Review Paper
GENETIC AND GENOMIC PROSPECTS FOR CAMEL MEAT PRODUCTION
S. Sabahat1, A. Nadeem*1,2 and J. Maryam1
1Institute of Biochemistry and Biotechnology, University of Veterinary and Animal Sciences Lahore, Pakistan
2Department of Biotechnology, Virtual University of Pakistan
*Corresponding Author: asifnadeem@uvas.edu.pk; asif.nadeem@vu.edu.pk
ABSTRACT
Camelids, originating in the Eocene Epoch, have diversified and the modern dromedary occurred during the Holocene in the Arabian Peninsula where it was subsequently domesticated. It provides meat, milk and other products, and Pakistan ranks ninth in the world in terms of camel population. It is well adapted to an arid environment and has a huge potential for food and economic security in the context of climate change, particularly via a genetic improvement for meat production. This article reviews the current knowledge on camel genetics, in relation to genetic improvement. It considers genetic diversity to understand breed differences and their historical origins, using molecular markers, as well as identification of sites of genetic polymorphisms in the camel genome, essential tools for genetic study. Several methods useful for genetic improvement in the camel are considered, including use of candidate genes such as GH, IGF-1, GHR likely to affect camel growth. The use of marker assisted selection (MAS) as well as modern methods including genome-wide association studies (GWAS) and genomic selection also been reviewed. Using genetic and other modern technologies, modern farming systems for camels can be introduced making a significant contribution to the rural economy.
Keywords: dromedary; genetic improvement; genomic markers; camel production season.
https://doi.org/10.36899/JAPS.2021.3.0253
Published online November 09,2020
INTRODUCTION
Camel is an important domestic animal in various countries for producing valuable food and for its adaptation to extremely harsh environments (Kadim and Mahgoub, 2008). It can survive in harsh environment with poor vegetation and water; even utilize feed that usually is not consumed by other domestic animals. Due to its multipurpose role, camel is gaining importance as meat producer. During the last decade, research on genetic study to improve camel meat quality has increased and it is vital component to sustain meat consumption. Development in the field of genomics have increased consumer’s awareness and motivated camel breeding companies take to more interest on meat quality as well as take account of quality traits as fundamental part of selection programs.
The current paper presents a reviewon camel diversity, breeds, classification, distribution, domestication, and genetics in general and selection for higher body weights, its subsequent effect on growth performance, production, reproductive traits, meat quality characteristics, carcass traits, growth and effects on genetic parameters. A brief review on genes and genetic polymorphisms associated with body weight and growth-related traits is also presented.
Current and Future Panoramas for Camel
History of Camels
Phylogenetic analysis: Camel ancestors were found in North America back to Eocene epoch (around 40 million year ago) (Burger, 2016). After separating into New World and OldWorld camels, the latter migrated to the Eastern Hemisphere by the Bering land bridge. The earliest Asian camel can be traced back to 5 million years ago (Mya) (Kozhamkulova, 1986). One-humped and two-humped camel divergence was estimated to occur between 5 and 8 Mya (Wu et al., 2014), and wild and Bactrian camel diverged between 0.7 and 1.5 Mya in the Pleistocene before domestication (4000-6000 ya).
ii.Classification of Camelidae: The taxonomic classification of camels is Class Mammalia, Order Artiodactyla (even-toed ungulates), Suborder Tylopoda (animals with padded feet), and Family Camelidae. Old world Camelids (Camelini) and New World Camelids (Lamini) are two subfamilies of Camelidae. Genus Camelus is comprised of two species, one humped and two humped camels. Camelus dromedarius (one humped camel) is mainly distributed in the warm arid region of the Middle East and Africa while Camelus bactrians (two-humped camel) is distributed in China and central Asia. It is also found in Mongolia and China where it is known as Camelus bactriansferus.
iii.Domestication: Old world ancient civilizations and numerous rural and roaming societies of Asia and Africa depended on camel for milk, meat, transportation and wool production. The dromedary, Camelus dromedarius ranges from East and North Africa to Pakistan and was also introduced into Australia for inland transport in the early 19th century. Global structure of East African and south Arabian dromedary was differentiated at nuclear level from North Arabian, North African and South Asian individuals (Almathen, 2014; Charruau, 2012; Schulz et al., 2010) During the Holocene epoch, presence of bones of wild dromedary indicates their presence on Arabian Peninsula along hunting site of eastern coast (Von den Driesch and Obermaier 2007). A zooarchaeological study also revealed that domestication took place in the coastal southeast Arabian Peninsula (Grigson, 2012; Grigson, 2014; Uerpmann and Uerpmann, 2002; Uerpmann and Uerpmann, 2012). A genetic study of recent and ancient DNA confirms this region as the possible place of domestication.
