Evidence for eumelanin and pheomelanin producing genotypes in the Arabian horse

The ultrastructural imaging of melanocytes coupled with analyses to detect sulfur-containing melanosomes by energy-dispersive X-ray spectroscopy were used to test the hypothesis that the yellowish-red and black pigments found in Arabian horses result from pheomelanogenesis and eumelanogenesis, respectively. These procedures detected pheomelanosomes in follicles at the base of hairs in chestnut horses and eumelanosomes in follicles at the base of hairs in black horses. By analyzing tissue obtained by skin biopsy, these procedures also demonstrated that skin melanocytes in a chestnut horse produce eumelanosomes, and follicular melanocytes in the same horse produce pheomelanosomes. It was also shown that the type of follicular melanosome present in light bay horses is correlated with the color of the hair. The results of this study give experimental evidence for the Odriozola-Adalsteinsson hypothesis that the e allele is responsible for the chestnut phenotype; they also give fine structure and chemical confirmation of the action of the A and E loci in the Arabian horse as currently proposed for the mouse and other mammals.