Taking into consideration that the growth hormone (GH) gene in rabbits is a candidate for meat production, understanding the genetic diversity and variation in this locus is of particular relevance. The present study comprised 86 rabbits (Oryctolagus cuniculus) divided into 3 groups: New Zealand White (NZW) outbred rabbits; first-generation inbred rabbits (F 1 ) and second-generation inbred rabbits (F 2 ). They were analysed by polymerase chain reaction-based restriction fragment length polymorphism method. A 231 bp fragment of the polymorphic site of the GH gene was digested with Bsh1236 restriction enzyme. Single nucleotide polymorphisms for the studied GH locus corresponding to 3 genotypes were detected in the studied rabbit populations: CC, CT and TT. In the synthetic inbred F 1 and F 2 populations, the frequency of the heterozygous genotype CT was 0.696 and 0.609, respectively, while for the homozygous CC genotype the frequency was lower (0.043 and 0.000), and respective values for the homozygous TT genotype were 0.261 and 0.391. This presumed a preponderance of the T allele (0.609 and 0.696) over the C allele (0.391 and 0.304) in these groups. In outbred rabbits, the allele frequencies were 0.613 (allele C) and 0.387 (allele Т); consequently, the frequency of the homozygous CC genotype was higher than that of the homozygous TT genotype (0.300 vs. 0.075). Observed heterozygosity for the GH gene was higher than expected, and the result was therefore a negative inbreeding coefficient (Fis=-0.317 for outbred NZW rabbits; -0.460 for inbred F 1 and -0.438 for inbred F 2 ), indicating a sufficient number of heterozygous forms in all studied groups of rabbits. The application of narrow inbreeding by breeding full sibs in the synthetic population did not cause a rapid increase in homozygosity.
The influence of antigen stimulation on the oxidative stress parameters in two groups of rabbits-inbred and outbred were explored by evaluation of the level of lipid peroxidation products (MDA) in the plasma membrane, and the activity of erythrocyte antioxidant defense enzymes superoxide dismutase (SOD) and catalase (CAT). There was not a significant difference between levels of MDA in inbred and outbred rabbits before immunization. However, SOD activity in inbred rabbits was significantly increased in comparison with that of outbred (p = 0.006). Significantly higher plasma levels of lipid peroxidation products were detected in both inbred and outbred rabbits during immune response in comparison to the corresponding groups before immunization (p = 0.008 and p = 0.002). SOD and CAT activities in erythrocytes of rabbits during immune response were also significantly increased compared to that before immunization. In addition, during immune response SOD and CAT activities were found to be positively correlated to each other in both inbred and outbred rabbits (r = 0.727 and r = 0.916). In conclusion, our results suggest the presence of an increased oxidative stress during the antigen stimulation accompanied by an adaptive increase of SOD and CAT activities. 30 days after immunization, the plasma levels of MDA and the activities of SOD and CAT in erythrocytes decreased and reached values close to the controls.
Five rabbit populations of New Zealand White (NZW), Californian (CAL), crossbred NZW×GW and two generations of the synthetic population – SPF<sub>1</sub> and SPF<sub>2</sub> reared in Bulgaria were included in the present study with the aim of detecting the genetic variability of the growth hormone encoding gene (<em>GH</em>) via polymerase chain reaction with the restriction fragment length polymorphism analysis and direct sequencing. The targeted region of the rabbit <em>GH</em> gene was amplified and a fragment of a total of 231 bp was obtained in all studied populations. Allele identification was determined after enzymatic digestion, where two fragments of 62 and 169 bp correspond to allele C and an undigested fragment of 231 bp corresponds to allele T. Two additional bands of 107 and 124 bp evidenced A/G genetic polymorphism in the rabbit <em>GH</em> gene. Thirtyeight percent of the studied rabbits were carriers of the double mutation (C/T+A/G) in the same locus as the studied <em>GH</em> gene. The sequence analysis revealed two nucleotide substitutions – g.111C>T and g.156A>G in the non-coding region between the regulatory TATA box and 5’ UTR region, and a novel g.255G>A genetic variant in intron 1 of GH gene. The A>G transition was most frequent (40.57%), compared to the other ones, G>A (28.57%) and C>T (10.80%), respectively. The most frequent genotype in the NZW population was homozygous TT (0.93), with a prevalence of the T allele (0.97) over allele C (0.03) for g.111C>T SNP site. The distribution of the allele and genotype frequencies at the sites g.156A>G and g.255G>A in this rabbit group was identical, with the highest value of 0.93 for alleles A and G, respectively. The rabbit populations CAL and NZW×GW showed equal frequencies of the prevalent T allele (0.83) and for homozygous TT genotype (0.67) according to g.111C>T SNP. The highest values were obtained for the allele А (0.83) and for homozygous AA genotype (0.67) at c.33A>G SNP in these rabbit groups. The highest values (0.67, 0.60 and 0.80) for the heterozygous genotypes at g.111C>T, g.156A>G and g.255G>A SNPs, respectively, were detected among the SPF<sub>2</sub> rabbit population, compared to the both homozygous genotypes. The results obtained in the present research indicates a significant degree of genetic variability of the studied polymorphic <em>GH</em> locus in the SPF<sub>2</sub> rabbit group.
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