to a whorled phyllotaxis. B. beckleri and B. corallina represent prototypes of a gradient shift towards more regular core eudicot flowers with a combination of ancestral and highly specialised features.