Genetic factors of horn-related traits in small ruminants with special reference to the polled intersex syndrome (PIS) in the goat

Abstract

Bovidae belong to the family of ruminants which unite a range of even-toed ungulates like cattle, sheep, goat and antelopes. The group of pecorans is very heterogeneous in their phenotypic appearance, but have a common feature, the horns, which appear in pairs. The characteristics associated with the presence or absence of horns are as diverse as the group itself. In livestock, especially cattle and goats, polledness is often seen as a desirable trait, as it can reduce the risk of injury to humans and flockmates. In order to achieve this, young horned animals are or have been mechanically dehorned. In view of the ever-increasing controversy and tightening of the legal situation in this regard, for example in Germany, breeding and selection for genetically polled livestock offers a possible alternative. While knowledge about inheritance and underlying gene variants for horn-specific traits is extensive for cattle, there are still some gaps in knowledge for small ruminants (sheep and goats) - even for long-known phenomena. The polled intersex syndrome (PIS) in goats, which describes the fact that female, homozygous polled goats are infertile hermaphrodites, has been described since the 1940s. Due to the wide variability in phenotypic expression, affected animals are often difficult to identify. This restricts breeding for polledness in goats. Despite the published association with an 11.7-kb deletion on chromosome 1 that affects the transcription of the PISTR1 and the FOXL2 gene, it was not possible for a long time to develop a genetic test for the early detection of affected individuals that could be used in practice. The identification of a complex rearranging structural variant, consisting of the named deletion in combination with an inverse inserted duplication, associated with PIS was made possible through the use of long-read whole genome sequencing in the context of this work. This finding made it possible for the first time to develop an early applicable genetic test to identify all three possible genotypes, as well as the sex. Subsequent publications have confirmed this variant for all goat breeds examined worldwide. Also for the trait polledness in sheep a causal variant in form of an 1.78-kb sized insertion in the 3‘-UTR region of the RXPF2 gene on chromosome 10 has been published for some time. However, it has been shown several times that this does not segregate with the trait in all breeds. This applies in particular to breeds with variable or sex-specific horn status. This was mainly confirmed for the Icelandic sheep in this study. In sheep, the RXFP2 gene has been shown to be not only associated with the presence/absence of horns but also with other traits related to horns, such as size and shape. Horn size showed some association with a previously published RXFP2 variant in the Icelandic sheep as well. The inclusion of polycerate Icelandic sheep in the present work confirmed the segregation of a previously published 4-bp insertion in HOXD1 (ovine chromosome 2) with this trait, but also brought new findings. For the first time, this insertion was detected in polled individuals of polycerate origin. And a simultaneous observation of the HOXD1 and RXFP2 variants mentioned allows the assumption that polledness in Icelandic sheep of polycerate origin is not controlled by the RXFP2 insertion described. This work has helped to solve some unknowns, especially with regard to the PIS of the goat. However, insights were also gained that raise further questions, such as polledness in sheep of polycerate origin. These should be investigated more intensively in other breeds and larger samples in the future. The further development of molecular genetic methods can also help to clarify further horn-specific traits and verify such initial indications.