The coat colour of animals is an extremely important trait that affects their
behaviour and is decisive for survival in the natural environment. In farm
animal breeding, as a result of the selection of a certain coat colour type,
animals are characterized by a much greater variety of coat types. This makes
them an appropriate model in research in this field. A very important aspect
of the coat colour types of farm animals is distinguishing between breeds and
varieties based on this trait. Furthermore, for the sheep breeds which are
kept for skins and wool, coat/skin colour is an important economic trait.
Until now the study of coat colour inheritance in sheep proved the dominance
of white colour over pigmented/black coat or skin and of black over brown.
Due to the current knowledge of the molecular basis of ovine coat colour
inheritance, there is no molecular test to distinguish coat colour types in
sheep although some are available for other species, such as cattle, dogs,
and horses. Understanding the genetic background of variation in one of the
most important phenotypic traits in livestock would help to identify new
genes which have a great effect on the coat colour type. Considering that
coat colour variation is a crucial trait for discriminating between breeds
(including sheep), it is important to broaden our knowledge of the genetic
background of pigmentation. The results may be used in the future to
determine the genetic pattern of a breed. Until now, identified candidate
genes that have a significant impact on colour type in mammals mainly code
for factors located in melanocytes. The proposed candidate genes code for the
melanocortin 1 receptor (
A large coat colour variation in animals is due to both genetic and environmental backgrounds. In the natural environment, dark/brown coat colours are preferable, whereas for economic purposes, pale coats (i.e. white and yellow) are desirable. Wild species are usually uniform in phenotype and show species-specific colours and patterns. By contrast, domesticated animals have a higher extent of coat colour variation compared to their wild ancestors. Therefore, the domesticated breeds are preferably used as the most suitable animal model for coat colour variation (Cieslak et al., 2011).
The colour diversity results from the presence and biochemical activity of melanocytes, the cells derived from the ectoderm. These cells are specialized in producing melanins – pigments protecting organisms from ultraviolet radiation (Solano, 2014). Melanins are divided into eumelanins (black/brown, pigmented phenotype) and pheomelanins (red/yellow, non-pigmented phenotype). In embryonic development, melanoblasts arise from the neural crest, migrate to the skin within a certain time frame, and then develop into mature forms of melanocytes (Parichy et al., 2006). Within 6 months of embryo development, the number of melanocytes is established (Costin and Hearing, 2007). If the melanoblasts fail to reach certain skin parts, they will lack pigment cells, and this is expressed as white patches (a phenotype leucism). This explains why those parts of an animal's body that are furthest away from the neural crest (i.e. the forehead, legs, belly) are usually white in colour.
For many years, only the phenotypic classification of breeds has been used. Coat colour phenotypes can be divided into two main types: patterned and non-patterned phenotypes, which are determined by the presence of eumelanin (brown and black) and pheomelanin (red or yellow). The coat colour of breeds was often the only visual trait in the morphological selection for breed identification (Hubbard et al., 2010). Thus, in the classic genetics of mammalian coat colour, the genes/loci names are mostly reflected in their phenotypic effect in the form of skin/coat colour.
Ryder and Land (1974) were among the first to highlight the complexity of the
coat colour inheritance process in sheep. Based on an analysis of the
proportion of differently coloured sheep performed on Soay, Orkney, and
Shetland, these authors suggested that coat colour traits were determined by
multiple allelic series within a locus, which can be compared to the agouti
gene in mice. Several alleles correspond to variation in coat colour, and it
was determined that white (A
The comprehensive study of the inheritance pattern of coat colour in sheep was investigated by Renieri et al. (2008) on lambs which were descended from crosses and backcrosses between seven pigmented rams and 166 full white ewes. It was found that there is no relationship between coat colour (white, black, or brown) and the pigment coat distribution (uniform or regular spotting), on the one hand, and the sex of the individuals on the other. Moreover, it was noticed that the full white phenotype dominated over black and brown and that the black colour is dominant over brown.
