Abstract
Despite massive research efforts, the molecular etiology of bovine polledness and the developmental pathways involved in horn ontogenesis are still poorly understood. In a recent article, we provided evidence for the existence of at least two different alleles at the Polled locus and identified candidate mutations for each of them. None of these mutations was located in known coding or regulatory regions, thus adding to the complexity of understanding the molecular basis of polledness. We confirm previous results here and exhaustively identify the causative mutation for the Celtic allele (PC) and four candidate mutations for the Friesian allele (PF). We describe a previously unreported eyelash-and-eyelid phenotype associated with regular polledness, and present unique histological and gene expression data on bovine horn bud differentiation in fetuses affected by three different horn defect syndromes, as well as in wild-type controls. We propose the ectopic expression of a lincRNA in PC/p horn buds as a probable cause of horn bud agenesis. In addition, we provide evidence for an involvement of OLIG2, FOXL2 and RXFP2 in horn bud differentiation, and draw a first link between bovine, ovine and caprine Polled loci. Our results represent a first and important step in understanding the genetic pathways and key process involved in horn bud differentiation in Bovidae.
Highlights
IntroductionMade of a pneumatized bony core fused with the skull frontal bone and covered by a continually growing keratin sheath, are a distinctive feature of the Bovidae family
What is more natural for cattle than to have horns? Horns, made of a pneumatized bony core fused with the skull frontal bone and covered by a continually growing keratin sheath, are a distinctive feature of the Bovidae family
While tens of thousands of mutations affecting horn development must have occurred in horned populations since that time, it is intriguing to note that only the Celtic and Friesian mutations have been transmitted through the generations until today
Summary
Made of a pneumatized bony core fused with the skull frontal bone and covered by a continually growing keratin sheath, are a distinctive feature of the Bovidae family. These appendages have become undesirable in the modern cattle industry. While the Polled mutation has been mapped on bovine chromosome 1 (BTA01) in more than ten breeds [11,12,13,14,15,16,17], fine characterization of this locus has proved more difficult than expected and, to date, neither the causal mutation(s) nor the molecular etiology of this phenotype have been definitively identified. Among the major issues encountered in this process are: (i) the lack of appropriate (i.e., horned) model species, which make it impossible to identify functional candidate genes from previous studies; (ii) the absence of candidate polymorphisms in the coding sequences of any of the positional candidate genes [18,19,20]; and (iii) the absence of differential expression of the same genes between horn buds from polled and horned newborn calves [21]
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