Current camel & world-wide breed distribution: The world camel population is estimated to be about 28 million (FAOSTAT, 2017), out of which 85% are distributed in Africa and 15% in Asia. Out of that, 70% of the African population is found in Somalia and Sudan while Kenya, Ethiopia and Chad contain 15% of camels in Africa. Pakistan ranks about the ninth as a major camel rearing country in the world after Somalia, Sudan, Ethiopia, Niger, Mauritania, Chad, Kenya and Mali. Since 1961, the world camel population was more than doubled with a yearly growth rate of 3.4% (Faye, 2015). There are about 50 camel breeds in the world, out of which 27 breeds are present in North, West and East Africa, Arabic countries and Pakistan. These are as Gabbra, Nord, Deshi, Ait, Steppe, Chumbi, Khawar, Manga, Somalie, Azmiyah, Dera Ismail Khan, Adrar, Sidi, Mekrani, Khiva Turkana, Cheikh, Mowallad, Tibesi, Ouled Targui, Arab, Soudani, Ajjer, BerabicheKhebbach and Benodir. West and North Africa, India, and the Arabian Peninsula contain 20 breeds, namely Gandiol, Azaouak, Bekaneri, Umaniyah, Guban, Indi, Mudugh, Turkana, Sahel, Rashaidi, Pellahi, Fleuve, Bekaneri, Reguibi, Riverine, Urfilla, Grain, Donkali, and Arabi. Bishari, Air and Anafi camel breeds are found in Africa and Sudan (Blanc and Ennesser 1989; Sakandar et al., 2018).
Camel distribution in Pakistan: The dromedary, Camelus dromedarius, is the most widely distributed and numerous camel species in Pakistan and is found in hot and arid regions of the country whereas the smaller population of the Bactrian camel, Camelus bactrians, is found in the northern areas. The camel population is about 1 million in Pakistan (Government of Pakistan, 2017). The distribution of the dromedary varies across the country. Baluchistan has the highest population (41%) followed by Sindh (30%), Punjab (22%) and KPK (7%). Pakistan has four ecological zones of camel distribution according to Qureshi et al., (1993), namely 1) coastal mangroves (Badin, Thatta and Karachi district of Sindh); 2) sandy deserts (Cholistan and Thal in Punjab and Thar in Sindh); 3) mountainous tracts of Baluchistan, D.I. Khan and D.G. Khan districts of KPK and Punjab, respectively); and 4) irrigated plains of Punjab and Sindh.
Camel breeds in Pakistan: Twenty breeds of camel have been documented by Isani and Baloch (2000) in Pakistan, seven breeds in Baluchistan, four in KPK four in Sindh and five in Punjab. Pakistani camels are Kachhi(Jacobabad &Sibi region), Brahvi(Chagi district & Sindh region region), Makrani(Makran, Lasbela, Kharan, Dadu& Karachi region), Gaddi (Lucky Marwat WaziristanAgency & D.I. Khan region), Bagri (Cholistan&Thal deserts region), Lassi (Lasbela, Balochistan& Sindh region), Rodbari(Makran, Pasni, Turbat& Khuzdar region), Ghulmani(Dera Ismail Khan, D.G. Khan &Zhoberegion), Maya (KPK Tribal areas region), Kala-chitta(Pabbi, Sohawa& Salt Range region), Khader (NWFP, extend between Sulaiman Range and Indus River region),Marecha(Cholistan, Thal and D.I. Khan region),Brela (Jhang, Muzaffargarh, Multan, Mianwali includes Thal area region),Kharai(Chohar, KharoChhan& Jamali, Thatta, Badin & coastal areas of Karachi and Kacchregion), Larry (Hyderabad and Badin region),Dhatti (thari) (Tharparkar, Umerkot&Mirpurkhas. Sanghar& Badin region), Sakrai (Mirpur Sakro to SujawalTallukas of Thatta district region), Cambelpure(Attock, Rawalpindi, Chakwal JehlumMianwali and Sargodha region), Kharani(Kharan, Jhalawan and Bordering Kala region) and Pishinfrom Pishin and Quetta region.
Camel population trends in Pakistan: For 18 years (1990-2007), a mixed trend of positive and negative population growth was observed. The camel population was 1 million during 1990-1994 and declined to 0.8 million in 1995 and a further decline in population to 0.7 million in the next two years, i.e. a decline of 29% between 1994 and 2004. In 2007-08, camel population again increased to one million (Government of Pakistan, 2008). The camel population remained stable since the last decade and is now 1.2 million (Government of Pakistan, 2017).
Current and Future Prospects for Camel Production
Economic importance of the camel: According to Iqbal (1999) and Raziq (2009), the camel has great economic importance in the developing world as it provides an inexpensive source of power in ploughing, drawing water from wells and land levelling. On Eid ul Ezha, the camel is slaughtered blissfully by Muslims and gains a good price at this event. Pakistan exports camels to Egypt, Saudi Arabia, Libya, the Gulf and other Arab countries. The camel industry flourishing in Arab states due to camel racing. In Sudan, a racing camel can have a price up to 15 million dirhams (Rs 550 million). An ordinary camel in a market can fetch 2–3 thousand Dhs/male (Rs 70-110 thousand) and 4–6 thousand Dhs/female (Rs 150-200 thousand) (Manefield and Tinson, 1997).
