Abstract
The molecular regulation of horn growth in ruminants is still poorly understood. To investigate this process, we collected 1019 hornless (polled) animals from different cattle breeds. High-density SNP genotyping confirmed the presence of two different polled associated haplotypes in Simmental and Holstein cattle co-localized on BTA 1. We refined the critical region of the Simmental polled mutation to 212 kb and identified an overlapping region of 932 kb containing the Holstein polled mutation. Subsequently, whole genome sequencing of polled Simmental and Holstein cows was used to determine polled associated genomic variants. By genotyping larger cohorts of animals with known horn status we found a single perfectly associated insertion/deletion variant in Simmental and other beef cattle confirming the recently published possible Celtic polled mutation. We identified a total of 182 sequence variants as candidate mutations for polledness in Holstein cattle, including an 80 kb genomic duplication and three SNPs reported before. For the first time we showed that hornless cattle with scurs are obligate heterozygous for one of the polled mutations. This is in contrast to published complex inheritance models for the bovine scurs phenotype. Studying differential expression of the annotated genes and loci within the mapped region on BTA 1 revealed a locus (LOC100848215), known in cow and buffalo only, which is higher expressed in fetal tissue of wildtype horn buds compared to tissue of polled fetuses. This implicates that the presence of this long noncoding RNA is a prerequisite for horn bud formation. In addition, both transcripts associated with polledness in goat and sheep (FOXL2 and RXFP2), show an overexpression in horn buds confirming their importance during horn development in cattle.
Highlights
Permanent horns (Figure 1A) are a typical feature of domesticated ruminants like cattle, sheep and goats
The analyses revealed two different polled associated haplotypes overlapping within the same chromosomal region on BTA 1
For subsequent genotyping of more than 400 horned and more than polled Holstein cattle (Table S2) we focused on the 43 variants located within the critical region of 441 kb initially mapped in Simmental cattle, as we assumed that the second polled mutation in Holstein should affect the same chromosome segment as the Simmental mutation and as it is the segment within which the (Holstein) polled mutation had been mapped in a previous publication [3]
Summary
Permanent horns (Figure 1A) are a typical feature of domesticated ruminants like cattle, sheep and goats. Besides the absence of horn growth (polledness, Figure 1C) these animals may show atypical eyelashes and defects of the genital tract in males [3]. Since there is no need for self-defense in modern intensive cattle production, and new housing systems as free stalls with headlock barriers have been established, horns have become an undesired trait in beef and dairy cattle. Due to animal welfare [4] and economic reasons, efforts are taken to breed hornless (polled) cattle. There are several cattle breeds such as Angus and Galloway, which are fixed for the autosomal dominant polled allele [5,6,7]. In other breeds natural polledness has until now just been accomplished in a small part of the populations by cross breeding or selection of polled individuals
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