Figure 5. A possible role for growth as a modifier of pigmentation pattern during evolution.

Change in pigmentation pattern dimensions can result from modifications in early pre-patterning mechanisms that divide the petal primordia in subregions and/or local variation in cell proliferation or expansion in developmental stages following the pre-patterning phase. (A) Schematic representation of a theoretical petal primordium at very early stages of development, prior to pigmentation pattern formation. In this early phase, pre-patterning mechanisms act to divide the petal surface in subregions that can later develop distinct cell fates. Here proximal and distal regions are specified: the white triangle indicates boundary position along a line of 8 cells going from the base to the tip of the petal. As development progresses, cell division increases total cell number and cells acquire distinct identities in each compartment: cells in the distal region synthesises pigments while cells in the proximal domain remain unpigmented, forming a visible bullseye pattern. (B) Larger bullseyes can be obtained by at least two modes: in (1), the early pre-patterning processes remain unchanged and the boundary between proximal and distal regions is specified as in (A) but in later stages, an increase in either cell division rate (top lane) or cell expansion (bottom lane) in the proximal region would modify the final proportions, yielding a larger unpigmented region. Alternatively, in (2), the early boundary is specified higher along the proximo-distal axis (e.g. increase in morphogen production) so that a larger proximal compartment is specified. (C) Similar scenarios can lead to a reduction in bullseye size: (3) cell division (top lane) or expansion (bottom lane) can be increased in the distal region post-patterning or (4) the early developmental boundary can be specified closer to the petal base during the pre-patterning phase.

White triangle: position of the early developmental boundary; purple triangle: position of the bullseye boundary in the mature petal; squares and rectangles: individual cells along the proximo-distal axis; numbers under each line of cells: cell number in each petal subregion.