FLOWER DEVELOPMENT OF TRISTERIX CONFIRMS IRREGULAR CALYX FORMATION AND OBHAPLOSTEMONY AS PLESIOMORPHIES IN NEW WORLD LORANTHACEAE (SANTALALES)

摘要

Premise of research. The New World Loranthaceae correspond to a polyphyletic assembly of hemiparasites that exhibit a high floral diversity in terms of size, merism, groundplan, symmetry, and pollination strategies. Homology assessment as well as evolution of floral traits and ontogenetic pathways related to calyx, corolla, and androecium in the family have been variously interpreted. The calyx has been considered absent and replaced by a calyculus of bracteolar origin, or reduced and vestigial, or fully developed and fused to the ovary. Also, the corolla and the androecium have been described either as biseriate or uniseriate. Methodology. Here we use SEM to study floral development in Tristerix, a large-flowered genus that precedes the diversification of most New World genera. Pivotal results. We found that flowers are sitting on a cupular pedicel subtended by a partially recaulescent bract, clearly distinct from the calyx. Calyx development is irregular, yet merism is conserved, as five sepals alternating to the petals are evident; five petals arise in a single whorl, as do the five epipetalous stamens. Uneven petal elongation causes sigmoidal curvature in Tristerix secundus. Conclusions. Floral groundplan fits Hofmeister's rule between sepals and petals on one side, and stamens and carpels on the other, but such rule is disrupted between petals and stamens. Irregular calyx development, uniseriate corollas and androecia, epipetalous stamens, and actinomorphic corollas appear to be plesiomorphic in Loranthaceae.