How to lose a whorl: the evolutionary and developmental biology of apetaly

The general plan of the core eudicot flower involves two perianth whorls, a calyx, and a corolla. However, numerous eudicots, in multiple lineages, have only one perianth whorl, generally assumed to be the calyx, resulting in apetaly. Historically, these plants were placed in the polyphyletic taxonomic group 'Monochlamydeae' and, unsure about the nature of the single perianth whorl, the 19th century botanist de Candolle coined the word 'tepal' to indicate this uncertainty. This review surveys the molecular mechanisms of apetaly, its evolutionary drivers, and its possible role as a gateway to the unisexual flower through the B-switch hypothesis. The pollination biology of the petal (and petaloidy) is considered. Many apetalous plants have evolved petaloidy of the remaining (calyx) whorl. It is suggested that more attention be paid to the molecular dissection of the three-whorl floral ground plan. Fortunately, excellent genomic resources to aid further research are now available for species with a unipartite perianth, such as Beta, Cannabis, Fagopyrum, Portulaca, Spinacia, and Urtica.