Spontaneous mutations in KNOX genes give rise to a novel floral structure in Antirrhinum.

Background: 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.