Due to global warming, the camel is gaining importance as the most favourable animal to be developed genetically, due to its climatic adaptability. Annually, 0.24-million -ton of milk and 50-thousand-ton of meat form the camel is produced in Pakistan which valued at Rs 2.4 billion and Rs 250 million respectively (Government of Pakistan, 2008). Camel hides are used to manufactures beautiful decorative articles, sandals and saddles which are exported. Twenty-thousand tons of camel hair are used for manufacturing blankets, tent cloth, ropes and floors mat annually. Soft woolly fleece of newborn calves is mixed with hairs and used for manufacturing blanks (Khan et al., 2003) These suggest that by improving conventional management system with modern farming practices, the life of nomads can be improved whose lives depend on raising camels.
General characteristics of camel meat: The chemical composition and nutritional value of camel meat distinguish it from beef and mutton. Camel meat is considered best at the age of 4-5 years. Due to the presence of glycogen, it has a slightly sweet taste and its colour varies from red to dark brown. On the basis of its low fat and high moisture contents, it can be distinguishing from beef and mutton. Camel meat is rich in proteins and minerals including Vitamin E has a high water-holding capacity compared with beef (Soltanizadeh et al., 2010). Camel meat is an excellent source of vitamins especially vitamin B complex and various minerals such as calcium, iron and phosphorous. It is considered a healthy food for humans due to the low level of cholesterol (75-86 mg/100 g) and high amino acid contents compared to other animals.
Camel meat as medicine: Camel meat is also used in medical purposes to reduce risk of diseases such as hypertension, hyperacidity, pneumonia and respiratory diseases (Kurtu, 2004). Camel meat has been used as traditional medicine for the last sixteen centuries in China. It is used to improve resistant against diseases and is helpful to give strength of muscles and bones, to reduce pain and give moisture to skin. Camel hump fat is used efficiently to relieve pain and swelling (Qiu et al.,2002). Due to presence of linoleic unsaturated fatty acid, it has ability to protect from cancerous tumour reported by some researchers. Studies have proved that camel meat can be useful in treatment of cancer, infections, stork, and sciatica (Abrhaley and Leta, 2018). Camel meat has high unsaturated fatty acid content as compared to beef and due to its unique features (Soltanizadeh et al., 2010).
Potential of meat production in camel: As a meat producer and due to its adaptability, the camel is becoming a more significant livestock species. Camel meat is typically produced by old conventional production system and its meat is mostly obtained from old animals that became less valuable and unable to provide milk, transportation and breeding (Tandon et al., 1988). Under these conditions, camel meat is considered coarse, tough, watery and sweet in flavour as compared to other meat sources.
There is evidence to show high demand for fresh camel meat when it is used for blended products, however, the marketing system of camel meat is not well organised (Pérez et al., 2000). To fulfil the growing demand of developing countries, camel meat provides a good and cheap option to accomplish their meat requirements (Saparov and Annageldiyev, 2005). However, as camels are raised primarily in under-developed countries, limited research work has been done to improve their reproductive and productive characteristics.
Camel growth rate: The dromedary camel is one of the most important domestic animals in the arid and semi-arid regions as it is equipped to produce high quality food at comparatively low costs under extremely harsh environments. The camel has great tolerance to high temperatures, high solar radiation and water scarcity. Tandon et al, (1988) noted that the camel is likely to produce animal protein at a comparatively low cost in the arid zones based on feeds and fodder that are generally not utilized by other domestic species due to either their size or food habits. The average birth weight of the dromedary camels is about 35 kg, but it varies widely between regions, breeds and within the same breed. Reports on camel birth weights range between 27 kg and 39 kg, which is comparable with that of tropical cattle breeds (Bakheit et al., 2012). Hammadi et al., (2001) reported camel body weights of 27, 48, 65, and 79 kg at birth, 30, 60 and 90 days of age, respectively, which indicates a daily growth rate of 580 g/day between birth and 90 days of age. The limited work carried out on improving camel nutrition demonstrated significant relationships between daily gain and daily intake of concentrates for dromedary camels. (Kamoun, 1995).
Genetics of Camel
Genetic improvement of camel meat: With the potential role of contributing to food security, as well as consumer interest and acceptance of camel meat, there is a need to increase meat production, and this may in part be achieved by genetic improvement of camel meat quality and quantity. Meat quality can be improved genetically by applying molecular marker technology which in part removes the need for extensive phenotyping. Proteomics and functional genomics are very helpful tools for understanding function and regulation of genes related with specific trait. Gene expression, DNA sequencing, protein analysis, microarray analysis are advanced techniques that can be used for genetic improvement of meat. Thus, by using molecular marker technologies, meat quality can be improved (Gao et al.,2007).
