In the present study, SNP genotypes were used to assess the distribution and characteristics of ROH and ROHet and identify selection signatures associated with adaptation within and between three SA indigenous cattle breeds. The distribution and number of ROH observed was consistent with expectations considering the genotypes available for each breed with the DRB (n = 1 118: nROH = 57 885) exhibiting the most ROH, followed by NGI (n = 377: nROH = 13 533) and TUL (n = 214: nROH = 10 260). A high frequency of ROH < 4Mb suggests that these runs likely originated from ancient generations (Peripolli et al., 2018) or recent admixture leading to the breakdown of longer ROH (Liu et al., 2021). However, Ferenčaković et al. (2013), indicated that medium density SNP arrays may overestimate shorter segments due to their limited sensitivity to detection. The high proportion of ROH > 16Mb in the TUL breed suggests that recent inbreeding events may have occurred in this population, given that the TUL breed has the smallest population compared to other indigenous breeds in SA (SA Stud Book, 2022). Despite the low proportions of ROH (ROH > 16 Mb), in NGI (0.101) and TUL (0.249) breeds, routine monitoring is necessary especially in cases where high-impact bulls are used for mating.
The FROH is a reliable metric for assessing autozygosity, providing insights into both recent and ancient inbreeding patterns (Ferenčaković et al., 2013). In the ROH < 4Mb category, higher FROH values, such as DRB (FROH<4Mb = 0.055) and TUL (FROH<4Mb = 0.049), suggest ancient inbreeding (Hulsegge et al., 2022). The FROH values align with the distribution of ROH in the DRB and TUL populations, showing decreasing FROH values with increasing ROH length.
Although ROHet have been linked to specific regions, they remain relatively unexplored compared to ROH (Tsartsianidou et al., 2021). These regions have been associated with loci that prevent the detrimental effects of continuous homozygosity, and the benefits thereof were reported in immune-related genes, as well as in traits related to productivity and reproduction (Chen et al., 2022; Ruan et al., 2022). Typically, ROHet are concentrated in genomic regions associated with disease resistance, where increased diversity can aid populations in addressing potential health challenges, especially when facing novel threats (Sanglard et al., 2021). This is clear in the current study, where ROHet islands were mainly associated with genes related to adaptation and immunity, while ROH islands primarily contained genes associated with reproduction, body size, stature, and production traits.
The high prevalence of ROHet in the DRB aligns with the expectations, as the breed has previously been characterized as an admixed breed with a genetic composition comprising 46% European, 38% African taurine, and 15% indicine ancestry (Makina et al., 2016). Furthermore, the substantial disparity in ROHet prevalence between the DRB population and the other two breeds (NGI and TUL) may be attributed to differences in sample sizes. Unlike several previous studies, such as those conducted by Biscarini et al. (2020) and Lashmar et al. (2022), which reported higher proportions for regions within the ROHet category 0.5–1 Mb, the current study showed that ROHet ≤ 0.25 Mb exhibited the highest mean proportions across all populations. This observation indicates selection favoring heterozygotes, possibly due to admixture within Sanga cattle populations.
Using ROHet approach, the FKBP4 gene was identified in the DRB breed. This gene was previously associated with thermotolerance in Gir cattle, where it plays a role in heat shock protein binding (Saravanan et al., 2021). Heat shock protein (HSP) is a cellular and tissue defense mechanism and is expressed in high volumes during heat shock (Archana et al., 2017). If overexpressed, it protects the animal against hyperthermia during heat stroke which signifies the pivotal role of HSP in cytoprotection (Dangi et al., 2017). The LMAN2 gene identified on BTA 7 using the FST approach in the NGI-versus-TUL pairwise comparison, was also previously associated with functions of heat shock protein binding (GO:0031072) in beef cattle (Ghebrewold, 2018). This gene responds to heat shock through intracellular and extracellular signals, which activate the expression of heat shock proteins such as HSP27 (Shibata, 2014), HSP70 (Bhat et al., 2016) and HSP90 in cattle (Archana et al., 2017). Using the same approach, NMU gene was identified in the NGI-versus-TUL pairwise comparison, previously associated with regulating stress responses and thermoregulation in cattle (Yayou et al., 2009).
Mammalian skin and hair play important roles in providing physical protection and regulating body temperature (Jian et al., 2014). Cattle adapted to tropical environments often display light-colored, sleek, and shiny coats, which effectively reflect a higher proportion of incident solar radiation, helping in reducing heat load (Hansen, 2004). In the current study, the TUBB3 gene was identified in the DRB-versus-NGI pair using the FST approach. This gene, previously associated with coat colour (Taye et al., 2018), is involved in regulating melanogenesis, important for pigmentation and inflammation processes (Taye et al., 2018; Goud et al., 2020). SA Drakensberger cattle are distinguished by their solid black coat colour, although they can also be red (https://drakensbergers.co.za/English/, 2023), while Nguni cattle exhibit a variety of coat colors, including black, brown, and red as primary hues (Olson, 1999). These diverse coat colours carry significant cultural and breed-specific importance, especially among the Nguni ethnic community, influencing their selection preferences (Oosthuizen, 1996; Kunene et al., 2022). The TUBB3 gene has also been associated with high-altitude adaptation in Ethiopian sheep (Edea et al., 2019).
