Evolution and development of staminodes in

Premise of research. Staminodes are infertile stamens that have evolved numerous times in angiosperms and exhibit a vast array of forms and functions. Variation in staminodes suggests that numerous evolutionary processes underlie their origins, but to understand how and why they evolved, comparative studies of groups of closely related species are needed. Identifying structures as staminodes, however, 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.Methodology. The evolution and development of staminodes were compared across species of Paronychia. We tested the hypotheses that structures were either petals or staminodes by evaluating the floral development of 14 species with scanning electron and light microscopy and conducted ancestral state estimations across phylogenies to infer when staminodes evolved.Pivotal results. Staminodes developed between the fertile androecial whorl and carpel, indicating an androecial origin. In eight species, staminodes developed similar to filaments of a fertile stamen in shape, length, and time, suggesting homology to them. In four species, staminodes are highly reduced through the heterochronic termination of development following initiation. In two species, broader staminodes, compared with filaments in the same flower, were interpreted as functional co-options. A vascular strand was not observed in any species examined, indicating that staminodes are vestigial before co-option. Staminodes evolved before the origin of Paronychia and were lost at least three times.Conclusions. Staminodes in Paronychia began as vestigial stamens following the loss of anthers and were highly reduced, remained vestigial, or were co-opted; we call this 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.