['Petal spurs-tubular outgrowths that collect nectar-are considered key innovations because of their ability to change pollinator specificity and so cause reproductive isolation and speciation. Spurs have arisen frequently and rapidly in many taxa. To test their potential origins, we isolated spontaneous dominant mutations at two loci, HIRZ and INA, that cause novel outgrowths from Antirrhinum petals, resembling the petal spurs of closely related genera.', 'HIRZ and INA were isolated and shown to encode similar KNOX homeodomain proteins that are normally expressed only in apical meristems and are likely to act redundantly. Both dominant mutations were caused by transposon insertions in noncoding regions that caused ectopic expression of functional transcripts, either in petals or in all lateral organs with more pleiotropic effects. Formation of a spur-like outgrowth, which resembled an ectopic petal tube, was dependent both on KNOX gene expression and dorsiventral asymmetry of the flower.'] Do spontaneous mutations in KNOX genes give rise to a novel floral structure in Antirrhinum?