Genetic diversity: Genetic diversity between individuals of Camelidae family such as the Bactrian camel (Chuluunbat et al., 2014; Jianlin et al.,2004), dromedary breed (Mburu et al,.2003), llama, guanacos and alpacas (Kadwell et al,. 2001) were analysed by using molecular markers. Phylogenetic analysis was performed for determination of evolutionary lineage of llama and alpaca. (Kadwell et al., 2001; Stanley et al., 1994). Two closely related species of Mongolian domestic and wild Bactrian camels show 2.9 % genetic diversity in the mitochondrial region. Genetic studies in camels have verified that there is high sequence variation at both mitochondrial (Ji et al., 2009; Silbermayr et al., 2010) and nuclear loci (Shah et al., 2006; Wang et al., 2012). Dromedary and Bactrian camels have undergone full genome sequencing and analysis. Scientists from Saudi Arabia, and China sequenced 2.2 billion nucleotides in the camel's genome (Mahmoud, 2010).
Molecular markers have been successfully amplified in camelids to study genetic distance between Mongolian and Chinese domestic Bactrian camels (Charruau, 2012; Jianlin et al., 2004). Babar et al. (2015) studied genetic diversity in Barela and Marecha camel breeds of Pakistan. Cyto-b gene and D-loop analysis indicated low genetic diversity in these two breeds. The phylogenetic analysis indicates dromedary and Bactrian camel as two different clades that originated from divergent lineage and have distinctive genetic distance.
Genetic diversity and association among Kenyan, Pakistan, Saudi Arabia and United Arab Emirates dromedary camels were studied by using microsatellite loci. Phylogenetic analysis showed that Kenyan dromedaries are distinctive from Pakistani and Arabian population (Vijh et al., 2007). Almathen (2014) reported on the geographic distribution of genetic diversity in dromedary camels from 21 countries, demonstrating the wide range of this breed. Microsatellite markers and mitochondrial DNA were used for analysis.
Effective population size: The effective population size (Ne) is one of the most important parameters in conservation and evolutionary processes, as a reduction in Ne may cause a loss of genetic variation, increase in frequency of lethal mutations, and a decline in the ability to adopt to natural and human-induced environment change. Reduction of actual population size and Ne threatens population viability due to modification, exploitation, habitat loss and altered population dynamics (Allendorf and Luikart, 2007). While it is very difficult to estimate Ne directly because it is hard to measure all the demographic factors that influence this parameter in wild populations (Frankham, 2005), using genetic data has become a popular and convenient method to estimate Ne. Various methods are available (Ovenden et al., 2007; Schwartz et al.,1998) that help to estimates Ne such as gametic disequilibrium (England et al.,2006; Hill, 1981; Waples, 2006), and linkage disequilibrium (LD) (Tallmon et al., 2008; Wang et al., 2012). Population structure was studied by using mitochondrial and nuclear microsatellites in Mongolian Bactrian camels. The clustering results reported that 97% of nuclear and 73% of mitochondrial variation occurred within populations, it between. These results show low population differentiation and high levels of gene flow among Mongolian camel breeds (Chuluunbat et al.,2014).
Linkage disequilibrium: The term ‘linkage disequilibrium’ (LD) was first used by Lewontin and Kojima,(1960) and simply used to indicate non-random association of alleles at different loci. LD has gained importance in human genetics, agricultural genetics and evolutionary genetics as it provides information about past history of a species and it constrains the potential response to both natural and artificial selection. The pattern of LD reflects the breeding system, geographic distribution, and population history throughout the genome but in each genomic region it reflects gene conversion, natural selection, mutation and rate of evolutionary change. Nowadays, LD is mainly considered in relation to SNPs that are diallelic and mutate in very low rates (Slatkin, 2008).
Genetic polymorphisms: A genetic polymorphism is the existence of more than two alternative forms of DNA sequence that are genetically distinctive and structurally different. These mutations range from change of a single nucleotide to hundred bases. A SNP is the simplest form of polymorphism that can be situated in the regulatory region of the genes or whole genome. These polymorphisms are significant for their use to differentiate DNA markers and have many applications in the field of molecular genetics for Genetic testing, Gene mapping, Detection of heterozygous disease in livestock, Paternity testing, DNA fingerprinting, Donor-recipient matching for tissue and organ transfer and Determination of genetic relationships among different strains, breeds and breeding lines of livestock(Yahyaoui et al., 2001). Various types of molecular markers are now used for specific traits in livestock animals.
Candidate Genes for Improving Growth in Camels
Growth hormone
Growth hormone (GH), also known as somatotropin, is a polypeptide hormone produced and secreted by somatotropic cells of anterior pituitary gland. It plays an important role in development and growth of mammals (Butler and Roith, 2001).
