Globally, sheep contribute significantly to the animal production industry with various products including, milk, meat and wool. The total sheep population in Egypt is about 5.1 million heads that produce about 11 217 metric tons of greasy fleece annually (FAOSTAT, https://www.fao.org/faostat/en/, last access: 22 November 2021). In Egypt, Barki, Rahmani and Ossimi are considered the three major sheep breeds (Galal et al., 2005), representing about 65 % of the Egyptian sheep population (Elshazly and Youngs, 2019). They are distributed along the western Mediterranean coastal region, the middle of Egypt and the northern Nile delta, playing an important role in the livelihood of large portions of Egyptians (Sallam, 2019). Egyptian sheep are considered a source of low-quality woolens with coarse fleece and include some kemp (Helal et al., 2019), which may be appropriate only for handmade carpets. Under extensive conditions where high lamb production is not allowed, wool could be an important source of increasing the impact of raising these animals (Thomas, 2015). Several attempts were made to improve wool production of local breeds by crossing with imported breeds, such as Merino, Romanov and Finn sheep (Almahdy et al., 2000).

Obviously, wool is the only source of woolen products and contributes significantly to the worldwide textile industry. Generally, producing high-quality wool will maximize the profitability of the wool industry. Wool characteristics greatly affect wool's suitability for processing, such as fiber diameter, staple length and strength, color, and other visually assessed traits (Mortimer et al., 2009; Helal et al., 2019). Wool traits are quantitative traits that are affected by both environment and the animal genotype. Additionally, multiple genes contribute to traits with different biological mechanisms. Understanding the genetic contribution and biological functions of a particular trait is essential for subsequent selection and genetic improvement programs (Oldenbroek and Van der Waajj, 2015; Noelle and Chunhua, 2015).

Recently, the advent of genotyping technologies enabled breeders to pinpoint the genetic architecture of economically important traits (Sallam et al., 2018). Over the last 2 decades, multiple candidate genes harboring single-nucleotide polymorphisms (SNPs) for various production traits were reported in livestock (Wilkening et al., 2009). Association analysis using SNPs is the most effective approach to identify genetic markers related to a trait of interest (Zhu and Zhao, 2007). SNPs screen the candidate genes, which may be biologically related to the desired trait, for putative mutations (Patnala et al., 2013). Subsequently, these mutations are genotyped in a group of individuals and correlated with certain phenotypes (Kwon and Goate, 2000). Therefore, SNPs became the most common genetic marker used in selection and evaluation programs in livestock (Dominik et al., 2021). Conversely, scarce information is available about the genetic background of wool traits as few genes (e.g., KAP1.1, KAP1.3 and KAP6) were identified for wool traits in Egyptian sheep (Farag et al., 2018; Sallam et al., 2021).

KAP22-1 is a part of the keratin-associated proteins (KAPs), which are the main structural proteins of wool and hair fibers (Parsons et al., 1994; Powell and Rogers, 1997). The physicomechanical properties of the wool fibers are determined by these proteins (Powell and Rogers, 1997). The KAPs contain high levels of cysteine, glycine and tyrosine, which are predominantly located in the wool fiber cortex. It was reported that different numbers of KAPs are found in different wool types, which raises questions about their function in identifying wool fiber characteristics (Ullah et al., 2020b). The gene is located at chromosome 1, spanning 391 base pairs and consisting of three exons (Kinsella et al., 2011). The gene was identified as a candidate gene for wool traits (Li et al., 2017), harboring putative SNP markers for wool traits (Gong et al., 2016). The objective of this study was to investigate the genetic polymorphisms in the KAP22-1 gene and then correlate these variants with important wool traits in three major sheep breeds in Egypt.

