Figure 5. Models for pattern formation and evolution.

(a) Network of interactions amongst iridophores (i), xanthophores (x) and melanophores (m).Interactions are positive (→) or negative (⊣); long-range are indicated by dashed lines. Csf1 and TH both promote xanthophore development; Csf1 is also supplied by iridophores to xantho-phores (i→x)²². When xanthophores are highly abundant, these cells can repress iridophore development (1). Iridophores attract melanophores (i→m), but also repress melanophore survival and localization, and terminate stripe development (2). Simplified interaction diagram at lower right, corresponding to (b).

(b) When Csf1 is overexpressed In D. rerio, melanophores encounter more xanthophores and fewer iridophores, participate in only a subset of potential interactions, and fail to receive positional information necessary to terminate primary stripes or initiate secondary stripes. Interactions between xanthophores are hypothetical.

(c) Pattern development in D. rerio. (1) Iridophores differentiate at the horizontal myoseptum. (2) Melanophores arise dispersed over the flank while iridophores express Csf1, which is also expressed in the skin. (3) Locally high levels of Csf1 promote xanthophore differentiation in the interstripe as interactions involving all three classes of pigment cells refine the pattern of stripes and interstripes and additional iridophores infiltrate the stripe. (4) Iridophores begin to emerge beyond the stripe and initiate a secondary interstripe. (5) Iridophores of the secondary inter-stripe terminate the primary stripe, promote differentiation of secondary interstripe xanthopho-res, and specify the location of secondary stripe development. Adult xanthophore precursors, and, later, incompletely differentiated xanthophores are distributed sparsely in stripes as well^26,50 (not shown).

(d) Pattern development in D. albolineatus. (1) Initially high expression of iridophore-independent Csf1 promotes early, widespread xanthophore development. (2) A few iridophores develop near the horizontal myoseptum posteriorly. (3) Melanophores begin to appear scattered over the flank as do additional iridophores. (4–5) Residual stripes begin to form posteriorly around the residual interstripe³⁵ but this pattern is obscured by widespread melanophores and xanthophores that fail to resolve into stripes and interstripes.