Fig. 4. Segregation analysis supports an essential role for fms in D. albolineatus xD. rerio hybrid stripe development. (A,B,D,E) Tester fms^j4e1/+ hybrids exhibit either of two easily distinguishable phenotypes, one in which stripes are well organized (A,B), and another in which stripes are weakly organized with irregular borders and fewer xanthophores (D,E). Differences are evident during pigment pattern metamorphosis (A,D) but are most apparent in adults (B,E). Details showing well-organized (C) and weakly organized (F) stripes in hybrid adults. Pigment cell complements in hybrids with weak stripes were significantly reduced compared with hybrids with strong stripes, with 84% of the number of melanophores (F_1,16=8.76, P<0.01) and only 14% of the number of xanthophores (F_1,16=20.85, P<0.0005). The reduction in melanophore numbers is comparable to that observed in homozygous fms^j4e1 mutant D. rerio and fms¹⁷⁴ mutant D. rerio at restrictive temperature; however, neither D. rerio mutant allele retains xanthophores (Parichy et al., 2000b; Parichy and Turner, 2003a). (G,H). Primer extension genotyping for fms alleles. (G) In D. rerio, an extension primer adjacent to the fms^j4e1 lesion yields additions of two nucleotides in homozygous wild-type individuals, additions of four nucleotides in homozygous fms^j4e1 individuals, and additions of both two and four nucleotides in heterozygous fms⁺/fms^j4e1 individuals. (H) Primer extension genotyping for tester fms^j4e1/+ hybrids that have been classified as having `strong' or `weak' stripes. All hybrids with strong stripes carry the D. rerio fms^+(wik) wild-type allele and the D. albolineatus fms allele, resulting in two nucleotide additions to the extension primers. All hybrids with weak stripes carry the D. rerio fms^j4e1 mutant allele and a D. albolineatus fms allele, resulting in the addition of four and two nucleotides to the extension primer, respectively. Scale bars: in A, 200 µm for A,D; in B, 500 µm for B,E; in C, 200 µm for C,F.