Pigment pattern expression in the plumage of the quail embryo and the quail-chick chimaera

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Abstract
The pattern of pigmentation in birds is dependent on the migration and differentiation of a population of neural crest cells that develop into melanoblasts. On the basis of previous grafting experiments Rawles (1948) concluded that the pigment pattern of the chimaera is determined by the genotype of the donor melanocyte. This led Wolpert (1981) to suggest that melanoblasts from one bird can read the positional value of the ectoderm in the feather papillae of another bird. An alternative view is that an isomorphic prepattern in the feathers determines the pigment pattern. We have examined these ideas in relation to the local pigment patterns of the embryonic quail wing, distal to the elbow, where several rows of feather papillae are consistently unpigmented. Melanin pigment is first seen at stage 35. By stage 39 a characteristic pigment pattern has been established. Most of the dorsal feather papillae are heavily pigmented, whereas many ventral papillae are unpigmented. Of the ventral papillae three rows (E2, E3 and H2) are always unpigmented, and it is these three rows that form the basis of the quail local pattern. The DOPA reaction indicates that no melanoblasts are present in these white feathers, although they are present in all the feathers of the White Leghorn wing. When quail neural crest cells are grafted to the chick, either isotopically or to the wing bud, all or nearly all rows of ventral papillae become pigmented by stage 39. The only evidence of donor influences in the pattern is that, in some grafts, rows E2–3 have a high proportion of unpigmented papillae, and wings from earlier stages resemble the quail. When unpigmented papillae are present, histology shows that they contain undifferentiated crest cells. When introduced into a quail wing bud, chick crest cells enter all the feather papillae of the wing, including those in rows E2–3 and H2. We suggest that neither the positional information nor the prepattern theory alone can account for all of our findings. Contrary to previous claims, local cues may be important in determining crest-cell differentiation. We have established that crest cells migrate into all feather papillae of the quail-chick chimaera, including those that will remain unpigmented. We show that neither differential migration nor differential proliferation is involved in pattern formation in the quail-chick chimaera.

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