Nested tandem duplications of the gene Melanoma antigen recognized by T-cells (Mlana) underlie the sexual dimorphism locus in domestic pigeons

Birds have classic examples of exaggerated sexually dimorphic traits, including colors. Sexes among the wild rock dove and its derived domestic breeds, however, are quite indistinguishable and sex can only be ascertained through genotyping or egg laying and successful hatching of eggs. Yet, the pigeon fancy has discovered sexually dimorphic traits and harnessed some of these traits in some auto-sexing breeds. Early genetics pioneers characterized the sex-linked Stipper locus and showed it to be linked to the pigeon Z linked B-locus (Tyrp1). The alleles of the Stipper locus have variable dominance relative to wild-type (more severe alleles are incompletely dominant, less severe alleles are reportedly fully dominant), characterized by a continuum of lightening in homozygotes and increased variegation in heterozygotes corresponding with severity. We leveraged this positional information and population structure among breeds in a candidate gene approach to identify the genetic mechanism of de novo pigeon sex dichromatism. A large tandem duplication (77 kb) centered on the gene Melanoma antigen recognized by T cells (Mlana) is completely associated with alleles of the Stipper locus. Copy number of the 77 kb genetic lesion was not correlated with allele severity suggesting that other mechanisms, including epigenetic regulation could underlie both allele severity and degree of variegation.