QTL and association analysis for skin and fibre pigmentation in sheep provides evidence of a major causative mutation and epistatic effects.

The pursuits of white features and white fleeces free of pigmented fibre have been important selection objectives for many sheep breeds. The cause and inheritance of non-white colour patterns in sheep has been studied since the early 19th century. Discovery of genetic causes, especially those which predispose pigmentation in white sheep, may lead to more accurate selection tools for improved apparel wool. This article describes an extended QTL study for 13 skin and fibre pigmentation traits in sheep. A total of 19 highly significant, 10 significant and seven suggestive QTL were identified in a QTL mapping experiment using an Awassi × Merino × Merino backcross sheep population. All QTL on chromosome 2 exceeded a LOD score of greater than 4 (range 4.4-30.1), giving very strong support for a major gene for pigmentation on this chromosome. Evidence of epistatic interactions was found for QTL for four traits on chromosomes 2 and 19. The ovine TYRP1 gene on OAR 2 was sequenced as a strong positional candidate gene. A highly significant association (P < 0.01) of grandparental haplotypes across nine segregating SNP/microsatellite markers including one non-synonymous SNP with pigmentation traits could be shown. Up to 47% of the observed variation in pigmentation was accounted for by models using TYRP1 haplotypes and 83% for models with interactions between two QTL probabilities, offering scope for marker-assisted selection for these traits.

Albinism in a Suffolk sheep.

This report introduces the second form of true albinism to be documented in sheep, which appears mild enough not to cause serious undesirable side effects yet apparently effective enough to have the potential for general usage in the sheep industry. Based on the matings conducted to date, the albinism is inherited like an autosomal recessive. Histochemical tests reveal a defective melanin synthesis involving a block to the conversion of tyrosine to dopa but not the subsequent reactions that lead to melanin. The enzyme tyrosinase is a product of the C locus and catalyzes the conversion of tyrosine to dopa and the following reaction (dopa to dopaquinone). Therefore, it is proposed that the albinism arises from a gene in the C locus that encodes a defective tyrosinase. The gene is provisionally named albino marrabel, the gene symbol is cmar, and the locus allele symbol is Ccmar.

Pigmented spots in the wool-bearing skin on white merino sheep induced by ultraviolet light.

Black-grey pigmented skin spots, some of which contained pigmented wool fibres, were observed in a flock of 8.5-year-old white Merino ewes. The spots were concentrated along the backline and increased in number following shearing, suggesting exposure to sunlight to be of importance in the development of these non-congenital pigmented skin spots in genetically white Merino sheep. To test the effect of ultraviolet light, white Merino sheep, ranging in age from 3 to 8 years, had a closely clipped midside area of wool-bearing skin irradiated on each of 28 consecutive days. Pigmented skin spots developed in 6 of the 16 white Merino sheep irradiated. Spots first appeared after 10 days of irradiation, the number subsequently increasing with time, and two skin spots were found to contain sparse numbers of black-grey pigmented wool fibres. Histological examination showed both the naturally occurring and irradiation-induced pigmented skin spots resulted from an increase in both number and activity of melanocytes localized along the epidermal-dermal border of the epidermis. With time, the melanocytes were observed to have entered, to varying depths, the outer-root sheath of follicles still producing white wool fibres. These ultraviolet-light-induced changes to epidermal melanocytes in white Merino sheep presumably occur due to alterations within the local tissue environment in which the melanocytes lie.

Characterization of melanocytes in wool-bearing skin of Merino sheep.

The distribution and character of melanocytes in the wool-bearing skin of Merino sheep of known genotypes were examined by light and electron microscopy. In black Merino sheep (ww, homozygous recessive), melanocytes were localized within three regions of the skin: epidermal-dermal border, outer root sheath and follicle bulb. Melanocytes within these regions were found to be actively producing melanin, had numerous dendritic extensions and were able to transfer melanin to adjacent keratinocytes. In a black Merino sheep whose fibres were white due to an experimentally induced copper deficiency the melanocytes were amelanotic. In contrast, for both WW (homozygous dominant) and Ww (heterozygous) white Merino sheep melanocytes were observed only at the epidermal-dermal border of the epidermis. The melanocytes appeared also to differ in character containing less melanin, appearing less dendritic in shape and having a reduced ability to transfer melanin to adjacent keratinocytes. The gene for white fleece (W), therefore, appears able to regulate pigmentation in Merino sheep, at least in part, by controlling the location and activity of melanocytes within the wool-bearing skin.