A coat/skin colour is an important economic trait in sheep breeding, and
in 2013 sheep skin and wool production amounted to over 374 thousand tonnes in Asia and 179 tonnes in
Australia and New Zealand combined (
The most common coat colour loci in farm animals are the A locus
(
Although classic genetics, which relied on “by eye” classification of coat colour type, was helpful in breeding schemes, it was not sufficient for different reasons. Firstly, different phenotypes can be caused by different alleles of one gene. Secondly, similar phenotypes can be caused by different alleles of many genes (Cieslak et al., 2011). Therefore, in order to define the phenotype, it is very important to determine its genetic complexity. Bennet and Lamoreux (2003) comprehensively reviewed the genetics of pigmentation in mice, pointing out that the most of the genes code for factors that are located in melanocytes. Following Cieslak et al. (2011), the candidate genes might be grouped into four categories depending on their activity in the following processes: (a) development of melanocytes, (b) melanogenesis, (c) pigment transport and transfer, and (d) survival of pigment stem cells.
For the coat colour of mammals, candidate genes have been proposed that code
for the melanocortin 1 receptor (
According to the classic genetics assumption, the presence of eumelanin or
pheomelanin is under the genetic control of
Some recessive genetic loss-of-function variants in the
The
In sheep, classic genetic studies have identified a two-allele series at the
The ovine
The recent studies using CRISP/Cas9 technology allowed us to confirmed that
the
In addition to
The tyrosinase-related protein 1 (
There had not been any evidence for the connection between the
The
Han et al. (2015) analysed the presumed relation and interaction between the
ovine
In the research conducted by Han et al. (2015) on Tibetan sheep, both the
Since the introduction of high-throughput genetic techniques, some additional genes as well as new factors that have an impact on pigmentation have been identified. The greatest advantage of the high-throughput sequencing methods is the possibility of obtaining a great deal of data of entire genomes. It gives the possibility of obtaining complex information about specific processes and of identifying the new genetic basis of important phenotypic traits such as coat colour in sheep.
The first time a microarray study on the pigmentation of sheep was conducted
was in Peñagaricano et al. (2012). Gene expression was studied on
black-spotted as well as solid-white skin samples of five Corriedale sheep.
The highest expression was indicated by, among others, C-FOS and CREB (cAMP
response element binding gene). Both genes are involved in melanogenesis:
C-FOS is a genetic factor stimulating melanin synthesis as the response to
ultraviolet (UV) light (Gordon et al., 1992), and CREB is a transcription
factor of numerous genes including
Fan et al. (2013) analysed skin transcriptomes from white and black merino
sheep. The authors obtained over 100 million raw reads from both skin colour
types using the Illumina sequencing technology. According to data analysis,
they determined 37 768 known genes, of which 2235 were differently expressed
in black vs. white sheep. Among the differently expressed coat colour genes,
The microarray study conducted on Finnsheep (Li et al., 2014) indicates a
significance effect of SNP s66432.1 in
According to the results of Fan et al. (2013), there were upregulated genes
encoding transcription factors regulating mRNA expression
(e.g.
Although some research on the genetic basis of wool colour has been conducted, the complexity of the issue remains. Identifying the genetic factors of coat colour in sheep is pivotal for improving the efficiency of artificial selection for preferable coat colour (or pattern) as well as for studying the evolutionary changes regarding the phenotypic variation in farm and wild animals.
Data are available from the corresponding author upon request.
All co-authors contributed to the preparation of the manuscript. Specifically, AK prepared Sect. 2 of the manuscript, with contribution from GS, KRM prepared Sect. 3 of the manuscript with contribution from DR, and AK drafted the entire manuscript.
The authors declare that they have no conflict of interest.
This study was supported by the National Research and Development Center (Poland) under the Strategic Research and Development Program “Environment, Agriculture and Forestry” – BIOSTRATEG, decision number BIOSTRATEG2/297267/14/NCBR/2016. Edited by: Steffen Maak Reviewed by: two anonymous referees