Integrating developmental and macroevolutionary approaches reveals a staminodial origin of all alternisepalous organs in Caryophyllaceae

PREMISE: The identity of sterile floral organs in second-whorl, alternisepalous positions within Caryophyllaceae remains contentious, having been described as staminodes, "petaloids," or petals homologous with those of other Pentapetalae. Limited ingroup and outgroup sampling and floral developmental data and inadequate phylogenetic comparative methods in previous studies have added to this uncertainty. Crucially, staminodes have not been analyzed as discrete characters, leaving their homology with other structures unresolved. METHODS: We reconstructed a chronogram of 775 species using five fossil calibrations for Caryophyllaceae and outgroups. Floral traits of 613 species, including number and position of petal-like structures, staminodes, and stamens, were quantified. Stochastic character mapping estimated ancestral character states and transition rates. Literature reviews provided developmental data to characterize common floral forms; transition models were used to assessed patterns of floral development and evolution. RESULTS: Staminodes evolved at the root of an apetalous Caryophyllaceae and were maintained throughout the backbone, being lost multiple times and becoming petal-like, especially in the Plurcaryophyllaceae in which a second whorl of stamens evolved in the alternisepalous position through the splitting of the alternisepalous primordia. The Caryophyllaceae crown ancestor likely had a single whorl of antesepalous stamens alternating with alternisepalous staminodes. CONCLUSIONS: Caryophyllaceae flowers are apetalous: All alternisepalous sterile structures are staminodes. Since their origin, staminodes have subsequently undergone multiple instances of loss, reduction, and elaboration. Their evolutionary history parallels patterns in early angiosperm lineages, making Caryophyllaceae a valuable system for investigating the role of staminodes in floral function and diversification.