When asymmetry mimics zygomorphy: flower development in Chamaelirium japonicum (Melanthiaceae, Liliales)

Pendent sessile flowers of Chamaelirium japonicum (Willd.) N. Tanaka appear zygomorpic, but they do not possess a bilateral symmetry. The flowers are subtended by a vestigial bract and lack a bracteole. The perianth consists of two small tepals on the abaxial side of the flower and four large tepals, none of which is median. Because the short tepals belong to different whorls, there is no symmetry plane. Despite the absence of a bracteole, the shape of the floral meristem before perianth inception resembles that of bracteolate monocot flowers. At early stages, all six tepals are equal in shape and size, and the flower is actinomorphic. The difference between the dorsal and ventral sides and the pendent nature of the flower become expressed during the gynoecium development. The absence of median organs allows to avoid collision of floral organs with the flower-subtending bract during flower curvature. Species of Chamaelirium reveal a set of different flower groundplans, which makes the genus a perfect model to investigate evolutionary changes in flower symmetry accompanied by differential tepal reduction.