Fig. 4. Isolation of a temperature sensitive fms allele. (A) fms^174A cDNA exhibits a tyr his substitution within the second immunoglobulin-like domain. Grey, untranslated regions. Green, signal sequence and transmembrane domain. Red, split kinase domains. (B) Primer extension analysis for genotyping fms^174A nucleotide substitution. (Upper trace) Wild-type (or fms¹, fms^blue) alleles result in the addition of 3 nucleotides (nt) to the extension primer. The peak at +0 nt represents excess extension primer without added nucleotides. (Lower trace) The fms^174A allele results in addition of 2 nt to the extension primer; shown is a chromatogram for a heterozygous fms^174A/fms⁺ individual. (C-F) Homozygous fms^174A individuals reared at 24°C (C,D) or 33°C (E,F). (C,D) At 24°C, hatchling larvae exhibit normal numbers of xanthophores (here evidenced by the yellow cast to the flank; C); adults exhibit melanophore stripes indistinguishable from wild-type (D). (E,F) At 33°C, hatchling larvae lack xanthophores (E) and adults both lack xanthophores and exhibit a severe disruption of melanophore stripes (F), resembling that seen in fms¹ or fms^blue (Fig. 1B). (G-I) Molecular marker analyses reveal that fms^174A conditionally affects the development of xanthophore precursors, as revealed by distributions of cells expressing the xanthophore lineage markers fms (G), gch (H), and xdh (I). (Upper panels) Presumptive xanthophore precursors are abundant over the myotomes of 60 h embryos at 24°C. (Lower panels) Presumptive xanthophore precursors are absent from over the myotomes of embryos reared at 33°C. Scale bars, (C,E) 600 µm, (D,F) 2 mm, (G-I) 40 µm.

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