Mapping of the recessive white locus and analysis of the tyrosinase gene in chickens.

An F(2) chicken population of 265 individuals, obtained from an intercross between the Japanese Game (colored plumage) and the White Plymouth Rock (the recessive white) and genotyped for microsatellite markers, was used for determining the locus of the gene responsible for the recessive white plumage phenotype in chickens. Two hundred twenty-five markers were mapped in 28 linkage groups. Linkage analysis revealed that the recessive white gene was mapped to chromosome 1. Detailed analysis using additional markers uncovered a significant linkage between 2 new markers, mapped to the flanking region of the tyrosinase gene, which is associated with skin and plumage color. The sequence of the tyrosinase gene was investigated in recessive white chickens and colored chickens. There were no obvious differences in the tyrosinase gene exons between the recessive white chicken and the colored chicken. However, sequence analysis of tyrosinase intron 4 in the recessive white chicken revealed a presence of an insertion of an avian retroviral sequence. The White Plymouth Rock and the F(2) generation with white plumage were identified as homozygous carriers of the retroviral sequence. Expression of the normal transcript containing exon 5 was substantially decreased in the recessive white chicken compared with the colored chicken. Some abnormal tyrosinase gene transcripts were expressed in the skin of the White Plymouth Rock: reverse transcription PCR products amplified from exon 3 to intron 4 and from retroviral sequence 3' long terminal repeat to exon 5. Based on these results, it was confirmed that an avian retroviral sequence insertion in the tyrosinase gene was the cause of recessive white phenotype in chickens.