Background: Understanding the evolution of novel features requires homology assessments at different levels of biological organization. In flowering plants, floral coronas that play various roles in plant-pollinator interactions have evolved multiple times independently, but are highly variable in their final position and overall morphology. Coronas of the Solanaceae species Jaltomata calliantha are found between the corolla and stamens, adjacent to the gynoecium, and form cups that house copious amounts of their characteristic blood red nectar. To test the hypothesis that J. calliantha coronas evolved as an outgrowth of stamens and therefore have staminal identity, we assessed their development, floral organ identity gene expression, and cellular morphology.

Results: Jaltomata calliantha coronas emerge after the initiation of all conventional floral organs on the abaxial side of the proximally modified stamens and then expand medially and laterally to form nectar cups. Overlapping expression of the B-class organ identity genes JcAPETALA3 and both JcPISTILLATA/GLOBOSA orthologs (JcGLO1 and JcGLO2), and the C-class-like gene JcAGAMOUS1-like, unites the stamens and corona. Epidermal cell shape also connects the adaxial surface of coronas and petals, and the stamen base, with remaining floral organs showing divergent cell types.

Conclusions: Our data, based on multiple lines of evidence, support a largely staminal origin for J. calliantha coronas. However, since slightly enlarged stamen bases are found in Jaltomata species that lack coronas, and J. calliantha stamen bases share cell types with petals, we hypothesize that stamen bases recruited part of the petal identity program prior to fully expanding into a corona.