In the interface of caesalpinioids and mimosoids: Comparative floral development elucidates shared characters in and (Leguminosae).

PREMISE OF THE STUDY

and (Leguminosae) have long been considered a possible enigmatic link between caesalpinioids and mimosoids because they both have an imbricate calyx and heteromorphic androecium, floral features that are rare among mimosoids but common among caesalpinioids. This study compared the developing flowers of and to determine whether the shared floral conditions also have the same ontogenetic origin.

METHODS

Buds of different sizes and flowers were processed for surface (scanning electron microscopy) and histological (light microscopy) examination.

KEY RESULTS

The floral meristem initiates five sepal primordia in a modified helical order in both species. The median sagittal sepal is adaxial. The overlap of the sepals during elongation culminates with the formation of the imbricate calyx. Heteromorphic androecia arise in the intermediate stages of development. In , the fertile pollen-bearing stamens are antesepalous, robust and short, and the anthers carry a robust apical gland; the staminodes are long and white with a vestigial apical gland. In contrast, in the fertile pollen-bearing stamens are antepetalous without glands and as long as the staminodes. The staminodes are thinner with an expanded apical region.

CONCLUSIONS

The imbricate calyx and the heteromorphic androecium in the studied species originated via distinct pathways, favoring the hypothesis of homoplasy of these conditions. The pathways observed in are more similar to those observed in caesalpinioids than to those observed in mimosoids, indicating that although the floral development differs between the species studied, it supports the basal placement of among mimosoids.