The Euphorbiaceae are notable for floral diversity and evolutionary complexity. Croton is the second largest genus in the family and exhibits particular diversity in its flowers. The aim of this study was to investigate the floral ontogeny and structure of three Croton species with distinct morphologies, with a focus on testing the hypothesis that the filaments of female flowers, which have received different interpretations in the literature and are currently described as reduced petals, are staminodes and part of a vestigial androecium. With the ontogenetic study we can understand the origin of the organs and associate these with flower evolution in the genus. Flowers in several stages of development were analysed using light microscopy and scanning electron microscopy. In the early stage of development, the sepals are the first structures to be formed, although they do not continue to grow in female Croton fuscescens Spreng. flowers. Petals are absent in female flowers, with filamentous, petaloid structures, interpreted here as staminodes, alternating with the sepals in Croton lundianus (Didr.) Müll. Arg. In Croton sphaerogynus Baill., the staminodes are located between the nectary lobes. The stamens exhibit centripetal development in the flower bud stage, and the carpels are post-genitally connate, with differences in style branching. Besides the ontogenetic interpretation for the filamentous structures, the genus shows transitional structures that we consider evolutionary reductions. Our results can explain how developmental alterations have influenced the suppression and modification of floral organs in the genus.
Genetics of flower development in Ranunculales - a new, basal eudicot model order for studying flower evolution