The present study identified candidate genes associated with immunity in three populations using three different approaches. The CYSTM1 gene, observed in both NGI and TUL breeds, has previously been linked to quantitative trait loci (QTLs) responsible for immune response (Fang et al., 2019). The CD14 gene, identified in NGI cattle is important for innate immunity and offers defense against a wide range of pathogens (Pal et al., 2011). Its roles in immune responses to diseases such as glomerulonephritis (Yoon et al., 2003), mastitis (Lee et al., 2003), and treponemiasis (Schröder et al., 2000) have been documented.
In the DRB breed, the WDPCP gene was also identified, associated with inflammatory response mechanisms to infections (de Las Heras-Saldana et al., 2019). Its involvement in collective cell movement and cilia formation has been linked to disease resistance mechanisms in Nelore cattle (Afonso et al., 2020). The DTX1 gene, identified in the NGI population, has been previously linked to the negative regulation of lymphocyte activation, suggesting its role in immune systems (Silva et al., 2022). The ELMO3 gene, located on BTA 18 in DRB, has been associated with phagocytosis and cell migration in postpartum dairy cows (Cheng et al., 2015). The ADAMTS12 gene, known for its participation in inflammatory responses and the regulation of the hepatocyte growth factor (HGF) receptor signaling pathway (Nakamura & Mizuno, 2010), has been associated with adaptation to high altitudes in pigs, suggesting its potential as an adaptive trait in NGI and TUL cattle (Ai et al., 2013).
Using ROH and FST approaches, three lysozyme genes, LYZ1, LYZ2, and LYZ3, were identified on BTA 5 in TUL and DRB-versus-NGI pair, respectively. These genes possess bacteriolytic properties and play diverse roles ranging from digestion to immune response (Wu et al., 2012). Overexpression of these genes have been reported to help sustain mucosal inflammation in the intestines, contributing to host resistance to intestinal worms (Li et al., 2015). CBFA2T3 gene, identified on BTA 18 in the current study, was previously under positive selection in Iraqi cattle (Alshawi et al., 2019). It plays a role in the innate immune response, particularly in response to mammary gland inflammation (Li et al., 2020). The Lymphocyte Antigen 96 gene (LY96), observed on BTA 14 in the DRB-versus-TUL pair, plays an important role in detecting lipopolysaccharide, serving as a pattern recognition receptor and positive regulation of phagocytosis (Dou et al., 2013). This gene is vital in the early identification of pathogens and the activation of immune signaling pathways engaging the adaptive immune response (Dixon et al., 2013). The identification of these genes implies that the three breeds under investigation likely possess a more effective mechanism for recognizing and responding to pathogens.
Several other genes observed were previously associated with immunity in species other than cattle, for example, the ROHet approach identified SLC4A9 gene on BTA 7 in TUL breed, previously linked to saliva secretion (Peña-Münzenmayer et al., 2015). Saliva serves to lubricate the oral mucosa and provide starch degrading enzymes, and the presence of this gene could be important in dry conditions where there is scarcity of water to sufficiently lubricate food and dilute salivary digestive enzymes (Peña-Münzenmayer et al., 2015). Huelsmann et al. (2019), reported the absence, loss, or inactivation of this gene in aquatic mammals, indicating adaptation to aquatic environments. Similarly, the EXOC3L1 gene, associated with several types of immune cells in humans (Zhang, 2023), was detected on BTA 18 in the DRB breed using same approach. Through the FST approach, the DEF8 gene was observed in proximity to the MC1R gene known for its role in influencing human pigmentation traits (Siiskonen et al., 2016). Using ROH approach, MYO1G gene was detected, primarily expressed in lymphocytes and associated with phagocytosis and exocytosis processes in mice (Maravillas-Montero et al., 2014). Furthermore, the current study observed DDX56 gene on BTA 4 in TUL using ROH approach, previously linked to an antiviral role against viruses such as Sindbis virus in humans (Taschuk et al., 2020). Similarly, MYO1F gene identified on BTA 7, known for its significance in immune cell motility and innate host defense against infection with Listeria monocytogenes (Kim et al., 2006; Sun et al., 2021) was observed in the TUL breed. Further research is warranted to validate these genes in cattle and other livestock species, given that many of them were initially identified in humans and mice. This cross-species validation could elucidate their functional significance and potential applications in beef production, and it may provide insights into adaptive mechanisms and breed-specific traits.
ROH analysis indicated varying numbers of ROH and lengths, providing insights into population history and recent admixture. The prevalence of shorter ROH was associated with potential selection signatures and ancient inbreeding. The presence of ROHet was also explored, revealing associations with traits related to thermo-tolerance and disease resistance. Selection signatures were identified based on FST, ROH, and ROHet, indicating candidate genes associated with traits of economic importance. Overall, this study provides valuable insights into the distribution of ROH and ROHet, as well as selection signatures within South African indigenous cattle breeds. It also gives insights into the domestication and evolutionary mechanisms that have fostered the diversification of numerous livestock breeds, adaptable to diverse environments and production systems, while also holding promise for enhancing livestock health, productivity, and resilience in the face of climate change.
Data availability
The datasets generated and/or analyzed during the current study are not publicly available but are available from the corresponding author on reasonable request.