Indel polymorphism in 3'-UTR of RXFP2 does not segregate with horns status in sheep breeds with a variable and/or sex-limited horns status

The inheritance mode of the horns status in sheep is more complicated than it may appear at first glance. It is influenced by sex, but also differs between breeds. Previous studies postulated that a 1.8-kb insertion within the 3'-UTR of the relaxin/insulin-like family peptide receptor 2 gene (RXFP2) on sheep chromosome 10 is causing polledness in sheep. We re-sequenced a region of about 250,000 bp covering the RFXP2 gene locus and the flanking upstream region in a total of 24 sheep from 6 completely horned and 6 completely polled breeds. We identified the same indel polymorphism published before segregating perfectly with horns status in these breeds. After establishment of a multiplex PCR for efficient genotyping, we tested for the indel polymorphism in a total of 435 sheep with breed-specific or individually known horns status from 1) 17 completely polled breeds (208 sheep), 2) 8 completely horned breeds (84 sheep), 3) single or multiple crossings of completely polled with completely horned breeds (18 sheep), and 4) 9 breeds with sex-limited and/or variable horns status (125 sheep). The RXFP23'-UTR indel polymorphism segregated perfectly in completely polled and completely horned breeds. A sex-dependent horns status of sheep heterozygous for the indel polymorphism was observed for completely polled breeds and for their crossings with completely horned breeds: Male sheep carrying the genotype del/ins were horned, whereas female sheep with this genotype were polled. However, one heterozygous multiple crossed ewe with horn rudiments disputes complete association. Unexpectedly, this segregation pattern was not or at least not completely reproducible in breeds with sex-limited and/or variable horns status. For the breeds Alpine Steinschaf and Bavarian Forest, it can be summarized that the occurrence of the del/ins genotype on the one hand, and a scurs/small horns phenotype on the other hand showed no regular pattern. Moreover, our samples originating from even more southern European regions (Bovec sheep, Cres sheep, Travnicka Pramenka and Walachenschaf) as well as from Africa (Dorper and Kamerun sheep) clearly disclaim causal relationship or segregation of the RXFP23'-UTR indel polymorphism with horns status. In this group of sheep, we observed almost all combinations of indel genotypes, sex and phenotypes of horns status. Therefore we conclude that the 3'-UTR RXFP2 indel polymorphism is not a useful marker for horns status in sheep breeds with sex-limited an