Which pigment appears first in the corolla—patterned or background?

Abstract Flowers display a diversity of pigment patterns on petals – spots, stripes, blotches, and varying combinations of these. Such pigment patterns are accompanied and surrounded by a background that is a contrasting shade or colour sometimes white. We ask the question: Do the pattern and background colours appear simultaneously or successively, and if the latter, is there a bias in which one appears first? We studied the morphological development of flowers of 35 species containing both types of pigmentation, sampled from clades across angiosperms (monocots, Ranunculales, Caryophyllales, rosids and asterids) to address this question of timing of occurrence of the two types of pigmentation. In 28 of the species studied, pigment pattern started appearing in the corolla earlier than the background colour. Pigment pattern appeared later in four cases, and simultaneously with background colour in three cases. Thus, our results reveal, for the first time, variation in developmental sequence of pattern and background colour, with an apparent tendency toward earlier appearance of pigment pattern in the corolla. We hypothesize that the mechanisms involve the imperatives of pigment types, reaction kinetics, differential gene expression, and reaction-diffusion models.