Improving meat yield characteristics such as rapid growth, high carcass yield, increasing carcass and meat quality are among the most important topics in breeding studies regarding the farm animals raised for meat yield. In recent years, there has been an increasing interest in the use of genetic markers in the studies conducted for this purpose. These studies obtained significant results especially in species such as cattle and pigs, while studies on sheep, which includes 25% of the known mammalian farm animal breeds, are relatively limited (Sheriff and Alemayehu, 2018). Sheep offer great opportunities in meeting the future demand for food of animal origin and responding to changing market needs because they are more flexible farm animals compared to cattle due to their adaptability to very different geographies (Wollny, 2003).
In addition to being resistant to bad environmental conditions and having the capacity to make high use of low-yielding pastures, fat-tailed sheep breeds store excess calories as tail fat, which also has commercial value, in the season when grass is abundant. Due to these characteristics, fat-tailed sheep breeds have adapted well to Anatolian geography, where steppe climate conditions prevail in general, compared to thin-tailed breeds (Ertugrul et al., 2009). For this reason, 87% of domestic sheep population in Turkey consists of fat-tailed sheep breeds (Balcİoğlu et al., 2010). With narrower breeding areas, there are also thin-tailed domestic sheep breeds such as Kivircik and Karayaka, which store fat in body cavities and between muscle fibers instead of the tail, and therefore have higher quality meat compared to fat-tailed sheep breeds (Ertugrul et al., 2009; Kaymakç et al., 2009).
The Akkaraman breed, one of the fat-tailed sheep breeds in Turkey, is mainly bred in the Central Anatolia Region, where sheep breeding is most intense but this breed is also common in the transition regions close to this region. Therefore, it is the sheep breed with the largest population size in Turkey (Ertugrul et al., 2009). Another fat-tailed breed bred in Turkey, the Awassi breed, is bred in the Southeastern Anatolia Region in Turkey, Iraq, Syria, Jordan, Israel and Palestine (Galal et al., 2008). The Kivircik breed, one of the thin-tailed domestic sheep breeds in Turkey, is bred in the Marmara Region and North Aegean region of Turkey and is superior to other domestic sheep breeds in terms of meat quality (Öner et al., 2014). Another thin-tailed sheep breed is the Karayaka breed, which, like the Kivircik breed, is better compared to the fat-tailed breeds in terms of meat quality and taste and is bred in the inner parts of the Black Sea Region (Ertugrul et al., 2009).
Compared to beef, sheep meat is a very important food of animal origin for human nutrition due to its high protein rate, low fat and cholesterol rate (Sun et al., 2016). In addition, sheep are the most important alternative source of red meat to beef in societies that do not eat pork. Muscle growth and development are important in the emergence of yield characteristics such as meat yield and quality in livestock bred for meat yield. It is thought that the information to be obtained as a result of investigating the relationship between the genes involved in the muscle development mechanism and the expression levels of the proteins encoded by these genes can be used in selection applications to improve the live weight gain and meat quality of farm animals (te Pas, 2004).
The importance of analyzing the expression power of genes and proteins in different tissues to determine the candidate genes that can be used in animal breeding studies is better understood today (Gebreselassie et al., 2020). However, there are still many unexplained issues in the transcription mechanism in which the genetic information in DNA turns into protein. One of these is the change of genes and their protein expressions in different tissues and organs based on factors such as race, age, gender and nutrition (Bagot and Meaney, 2010; Thompson et al., 2020).
One of the important challenges in molecular biology in understanding gene regulation is to reveal genetic information about the gene regulatory factors that control gene regulation, which is the basis of all biological processes and phenotype (Hobert, 2008). Transcription factors, an important family of gene regulatory proteins, play an important role in the growth, development and evolution of higher organisms (Chen and Rajewsky, 2007). Therefore, investigating the transcriptome profile of transcription regulatory proteins, especially in different muscle tissues, will provide useful information to improve meat production and quality in all livestock, especially sheep.
The fusion of myoblasts is required to produce multinucleated myofibrils (Krauss et al., 2017) during the formation of skeletal muscles, called myogenesis, which play an active role in intranatal and postnatal growth (Braun and Gautel, 2011). The myogenic regulatory factor (MRF) gene family, which includes myogenin (MYOG), myoblast detection protein-1 (MYOD1), myogenic factor 5 (MYF5) and myogenic factor 6 (MYF6), has very important roles in the muscle growth and development (Cheng et al., 2020; Rihan et al., 2020). During this process, MYOD1, MYF5 and MRF6 control the formation of myogenic cells (Hernández-Hernández et al., 2017), while MYOG is required for terminal differentiation of skeletal muscle (Cheng et al., 2020).
The expression levels of the genes in the MRF family vary according to the developmental stage of the embryo in the intrauterine period (Rihan et al., 2020). For example, while the expression level of MYF5 is high in sheep fetuses in mid-pregnancy, the expression level of MYF6 is higher in the later stages of pregnancy (Quigley et al., 2014). Members of the MRF gene family are also expressed postnatally, but the expression patterns of these genes vary according to species, breed, nutritional status, gender, and developmental period of the offspring (Lv et al., 2015; Sun et al., 2016; Cheng et al., 2020; Rihan et al., 2020; Sui et al., 2020).
Skeletal muscle growth and development is a complex process supported by transcriptome regulation involving regulatory networks and signaling pathways (Sun et al., 2016). To date, MRFs (Bryson-Richardson and Currie, 2008) in farm animals were associated with growth hormone (Gent et al., 2003), insulin-like growth factors (Duan et al., 2010) and myostatin genes (Abe et al., 2009) were reported to be associated with muscle growth and development. The transcriptome analyzes to be performed in different species and different muscle groups will contribute to the understanding of the effects of the candidate genes responsible for muscle development or the previously determined candidate genes in different muscle groups.
this study aimed to investigate MYOG, MYOD1, MYF5 and MYF6 genes and protein expression levels in longissimus dorsi (LD) and gluteal (GL) muscles in two fat-tailed sheep breeds (Akkaraman and Awassi) and two thin-tailed sheep breeds (Kivircik and Karayaka) from among the domestic sheep breeds in Turkey.