Biological function of GH: GH has diverse effects and its receptor is expressed in various tissues. GH function is stimulated by binding with GHR on the cell membrane of specific tissues. These receptors activate tyrosine kinase, particularly JAK2. Binding of JAK2 with receptors causes phosphorylation of kinase and activators of transcriptional family, particularly STAT5 which activate gene expression and causes production of IGF-1 (Argetsinger and Carter-Su, 1993).
ii.Organization of the growth hormone gene: The GH sequence of Camelus dromedarius was studied by Maniou et al., (2004). The length of camel GH is about 1900 bp and consists of five exons (lengths 71, 161, 17, 162 and 200 base pairs) and four introns (Maniou et al., 2001). The location in the camel genome is not known. However, it is located on Chromosome 19 atq26 in cattle (Hediger et al.,1990), and in buffalo; it is located on Chromosomal 19 from base pair 157215-159046.
iii.Polymorphism associated with the growth hormone gene: GH plays important role in selection of growth trait in livestock’s and serve as candidate gene (Daverio et al., 2012). Thomas et al., (2007) reported an association of polymorphism in GH with growth and carcass trait in Brangus bulls. In addition, an association between body weight and secretory function related with GH polymorphism in Japanese black calves (Katoh et al., 2008). Body measurement relationships of SNPs with GH gene was studied in six Sudanese breeds by Ishag et al. (2010).
Afifi et al., (2014) studied polymorphism association between the growth hormone gene and body weight gain in the camel. The association was studied in four Saudi Arabian camel breeds, viz.Majaheem, Saheli, Waddah and Homor breed. SNP T450C was found to be linked with body weight gain. Saheli camels had a heavier body weigh with CC genotype than CT and TT genotypes. This study verified that this SNP can be used as a marker for selection of higher rate of growth and meat production in camels. Abdel-Aziem et al., (2015) studied genetic polymorphism of GH gene in Somali, Sudany, Maghrabi, Mowaled and Falahy camel breeds in Egypt. Maghrabi camel that is used as dual-purpose camel shows a higher frequency for allele C (0.75) than other four breeds. These findings provide credibility to the hypothesis that GH is an ideal candidate gene for growth-associated traits in livestock.
iv.GH and IGF-1: There is an important association between GH and IGF genes to do with controlling growth and metabolism during foetal and postnatal development (Mertani and Morel, 1995). The GH-IGF axis consists of GH, GHR, the GH binding proteins (GHBP), IGF-I, IGF-II, IGF receptors and the six IGF binding proteins (IGFBP Both GH and IGF-I have a significant effect on the growth and physiology of an organism (Schuller et al.,1993).
v.GH-IGF in cell proliferation and apoptosis: GHR mRNA and protein was identified in bovine mammary gland tissue, (Plaut et al., 2003; Sinowatz et al, 2002; Zhou, 2007). GH stimulates proliferation in epithelial cells (Knight et al., 1990). During regulation of the cell cycle, IGF-1 can stimulate proliferation of cells (Evan and Vousden, 2001).
vi.GH and IGF-I in body regulation and muscle development: GH plays a key role in growth, development, lactation and reproduction. Laron’s syndrome in humans is caused by deficiency of GH, and that can affect growth during infancy and result in adolescence of short stature (Etherton and Bauman, 1998). Excess of GH causes gigantism that results in a condition called acromegaly that results in enlargement of bony tissue, carpal tunnel syndrome, hypertension headaches, cardiomyopathies and diabetes mellitus. The effect of GH is chiefly moderated by IGF-1 (Ayuk and Sheppard, 2006).
vii.GH-IGF in transport processes: GH affects amino acid transport and protein synthesis in various tissues. Transportation and utilization of amino acid in perfused rat liver can be simulated by GH. Concentration of seven amino acids such as serine, alanine, lysine proline, glycine threonine and arginine increase in media that contain GH (Jefferson et al., 1975). IGF can stimulate protein synthesis and enhance cellular uptake of amino acid (Dimitriadis et al., 1992, Nielsen and Jakobsen, 1993; Louveau and Gondret, 2004).
viii.GH and GHR: GH is polypeptide hormone which is produced and secreted by acidophilic cells of the pituitary gland (Wood et al., 1989). Binding of GH with two GHR induces signal transduction (Sundstrom et al., 1996). GHR consists of a single transmembrane domain, an extracellular domain and cytoplasmic domain (Gordon et al., 1983). Sabahat et al. (2020) characterized the genetic variability and reported the amino acid substitutions in growth hormone and growth hormone receptor genes mutants in Camelus dromedarius.
2.IGF-1: IGF-1 is a polypeptide hormone which belongs to the family of growth factors, and plays a key role in regulation of growth, differentiation and development (Martin and Stoica, 2002). IGF-1 is mainly produced by the liver as an endocrine hormone (Combes et al., 1997). IGF-1 can also be synthesized by various tissues and has the ability to stimulate cell growth by paracrine and autocrine fashion (Yee, 1994). IGF-1 production is stimulated by GH and is affected by nutrition. IGF-1 plays a significant role in juvenile and adult growth by binding with specific receptors of various tissues. IGF-1 acts in three different ways by the endocrine system (Butler and Roith, 2001).
