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Cytogenetic Characterisation of Nattukuttai - A Non-descript Cattle Population of Tamil Nadu
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First Online 13-08-2021|
Methods: Blood samples from ten Nattukuttai cattle (five males and five females) were utilized to study the chromosome profile through short-term lymphocyte culture method. Good metaphase spreads were selected for estimation of the relative length, arm ratio, centromeric index and morphological index.
Result: The diploid number was 60. All the 29 pairs of autosomes and Y-chromosome were acrocentric while X-chromosome was sub-metacentric. The mean relative length of autosomes ranged from 5.24 ±0.08 to 1.90±0.06. X-chromosome was the largest in the karyotype (5.64±0.12), while the Y-chromosome was the smallest (1.85±0.03). The arm ratio, centromeric index and morphological index were 1.98±0.02, 0.33±0.03 and 4.06±0.4 respectively. The study revealed that the chromosome architecture of Nattukuttai cattle was similar to that of other breeds of Zebu cattle.
Characterisation of animal genetic resources at phenotypic level involves identification of distinct breed populations, description of their typical physical and production characteristic and documentation of any unique features in terms of adaptation and production (FAO, 2012). Molecular genetic characterisation, commonly undertaken using microsatellite markers, explores genetic variation between and within animal populations and facilitates in determination of evolutionary relationship among animal populations. FAO and International Society of Animal Genetics-FAO Advisory Group on Animal Genetic Diversity proposed panels of thirty microsatellite markers for cattle (FAO, 2011) and recommended the usage of all thirty microsatellites for genetic diversity analysis. Apart from phenotypic and molecular studies, cytogenetic studies are also incredibly useful in genetic characterisation and effective conservation of any species (Benirschke and Kumamoto, 1991).
Karyological methods are useful in differentiating cattle of exotic origin (Bos taurus) from indigenous cattle (Bos indicus) based on Y-chromosome polymorphism. Y-chromosomes of Bos indicus and breeds derived from Bos indicus bulls are acrocentric while those of Bos taurus, Sanga and breeds derived from these bulls are metacentric or submetacentric (Potter and Upton, 1979). The morphological difference between Bos taurus and Bos indicus Y-chromosome is the consequence of pericentric inversion (Pinheiro et al., 1980). Hence, karyotyping can be employed to take culling decisions at farms where cross-bred animals are required to have an exotic sire line. Karyotyping is also used in detecting numerical and structural abnormalities, chromosomal damage or irregularity in cell cycle which indicate toxicity and carcinogenic activity (Wójcik and Szostek, 2019). However, there are certain limitations towards the use of conventional karyotyping because it requires the culture of living cells and hence many factors may lead to failure in obtaining results, e.g. delay in transport of blood sample, exposure to extreme temperature, bacterial contamination, low lymphocyte count within the sample and low resolution limit. Despite these limitations, conventional banded karyotyping is recognized as the gold standard for detection of chromosomal abnormalities. Chromosomal studies are available for lesser number of breeds; hence the present study was undertaken to characterise Nattukuttai genetic group by cytogenetic norms with focus on chromosome morphometrics and cytogenetic screening.
MATERIALS AND METHODS
Slides containing good quality metaphase spreads were selected from further processing. Vernier caliper (Mitutoyo, Japan) was used for measuring the length of short arm (p), long arm (q) and total length of chromosomes. Arm ratio, centromeric index and morphological index were estimated. The relative lengths of each chromosome were measured as the percentage of it to the total haploid genome length (excluding Y-chromosome).
RESULTS AND DISCUSSION
The relative length of chromosomes descends uniformly as can be seen in the idiogram (Fig 2 and Fig 3). The mean relative length of autosomes for combined population varied from 5.24 to 1.90 per cent (Table 1) which is in accordance with the estimates reported by Girija (1994) in Vechur (5.431 to 1.757), Kumarasamy et al., (2008) in Umblachery (4.637 to 1.850), Bharathi et al., (2015) in Punganur (5.34 to 1.69), Longkumer et al., (2015) in Tho-Tho cattle (5.31 to 1.86), and Bharti et al., (2017) in Ongole breed (5.24 to 1.92). The X-chromosome contributed 5.64 per cent to the total genome which is higher than 5.002 per cent as observed in Umblachery (Kumarasamy et al., 2008) and 4.81 per cent as observed in Punganur (Bharathi et al., 2015); but in agreement with estimates of 5.591 in Vechur (Girija, 1994), 5.53 in Tho-Tho cattle (Longkumer et al., 2015) and 5.42 in Ongole (Bharti et al., 2017). The Y-chromosome had a relative length of 1.68 per cent, which is comparable to the previous reports, except Vechur, where the relative length was reported as 2.875 by Girija (1994).
The mean arm ratio, centromeric index and morphological index were 1.98, 0.33 and 4.06 respectively. The arm ratio value of X-chromosome in the present study is similar to values reported for Vechur (2.182), Malnad Gidda (2.12) Umblachery (2.035) and Ongole (1.87); but higher to value reported in Punganur (1.55) by previously mentioned researchers. Arm ratio of more than 1.00 also confirmed the submetacentric nature of X-chromosome. The value for centromeric index in Nattukuttai cattle is in agreement with values reported in the mentioned breeds. The morphological index, however, is lower in Nattukuttai chromosomes as compared to Punganur (5.12) and Ongole (5.25).
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