The horned condition in cattle is believed to be the wild type with morphogenesis primarily occurring after birth. The polled condition has existed since domestication and has been selected for its economic importance. The polled locus has previously been mapped by genetic linkage analysis to the proximal region of bovine chromosome 1. In order to help us eventually identify the causative mutation, the objective of the study was to structurally and functionally characterize the polled interval from IFNAR1 to SOD1 on BTA1. Our hypothesis was that the polled locus is a tissue specific transcription factor that is expressed in the developing horn buds and acts directly or indirectly upon SOX9. A 2.5 Mb BAC contig and STS content map of the polled interval was constructed. Three candidate genes encoding transcription factors were identified within this region but only C21orf66 was expressed in the horn buds from 1 d old Bos indicus influenced calves. The C21orf66 gene has 18 exons, spans 30,976 bp of genomic DNA, and 144 SNP were identified. No single SNP discovered in C21orf66 can be attributed as the causative mutation. None of the genes from the polled interval were differentially expressed in skin and horn from 1 d old Bos indicus influenced calves. However, there were significant differences in the levels of expression of RUNX2, SOX9, BMP4, PRKCA, and FOXL2 in these samples. Expression of RUNX2 was localized to the osteoblasts, and both RUNX2 and SOX9 were expressed in sebaceous glands of the horn at 1 d of age. Histological examination of horns and scurs from newborn, 5 to 6 mo, and ~1.5 yr old Bos indicus influenced cattle suggest that horns form through intramembranous ossification. Based on the data presented herein, we propose that the polled locus is upstream of RUNX2 and SOX9 in the osteogenic pathway, and could have its primary effect on the differentiation of mesenchymal condensations. The genes IL10RB, SFRS15, C21orf66, OLIG1, OLIG2 and HUNK remain candidates for the polled locus and warrant further investigation.

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