Organization of the IGF-1 gene: The location of IGF-1 gene in Camelus dromedarius is unknown. IGF-1 is present on chromosome 5 in cattle (Miller et al., 1991). IGF-1 plays a vital role in growth, embryogenesis, cell discrimination and regulation of metabolism. It is formed by arrangement of 70 amino acid and has a molecular weight of 7.5 kDa. (Dimitriadis et al., 1992). Humans, cattle, dogs and pigs have an identical amino acid sequence (Rinderknecht and Humbel, 1978). Exon numbers vary between different species, for example pigs, goat and sheep have six exons while human, rats and camels have five (Rotwein, 2017).
ii.Polymorphism associated with IGF-1 gene: IGF-1 considered as effective candidate marker for growth and meat production in livestock. A high IGF-1 concentration was reported in the blood of Simmental which is beef breed, compared with dairy Holstein cows. However, growth rate shows an association with concentration of IGF-1 in both breeds (Schlee et al., 1994). The association between growth rate and high level of IGF-1 also reported by Barash et al., (1998). However, a negative correlation was also observed between growth rate and level of IGF-1 in Angus cattle by Ge et al., 2003). Siadkowska et al. (2006) reported an association of polymorphism of IGF-1 gene with meat production traits in Holstein Friesian cattle. Three SNPs were reported in IGF-1 gene by Fatima et al. (2009) in three buffalo breeds of Gujrat.
Sixteen SNPs were identified in IGF-1 and GH genes of Holstein Friesian dairy cows and found to be associated with milk production, growth rate and fertility traits (Mullen et al. 2011). Kim et al., (2005) reported a novel SNP in the IGFBP-3 gene of Korean cattle. Choudhary et al., (2007) studied polymorphisms of IGFBP-3 gene associated with body weight and birth weight in Holstein Friesian and crossbred cattle. Different breeds have variable concentrations of IGF1 in plasma (Rhoads et al,. 2008).Ozawa et al., (1995) found different concentration of IGF-1 in heavy horses, light horses and ponies. IGF-1 concentration in serum has been reported in different breeds of horses including Thoroughbreds (Noble et al., 2007) Standardbred trotters (Champion et al., 2002), and Quarter horse (Ropp et al. 2003). Genetic polymorphisms in insulin-like growth factor-1 (IGF-1) gene of Pakistani Marecha camel breed were reported by Sabahat et al. (2020a). A significant finding was the occurrence of a T→C polymorphism in exon 5 that causes a substitution of an amino acid from Cysteine to Arginine.
In Nellore, Canchim and various other cattle breeds, significant associations between body weight and SNPs have been reported by Curi et al., (2005). A study evaluated SNPs in the promoter region (C-512T) of IGF-1 and SNPs found between exon 3 and 4 showed a positive association with growth traits in Angus cattle (Ge et al., 2001). A SNP in the promoter region of IGF-1 (IGF1/SnaBI) in Charolias cattle breed has been found to be associated with growth trait and can be helpful for genetic evaluation of this breed with MAS strategies (De la Rosa Reyna et al., 2010).
3.GHR: The growth hormone receptor (GHR) is a transmembrane protein and is considered a very important candidate for growth, meat and milk traits in livestock. The binding ability of GH on target tissues is affected by changing function of GHR (Di Stasio et al., 2003). Mutation of GHR gene causes Laron syndrome and idiopathic short stature in humans (Blair and Savage, 2002;Tixier-Boichard, 2002).
Organization of the GHR gene: The location of GHR is unknown in the camel. GHR gene is located from base pair 2960112-3196609 and about 236498bp. GHR comprise of 10 exons.
ii.Polymorphism associated with the GHR gene: Dybus and Grzesiak (2006) and Kmiec et al. (2007) reported several polymorphisms in bovine GHR. Growth hormone exerts its effect on growth and metabolism by interaction with GHRs that are present on the surface of target cells (Hradecka et al., 2008). GHR can alter activity of GH by changing its functional region and signalling pathway. Rahbar et al., (2010) reported SNPs in the promoter region of GHR and found a significant correlation with milk trait in Holstein cows. Five polymorphisms have also been documented in East Anatolian red cattle, Turkish grey cattle and South Anatolian red cattle (Akad et al., 2012). Andreas et al., (2010) studied genetic polymorphism of GH and GHR genes in buffalo to evaluate them as candidate genes in meat production
GH is major regulator of metabolism and growth and influences health, growth, aging and milk production by modulating the expression of IGF-1 (Lincoln et al., 1995; Sumantran et al.,1992) and it was also verified that GHR mediates the function of GH on specific cells by signal transduction across the cell membrane and transcription of various genes including IGF-1 (Argetsinger and Carter-Su, 1996; Rotwein et al,. 1994). Thus, GH and GHR genes are key candidate genes for the detection of genetic markers for growth, carcass, meat and milk traits in livestock (Ge et al., 2003).
