ASIP Variants in Livestock: It's Not Black and White

Fur and feather pigmentation play an important role in the survival and reproduction of many species, as well as in selective breeding and agriculture. The synthesis of pigment molecules, eumelanin and pheomelanin, is partially regulated by the agouti signalling protein (ASIP) encoded by the ASIP gene. Although ASIP has been widely studied, variants and associated phenotypes are often studied within individual species. This review synthesises current knowledge across mammalian and avian livestock, classifying ASIP variants by type and genomic location and linking them to characteristic pigmentation outcomes. Regulatory and structural variants primarily affect spatial and temporal expression, producing region-specific pigmentation or whole-body pheomelanic phenotypes, whereas coding variants are consistently associated with eumelanic phenotypes through loss of ASIP function. Comparative analysis highlights conserved relationships between variant classes and phenotypic outcomes across livestock species and emphasises the central role of regulatory architecture, including promoter usage, copy-number variation and fusion transcripts. This demonstrates the value of cross-species approaches for interpreting ASIP functional variants and provides a framework for understanding pigmentation diversity in livestock.