Staminodes are infertile stamens that have evolved numerous times in flowering plants and exhibit a vast array of forms and functions. Variation in staminodes suggests that numerous evolutionary processes underlie their origins, but to understand their how and why they evolved, comparative studies are needed in groups of closely related species. Identifying structures as staminodes is not always straightforward and sometimes requires corroborating phylogenetic and developmental evidence. Staminodial structures in Paronychia (Caryophyllaceae), for example, vary in shape and size and have been referred to as both petals and staminodes, rendering their homology uncertain. The development of staminodes was compared across species of Paronychia. We tested the hypotheses that structures were either petals or staminodes and by evaluating floral development of fourteen species with scanning electron and light microscopy and conducted ancestral state estimations across phylogenies to infer when staminodes evolved. Staminodes developed between the stamen whorl and carpel, indicating a true staminodial origin. Staminodes evolved prior to the origin of Paronychia and were lost at least three times. Staminodes in Paronychia began as vestigial stamens following the loss of anthers and were either highly reduced, remained vestigial, or coopted, which we term the vestigial intermediate hypothesis. Our results illustrate a dynamic history of staminodial evolution in Paronychia and that selection on the function of staminodes can differ across closely related species
