Genomic tools are now available for most livestock species and are used routinely for genomic selection (GS) in cattle. One of the most important developments resulting from the introduction of genomic testing for dairy cattle is the application of reasonably priced low-density single nucleotide polymorphism technology in the selection of females. In this context, combining genome testing and reproductive biotechnologies in young heifers enables new strategies to generate replacement and elite females in a given period of time. Moreover, multiple markers have been detected in biopsies of preimplantation stage embryos, thus paving the way to develop new strategies based on preimplantation diagnosis and the genetic screening of embryos. Based on recent advances in GS, the present review focuses on new possibilities inherent in reproductive technologies used for commercial purposes and in genetic schemes, possible side effects and beneficial impacts on reproductive efficiency. A particular focus is on the different steps allowing embryo genotyping, including embryo micromanipulation, DNA production and quality assessment.
Heterozygous carriers of Robertsonian translocations generally have a normal phenotype but present reproductive failure. In cattle, the t(1;29) Robertsonian translocation is very common and carriers show a 3–5% decrease in fertility. Some data suggest that female carriers have a higher decrease than male carriers but no direct studies of the chromosome content of oocytes from a t(1;29) carrier cow have been performed so far. Four heterozygous carrier cows underwent hormonal stimulations and follicles punctions and about 800 oocytes were matured in vitro. Six hundred metaphase II preparations were obtained and analysed by fluorescent in situ hybridization with bovine chromosome 1 and 29 painting probes. Proportions of different kinds of oocytes were assessed: 74.11% (292/394) were normal and balanced, 4.06% (16/394) unbalanced and 21.83% (86/394) diploid. For all cows, the number of normal oocytes was not significantly different from the number of translocated oocytes but the diploidy and unbalanced rate were significantly different between them. As found in bulls, the meiotic segregation pattern in cows has shown a preponderance of alternate products. However, the frequency of unbalanced gametes determined in females (4.06%) was significantly higher than the frequency observed in males (2.76%). The divergence in the rate of diploid gametes (0.04% vs. 21.83%) is mainly explained by the difference between males and females.
Rapid genetic improvement in cattle requires the production of high numbers of embryos of excellent quality. Increasing circulating insulin and/or glucose concentrations improves ovarian follicular growth, which may improve the response to superovulation. The measurement of anti-Müllerian hormone (AMH) can help predict an animal's response to superovulation treatment. The aim of the present study was to investigate whether increasing circulating insulin concentrations, through propylene glycol (PG) drenches, could improve in vitro embryo production in oestrus-synchronised superovulated heifers with different AMH profiles. Holstein heifers were grouped according to pre-experimental AMH concentrations as low (L) or high (H). The PG drench increased circulating insulin and glucose concentrations and reduced β-hydroxybutyrate and urea concentrations compared with the control group. AMH was a good predictor of follicle and oocyte numbers at ovum pick-up (OPU), and of oocyte and embryo quality (AMH H>AMH L). PG in the AMH H group increased the number of follicles and blastocyst quality above that in the control group, but did not improve these parameters in the AMH L group. These results indicate that short-term oral PG supplementation modifies an animal's metabolic milieu and is effective in improving in vitro embryo production, after superovulation-OPU, more markedly in heifers with high rather than low AMH concentrations.
Genomic tools are now available for most livestock species and used routinely for marker-assisted selection (MAS) in cattle. The detection of a large number of markers that are widespread over the genome is generally limited by the amount of genomic DNA available in an embryo biopsy of a small size not to be detrimental to embryonic survival. Amplification of DNA from such a biopsy is then necessary. In this study, the efficiency of embryo genotyping for 45 microsatellites (MS) following whole-genome amplification (WGA) was evaluated from samples of a variable number of cells isolated from cattle embryos. In a second part, this work aims to test the reliability of the MAS method for 45 MS and 13 single nucleotide polymorphisms (SNP) from bovine embryo biopsies under field conditions. In experiment 1, in vitro bovine morulae (n = 10) were produced, and 1, 5, and 10 embryonic cells were removed from each morula. Cells were dry frozen in tubes before further processing. Whole-genome amplification was performed using the commercial Qiagen REPLI-g® Mini Kit according to the manufacturer instructions (Qiagen, Valencia, CA, USA). WGA solution was then diluted, processed by PCR with 45 markers, and the resulting data were genotyped with GeneMapper software® (Applied Biosystems Europe). Accuracy and reliability of genotyping were assessed using different samples of cells from the same embryo. In experiment 2, after superovulation (10 cows), bovine embryos were in vivo-produced and collected at day 6 or day 7 of pregnancy. Only grade 1 embryos were washed and biopsied using a microblade. Biopsied embryos were either frozen or transferred back to synchronized recipients. Individual biopsies were transferred as dry samples to the laboratory. Genomic DNA was amplified using WGA, and embryos were genotyped. The results of experiment 1 clearly indicate that a conventional biopsy of 5 to 10 cells was sufficient for multi-markers detection after whole-genome amplification as 98% of the 45 markers were detected compared to 45% of marker detection using 1 cell (P < 0.01). In experiment 2, from 123 collected embryos, 79 were classified as grade I or II transferable embryos (64.2%) and 57 were biopsied (34 were classified as stage 4–5 and 23 as stage 5–6, according to the IETS criteria). Using the stereomicroscopic analysis, 44 biopsies had a number of cells ranging from 4 to 7 (5.6 ± 1.4) and 13 biopsies from 8 to 10 (8.4 ± 1.6). Overall, at least 95% of markers (MS + SNP) were detected in 49.1% of biopsies (28/57). The total detection rate for SNP was significantly higher than for MS; 70.2% (40/57) v. 31.6% (18/57), respectively, (chi-square, P < 0.01). The detection rate of the markers was not significantly affected by the embryo stage or the biopsy size. Our results confirm that genotyping a large number of markers from biopsy samples after whole-genome amplification is possible under field conditions. A larger number of biopsies is required to assess the reliability of this method that may allow the development of MAS from early embryo. This work has been performed through the programme TYPAGENAE (GENANIMAL 4-03) with the financial support of FRT/ANR and Apis-Genes.
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