Date Published: June 4, 2019
Publisher: Public Library of Science
Author(s): Ewa Wójcik, Małgorzata Szostek, Juan J Loor.
Chromosomal instability is a type of genome instability involving changes in genetic information at the chromosomal level. The basic tests used to identify this form of instability are sister chromatid exchange (SCE) tests and identification of fragile sites (FS). SCE is the process by which sister chromatids become fragmented as a result of DNA strand breakage and reassembly, followed by exchange of these fragments. FS can be observed in the form of breaks, gaps or constrictions on chromosomes, which often result from multiple nucleotide repeats in DNA that are difficult to replicate. The research material was the peripheral blood of ten breeds of cattle raised in Poland, including four native breeds covered by a genetic resources conservation programme, i.e. Polish Red, Polish Red-and-White, White-Backed, and Polish Black-and-White, as well as Polish Holstein-Friesian, Simmental, Montbéliarde, Jersey, Limousine and Danish Red. Two tests were performed on chromosomes obtained from in vitro cultures: SCE and FS. The average frequency of SCE was 5.08 ± 1.31, while the incidence of FS was 3.45 ± 0.94. Differences in the incidence of SCE and FS were observed between breeds. The least damage was observed in the Polish Red and White-Backed breeds, and the most in Polish Holstein-Friesians. The most damage was observed in the interstitial part of the chromosomes. Age was shown to significantly affect the incidence of SCE and FS. Younger cows showed less damage than older ones (SCE: 4.84 ± 1.25; 5.34 ± 1.24; FS: 3.10 ± 0.88, 3.80 ± 0.92).
Chromosomal instability is one type of the genomic instability . It involves changes in genetic information at the chromosome level . These changes may cause disturbances in the functioning of the cell . Basic cytogenetic diagnostic tests used to detect structural chromosomal instability include the sister chromatid exchange (SCE) test and identification of fragile sites (FS). Sister chromatid exchange is a natural phenomenon in the cell cycle [4,5]. SCE is a process involving the exchange of regions of parental strands in duplicated chromosomes between sister chromatids . They are the result of single- and double-stranded DNA breaks that are unrepaired or incorrectly repaired by repair mechanisms, especially homologous recombination. Sister chromatid exchanges take place just after replication and in subsequent stages of the cell cycle, when chromatids are held together in a cohesive complex that ensures the proximity necessary for exchange between homologous DNA sequences . SCE generation may also be caused by non-homologous recombination [8,9]. This system is fast but not accurate, and therefore any errors in repair are visible in the form of exchanges. These errors disturb the integrity of the genetic material of the cell and the organism . The SCE test is a very sensitive technique and should be carried out with great precision and care. The test result provides information on an animal’s health condition. It can also be used as a tool to assess the genomic stability of a given breed or species.
This study was carried out in strict accordance with the recommendations in the Directive 63/2010/EU and the Journal of Laws of the Republic of Poland of 2015 on the protection of animals used for scientific or educational purposes. The study was approved by the Polish Local Ethical Commission, Warsaw, Poland (Number: 51/2015) and by the Polish Laboratory Animal Science Association (Number: 3235/ 2015; 4466/ 2017).
The study evaluated the chromosome stability of ten breeds of cattle raised in Poland. Cytogenetic tests were used to identify instabilities in the cows, i.e. the frequency of sister chromatid exchanges and of fragile sites on chromosomes. Over 4000 metaphases and 240000 chromosomes were analysed.
Cattle are the most widespread livestock animal and the main source of food of animal origin. Over the years there has been a huge increase in cattle production for both milk and meat. This has been possible mainly due to intensive assessment of utility and breeding value as well as selection for milk or meat traits. At the same time, breeders’ exclusive focus on increasing productivity has entailed neglect of the functional features of cattle, leading to a decline in health, reproductive performance and biodiversity, as well as the emergence of defects and genetic diseases. Effective assessment and detection of abnormalities is extremely important, as they pose the greatest risk of economic losses for the breeder.
The analysis of chromosomal instability in cattle using the SCE and FS tests provided a more complete picture for the assessment of the genetic stability of these individuals. These tests also make it possible to assess the genomic stability of individual breeds, providing valuable information to be used in efforts to preserve biodiversity and to improve animals. Moreover, both the sister chromatid exchange test and the fragile site identification test can be used as bioindicators to identify less healthy animals, which have an increased level of damage in their genetic material. Rapid examination of individuals with genetic disorders allows them to be eliminated from breeding early on. This is particularly important for the breeder, in terms of both economics and breeding. The analysis of the prevalence of instability in various breeds, including indigenous breeds, has shown that native breeds have a more stable genome. These breeds can be crossed with other breeds of cattle to improve their genetic stability as well as their functional traits.