Additional Genetic Considerations
Mapping loci for meat quality: Meat quality is affected by number of quantitative trait loci (QTL) and various other factors. Genomic technologies are now used for mapping loci that affect meat quality (Koopaei and Koshkoiyeh, 2011). In association analysis, candidate genes and linkage mapping markers are used to detect QTL. Bioinformatic and genomic analysis are useful to differentiate genes on the basis of their function in livestock species including camel. Marker-assisted selection (MAS) by using information from meat (QTL) loci, can be useful in a breeding selection program. These technologies play an important role in genome mining and gene discovery. However, RAPD, SNP, RFLP, microsatellite, SSLP, VNTR are various techniques that are being used in the last decade. Biological researchers are taking great interest in modern technologies such as microarrays, high density SNP (HD-SNP) arrays to study genetics and genomics of animals (Fan et al., 2010). Genetic variation information within and across breeds can be obtained by using these techniques.
Genes associated with meat quality traits: The three candidate genes described earlier, GH, IGF-1 and GHR, are relevant to meat production, given their role in overall animal growth. However, there are some additional candidate genes to consider that are specific to meat quality traits. The myogenic factors (MYF) 5 and 6 considered essential for initiation and development of skeletal muscles and phenotypes maintenance in camel (Shah et al., 2006). FABP4 plays a major role in the regulation of glucose and lipid homeostasis. FABP4 gene polymorphism shows associations with economically important traits such as fat and marbling in cattle (Michal et al., 2006).
Leptin (LEP) also plays an important role in energy metabolism, food intake, body weight and energy expenditure (Woods et al., 1998), immune system (Lord et al., 1998) and reproduction (Garcia et al., 2002). Significant associations of LEP polymorphism with feed intake (Oprządek and Flisikowski, 2003), body fatness (Buchanan et al,. 2002) and marbling scores (Nkrumah et al., 2004) and milk yield (Buchanan et al,. 2003) have been detected. Thyroglobulin (TG) is the precursor of thyroid hormones which plays an important role in lipid metabolic regulation and, other functions (Barendse et al., 2001).
Making use of genetic markers for meat quality trait genes: Having identified genes associated with meat quality traits, the next issue is how to best utilize this information. Gene markers are advanced and powerful technology which will be helpful livestock breeders to attain their breeding target specifications. Gene markers are becoming available for a variety of performance, conformational and disease traits.
However, the markers need evaluation, and it is now necessary for breeders to utilize best gene marker technology (Davoli and Braglia, 2007). Three options are available to breeders to make optimal use of this technology. Firstly, breeders may simply buy sires that have been bred utilizing information on useful gene markers. Secondly, breeders can evaluate gene markers in their own herd to set up breeding stock with the most advantageous gene marker profiles for further use. Thirdly, breeders can buy sires with distinct gene markers and carry on breeding inside their herd to achieve an improvement in their livestock profiles (Meuwissen et al., 2013).
Marker assisted selection: Marker assisted selection (MAS) uses quantitative trait loci (QTL) information for livestock selection programs (Davis and DeNise, 1998). The first requirement in the selection of molecular markers is the identification of candidate’s genes related to the economically important trait. Candidates genes have potential a relationship with physiological and biochemical processes and allow the detection of single nucleotide polymorphism (SNPs) in the gene that are the basis for variation in a trait.
MAS has the ability to increase the genetic gain rate in animals where conventional phenotypic selection is less efficient (Abdel-Azim and Freeman ,2002). MAS could be useful in genetic gain, reduction in generation interval and expanding selection differentials. These goals can be attained by using MAS through selection of young bulls before progeny testing (Mackinnon and Georges 1998). Sabahat et al. (2020b) investigate patterns of variation in birth weight and weaning weight of Marecha and Lassi camels breed of Pakistan and developed spline-based growth models.
Meat quality enhanced by MAS: In the past, traditional phenotype-based selection shows a slow rate of genetic improvement with low accuracy but in modern farming animal systems, DNA-based technologies and genetic markers have changed the breeding abilities for livestock. Specific DNA variation associated with meat quality characteristic can be identified by QTL mapping and selective improvement undertaken by MAS. In farm animals, undesirable characteristics in the population can also be eliminated by using MAS. DNA markers related to softness and marbling of beef is commercially available, for instance GeneSTARTM Tenderness; these SNP markers are used to test for meat tenderness (Allais et al., 2014). Page et al., (2004) utilized GeneSeek Company’s MassArray SNP chip and identified two SNPs associated with meat tenderness in the Calpain 1 (CAPN1) gene.
However, there are some limitation of QTL mapping as it is difficult to find and prove a causative mutation associated with a QTL (Andersson, 2001). The main hurdle is to localize QTL, as fine-mapping can be a daunting task. Along with QTL mapping, various approaches such as genomics, proteomics and metabolomics are needed to understand the genetic architecture of complex traits (Liu and Cheng, 2002).