Limited information is available about the genetic contribution of wool characteristics of the Barki sheep and the Nile valley breeds (e.g., Rahmani and Ossimi). This study reported significant differences in wool characteristics among the major sheep breeds in Egypt. Barki sheep produce lustrous, finer wool with higher medullation and crimp percentages and slightly lower KS and SST. In comparison, Rahmani have more colored wool with coarse FD, the lowest medullation and SST, and the highest PF and FD. Likewise, wool produced from Ossimi sheep was characterized by higher SL and KS and tended to be lustrous. However, FD was higher in Rahmani wool, but Barki wool may be the best as it had low PF and KS and was whiter, which means higher dying flexibility during manufacturing. Therefore, it is expected that Barki fleece is more economically valuable compared to Rahmani and Ossimi fleece. The abovementioned characteristics indicate that a wide range of wool types are produced from the local Egyptian sheep (El-Gabbas, 1999). The mixed structure (i.e., fine and coarse) fibers found in the same fleece was also reported in Egyptian wool. Therefore, it is recommended to assess the wool subjectively and objectively (Helal et al., 2019).

Generally, the amplified fragment of the KAP22-1 gene tended to be polymorphic in the studied Egyptian sheep population as three alleles were identified (A, B and C). This result is consistent with other findings on KAP family genes (Li et al., 2017; Tao et al., 2017; Farag et al., 2018). Conversely, higher (Farag et al., 2018; Ullah et al., 2020a, 2021) and lower (Ullah et al., 2020b; Sallam et al., 2021) numbers of alleles were identified in other KAP genes and different sheep populations. KRTAP6-5 was the most polymorphic gene in this family, which was identified in goats with 12 variants (Li et al., 2021). However, the C allele was less common, with frequency less than 5 % in the current Egyptian population, similar minor allele frequency was identified in KAP21-1 in Merion × Southdown crosses (Li et al., 2017). This suggests that increasing the sample size in further investigations may succeed in identifying this allele.

Interestingly, allele A was identified as the common allele in the Ossimi breed that is recognized as producing whiter wool compared to other breeds (El-Gabbas, 1999). The presence of this allele significantly increased GCG in the wool (+0.40±1.73, P=0.01). Notably, allele B seemed to be important in wool production and characteristics. Animals that carry BB genotypes produce GFW more than non-carriers (1163.63±65.91 g versus 1054.76±47.7 g); however, this effect was statistically not significant. Furthermore, the wool produced from animals with BB genotypes had longer staples compared to other genotypes (8.38±0.20 cm versus 8.27±0.20 cm). This is consistent with the high frequency of the B allele in the studied population, which indicates that breeders may tend to select for it as a desirable allele to increase GFW (Asif et al., 2017). Likewise, it means that selection for the B allele at an early age may be worth it to increase wool production (Sallam et al., 2021). Conversely, wool produced from AA carriers had lower KS (0.45±1.87 versus 1.98±1.88, P=0.01) and increased the whiteness of the wool (+0.40±1.73 versus -0.62±1.74) compared to other genotypes.

So far, no information is available about the effect of KAP22 polymorphisms on subjective wool production traits in sheep worldwide. Accordingly, it is hard to compare findings of this study with results of other reports. Nevertheless, variants in a different gene from the same family (i.e., KAP6-1) were associated with GCG and SST in Barki sheep (Sallam et al., 2021). Results of this study suggest that genetic markers may significantly influence subjective wool traits as well as objective measurements and should be considered. As expected, animals that carry the BB genotype increased the desirable wool yield (i.e., GFW), but it was accompanied by an increase in the unfavorable KS (+1.98±1.88). This confirms the negative correlation between both wool traits. Such information is important to be considered in breeding programs for wool production.

It is notable that Egyptian sheep produce a wide diverse wool type even in the same individual. This may be useful commercially in blending different wool types to make an appropriate composite for the carpet industry. In the current study, genetic polymorphisms were detected in the KAP22-1 gene, and they strongly affect multiple wool characteristics. Significant associations were identified between the KAP22-1 variants and CR (P=0.01), SL (P=0.012), SDfd (P=0.018), KS (P<0.001) and GCG (P=0.01). Interestingly, animals with BB genotypes tended to produce more wool yield (1163.63±65.91) with high SL (8.38±0.20), CR (8.38±0.21) and KS (1.98±1.88). Hence, the KAP22-1 gene is a putative genetic marker for wool traits in the Egyptian sheep.