Floral Development of <i>Hillebrandia sandwicensis</i> (Begoniaceae) with Emphasis on the Ovary Structure and the Nature of the Petaloid Organs

Premise of research. Changes during floral development contribute to the diversity of flowers in angiosperms. Studying the causes for changes in floral organs helps in understanding the structure and evolution of flowers. The monotypic Hillebrandia sandwicensis has been recognized as different from Begonia mainly by its meristem, which is partly sunken and not a fully closed ovary, and by the presence of petaloid organs in both female and male flowers. The current study focuses on the process of floral development of H. sandwicensis and discusses the controversial characters from an ontogenetic perspective in order to understand the ovary position and the nature of the petaloid organs. Methodology. The process of floral development of male and female H. sandwicensis was primarily observed with scanning electron microscopy. Pivotal results. Sepals are initiated in a 2/5 pattern in both male and female flowers. Petaloid organs and stamens then emerge acropetally without interruption on the bulging floral apex in male flowers, while only petaloid organs emerge in female flowers. The ovary develops as five carpel primordia on the depressed floral apex, although the number can fluctuate because of pressure and lack of space. The ovary becomes semi-inferior by hypanthial growth. Conclusions. The ovary of H. sandwicensis is semi-inferior, with the placenta extending to the superior region. The homology of the petaloid organs is unclear, as evidence supports a staminodial origin of the petaloid organs in the male flower but a possibly different origin in the female flower. Further study on floral organ identity genes in Begoniaceae may help with understanding the origin and evolution of the petaloid organs.