Floral organogenesis and floral evolution of the Lecythidoideae (Lecythidaceae).

The subfamily Lecythidoideae of Lecythidaceae (Brazil nut family) is a dominant group in neotropical forests, especially those of Amazonia. New World members of the family have large showy flowers that are either polysymmetric or monosymmetric. In this study, floral organogenesis of all 10 neotropical genera was examined using SEM. Our observations of floral development are put into the context of a molecular phylogeny based on sequences of the ndhF and trnL-F genes (Am. J. Bot. 94: 289-301). Floral evolution of the subfamily is explained as having undergone four different levels of complexity in regard to floral symmetry. The basal most genera, Grias and Gustavia, have polysymmetric flowers. At level two, represented only by Couroupita, monosymmetry is established through the expression of abaxial dominance and the development of an androecial hood; at this level, abaxial dominance impacts the perianth and androecium, but not the gynoecium. At the third level, monosymmetry is developed in groups of Couratari and Cariniana domestica; but, in the Allantoma/Cariniana decandra lineage, a reversal back to polysymmetric flowers, resulting from a gradual weakening of abaxial dominance, and the loss of the hood has occurred. Finally, in level four, including Bertholletia, Corythophora, Eschweilera, and Lecythis, monosymmetry is so strongly expressed that the gynoecium is also influenced by abaxial dominance. In this group, the hood is complicated in both structure and function, and the floral axis is changed from straight to slightly inclined. This study demonstrates that the development of floral abaxial dominance is the proximate cause of monosymmetry in the Lecythidoideae. We suggest that monosymmetric flowers are more efficiently pollinated, and therefore the bees and bats that pollinate the monosymmetric flowers in this group are ultimately responsible for the monosymmetry.