Fig. 9. Excess xanthophores in D. albolineatus compared with D. rerio. (A-H) Xanthophores in D. rerio (A,B,F), D. albolineatus (C,D,G), and wild-type D. rerio xD. albolineatus hybrids (E,H). (A) In D. rerio, few xanthophores (arrowhead) are visible during early stages of metamorphosis (stage equivalent to Fig. 6A) and most initially are found ventral to the horizontal myoseptum (box). Arrow indicates iridophores in the prospective primary interstripe region. (B) During middle stages of pigment pattern metamorphosis in D. rerio (e.g. Fig. 6C), xanthophores occur in the developing interstripe region (arrow) and a few faint xanthophores can be seen along the dorsal myotomes. (C) In D. albolineatus, xanthophores (arrowheads) are widely dispersed over the flank during early pigment pattern metamorphosis (e.g. Fig. 6G). (D) During later pigment pattern metamorphosis in D. albolineatus (e.g. Fig. 6I), xanthophores (arrowheads) persist widely scattered over the flank. Reddish erythrophores have started to develop in the interstripe region (arrow) and persist into the adult. Lineage relationships of erythrophores to xanthophores are unclear. (E) In hybrids between wild-type D. rerio and D. albolineatus, excess xanthophores (arrowheads) develop over the flank compared with D. rerio, and reddish erythrophores develop in the interstripe region (arrow). (F-H) Higher magnification images of larval D. rerio, D. albolineatus, and D. rerio xD. albolineatus hybrids shown in B-E. (I-L) In situ hybridization for early markers of the xanthophore lineage (Parichy et al., 2000b; Ziegler et al., 2000). Shown is GTP cyclohydrolase I (gch), which encodes an enzyme required for synthesizing pteridine pigments of xanthophores. Results for xanthine dehydrogenase (xdh), encoding a second pteridine synthesis enzyme were similar (data not shown). (I,J) D. rerio larvae; (K,L) D. albolineatus larvae. (I,K) In both species, gch⁺ cells occur over the myotomes during early metamorphosis. (J,L) At an optical plane medially within the same larvae shown in I and K, relatively few gch⁺ cells are observed in D. rerio (J), whereas many gch⁺ cells are present in D. albolineatus (L). (M,N) In situ hybridization for fms mRNA does not reveal clear differences in the numbers or distributions of fms⁺ cells between D. rerio (M) and D. albolineatus (N). Scale bars: in A, 100 µm for A,C; in B, 160 µm in B,D,E; in F, 60 µm for F-H; in I, 60 µm for I-L; in M, 80 µm for M,N.