Development of a complex floral trait: The pollinator-attracting petal spots of the beetle daisy, Gorteria diffusa (Asteraceae).

Angiosperms possess a variety of complex floral traits that attract animal pollinators. Dark petal spots have evolved independently many times across the angiosperm phylogeny and have been shown to attract insect pollinators from several lineages. Here we present new data on the ontogeny and morphological complexity of the elaborate insect-mimicking petal spots of the South African daisy species, Gorteria diffusa (Asteraceae), commonly known as the beetle daisy, although it is fly-pollinated. Using light and scanning electron microscopy and histology, we identified three distinct specialized cell types of the petal epidermis that compose the petal spot. Sophisticated patterning of pigments, cuticular elaborations, and multicellular papillate trichomes make the G. diffusa petal spot a uniquely complex three-dimensional floral ornament. Examination of young inflorescence meristems revealed that G. diffusa ray florets develop (and probably also initiate) basipetally, in the opposite direction to the disc florets-a developmental phenomenon that has been found in some other daisies, but which contradicts conventional theories of daisy inflorescence architecture. Using these ontogenetic and morphological data, we have identified the mechanism by which G. diffusa patterns its insect-mimicking petal spots, and we propose a testable model for the genetic regulation of petal spot identity.