Evolution of the Inflated Calyx Syndrome in Solanaceae

Species that express the inflated calyx syndrome (ICS) are found in several genera of the Solanaceae. The MADS-box protein MPF2, together with the plant hormones cytokinin and gibberellin, has been shown to be responsible for this trait in Physalis floridana. We have used sequence data from 114 species belonging to 35 genera to construct a molecular phylogeny of Solanaceae. Apart from the 2 Witheringia species analyzed, species within a given genus cluster together on the resulting cladogram. Witheringia solanacea is embedded within the Physalinae, but Witheringia coccoloboides is placed basal to the Iochrominae. The ICS trait seems to be of multiple origins both within the Solanaceae and the Physaleae. Surprisingly, expression of MPF2-like genes in floral organs appears to be plesiomorphic in both the Physaleae and the Capsiceae. Some species in these tribes that show neither ICS nor calyx accrescence fail to express the MPF2-like gene in floral organs. Among those that do express this gene in the calyx are the species Capsicum baccatum, Lycianthes biflora, Tubocapsicum anomalum, W. solanacea, and Vassobia breviflora, all of which form small calyces that do not respond to externally applied hormones. The plesiomorphic nature of MPF2-like gene expression in the calyx of the Physaleae and Capsiceae raises the possibility that originally ICS also was actually a plesiomorphic character in these 2 groups. However, this trait might have undergone changes in a number of species due to secondary loss of components in ICS formation, like hormone response of calyx development. These findings are discussed in an evolutionary context of a molecular pathway leading to ICS.