Frequency of the beta-carotene oxygenase 2 (BCO2) allele associated with the yellow fat phenotype in rabbits: insights into the spread of a genetic alteration in a wide variety of breeds and populations

• Carotenoids are essential dietary compounds that can accumulate in fat if not metabolized completely. • A deletion in the beta-carotene oxygenase 2 ( BCO2 ) gene (AAT at codon 248) is associated with the yellow-fat phenotype in rabbits. • This mutation was genotyped in 1041 rabbits from 41 different breeds and populations. • The BCO2 mutation segregates in various breeds and populations, typically with low frequency except for the Lynx breed where it could be associated with its unique coat colour. • Comparing the results obtained with those from a study published more than 80 years ago, the frequency of the mutated alleles has declined. In various animal species, the accumulation of carotenoids that are not completely metabolized leads to yellow fat pigmentation. In rabbits, the yellow-fat phenotype was described by classic genetic studies at the beginning of the last century to be determined by a simple Mendelian recessive allele at the Yellow locus, in linkage with the Albino coat colour locus. More recent studies reported that a deletion of three nucleotides at codon 248 of the beta-carotene oxygenase 2 ( BCO2 ) gene (delAAT) is the causative mutation for the yellow-fat defect when in a homozygous state. In this study, we genotyped the BCO2 polymorphic site in 1041 rabbits from 41 breeds and populations, including albino rabbit breeds. Considering the overall genotyped population, the frequency of the delAAT allele was 11.96%. Homozygous delAAT/delAAT rabbits had a frequency of 3.55%, while the heterozygous rabbits had a frequency of 16.81%. In 12 out of 41 breeds/populations, all rabbits analysed were homozygous for the wild-type allele, while in 29 the mutated allele segregates. Only one breed, Lynx (with a pale silver-blue coat colour and yellow-red shades), was fixed for the delAAT allele, suggesting its potential functional role in determining the yellow-red secondary tint that characterises this breed. Comparing the molecular genetic results with the genetic information reported in earlier literature, the frequency of the mutated allele decreased over the last 80 years, probably due to culling of the carriers for the negative effect on carcass quality, due to the preference of consumers for white fat in rabbit meat.