Influence of breed on meat quality: Breed can influence meat quality in different ways such as meat physiology and muscle structure (Sañudo et al., 2004; Waritthitham et al., 2010). In cattle, it has been reported that native breeds have benefits such as high reproductive rate, crude feed tolerance, low maintenance, dressing percentage and growth rate are low (Liu et al., 2006; Shengli, 2009; Xie et al., 2012). Similar benefits may flow in to selecting appropriate breeds of camel for their environment. Cross-breeding may also be important, particularly if done using a marker-assisted introgression approach, binging desirable alleles over from one breed into another (Charcosset, 1997).
Application of genome-wide association study: While candidate genes are one approach to selecting useful genes for MAS, a genome-wide association study (GWAS) can identity a large number of potentially useful markers. GWAS is a modern technique for detection of genes associated with traits of interest, used in humans, as well as agricultural species including domestic animals. GWAS mainly uses SNPs linked with sequence variations of the genome along with pedigree and phenotype information to discover associations to recognize genes that play a key role for the trait of interest. It was first used in human disease analysis and made great progress (Hirschhorn and Daly, 2005). GWAS was expanded to the breeding and genetics of domestic animals when genomic sequences of various domestic animals were available, and the number of SNPs on a SNP chip increased as a result of sequencing and re-sequencing. There are various types of SNP chip commercially available for cattle (50,000 SNPs; Illumina BovineSNP50 BeadChip), sheep (56,000 SNPs), chickens (60,000 SNPs; Illumina ChickenSNP60 BeadChip). horses (54,602 SNPs; Illumina EquineSNP50 BeadChip), dogs (22,362 SNPs; Illumina CanineSNP20 BeadChip) and pigs (60,000 SNPs; Illumina PorcineSNP60 BeadChip) (Wang et al., 2012).
GWAS related to meat quality of cattle has been studied and reported by Bolormaa et al. (2014). A GWAS was carried by Sorbolini et al. (2015) to find out significant association of markers with carcass and meat traits in the Marchigiana breed of cattle. Sherman et al. (2009) studied associations between feed intake and markers and found associations of 161 SNPs with net feed intake. Growth, meat quality and carcass traits were measured on 490 bulls and genotyped by using the Illumina 50K Chip.
Marecha and Lassi are two important breeds of camel, and GWAS of these breeds with meat quality and growth trait will most likely result in improved camel meat production and helpful to fulfil requirement of meat industry in Pakistan.
Genomic selection and progress of livestock: In the early stages of QTL mapping, low-density microsatellite markers where used, and by the 2000s, more than 15,000 SNP markers have been developed on the one SNP chip (Khatkar et al., 2008). Today, SNP chips of more than 50,000 SNPs are available for association studies. It is now possible to select animals by using these SNPs markers simultaneously. Genomic selection was first of all proposed by Meuwissen et al. (2001). Many livestock companies are now planning for implementation of genomic selection in breeding and management programs. Application of genomics approach has moved quantitative genetics to molecular genetics and will move genetical genomics to system genetics (Kadarmideen et al., 2006). Genetic progress of livestock can be enhanced by using different practical ways, among which three are most beneficial, namely, (1) precision of selection (2) reduce generation interval and (3) enhance selection intensity. Whole genome selection is one of the effective tools that influence genetic progress as accuracy can be increased and generation interval can be reduced. Along with phenotypic and pedigree information, this approach becomes most powerful and practical for creating breed improvement though changes in DNA.
Application of functional genomics to improve camel meat quality: New opportunities have been created by recent technologies to study complex characteristics of meat quality traits in the camel. Instead of investigating the association of a single gene or DNA marker with a specific trait such as meat quality, researchers are now focusing their interest to reveal gene expression profiles, gene clusters and association that are characteristics of a specific phenotype.
With the improvement of advanced techniques such as proteomics and DNA arrays, DNA microarray and proteomics help in the study of regulatory events that control biological functions by gene expression profiling. For example, gene and protein expression in muscles has been studied in the MUGENE program (Hocquette et al., 2010) in which young Charolais bulls and steers were studied. During this study, new molecular tools such as DNA chips and dot blot quantitative tools were used for analysis of muscle and beef quality (Guillemin et al., 2011). These genomics technologies are now used in meat production sectors (Mullen et al,. 2006). Advanced technologies can be applied in livestock species for improvement of productivity, heath and genetic selection of animal by an integrated use of a range of molecular ‘omics’ tools (Suravajhala et al., 2016).
Modern Interventions to Improve Camel Production: Based on the natural resources and herd mobility, conventional camel farming systems are widespread in the world. Productivity of the camel is very low due to very sluggish reproductive cycle, elongated gestation period (13 months), delayed precocity for reproduction (not earlier than 3 years), extended calving interval (usually 2 years) and high mortality rate. The conventional farming system of camels shows high unpredictability in production and it requires improvement to meet increasing demands for products (Marai et al., 2009). This potential could be achieved by modernizing camel productivity through intensified systems. Modernized camel farming systems show evidence of improved milk and meat production as compared to traditional farming system. As indicated in the previous sections, application of genetics, and increasingly genomics, has the potential to increase camel productivity in Pakistan.
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