26 ASSESSMENT OF CHROMOSOME ABNORMALITIES IN SHEEP PARTHENOGENETIC AND NUCLEAR TRANSFER EMBRYOS: EFFECT OF 6-DMAP AND CYCLOHEXIMIDE ON PLOIDY

Abstract

In current somatic cell nuclear transfer (NT) protocols, the reconstructed embryos are activated by incorporation of secondary oocyte activation compounds such as 6-DMAP or cycloheximide (CHX). The effects of these compounds on the chromosome complement of sheep NT embryos have not been studied in detail. Therefore, the aim of this study was to assess the chromosome abnormalities using sex chromosome specific probes of Day 6 blastocyst-stage sheep embryos produced from parthenogenetic activation and NT. Following 20–22 h of IVM, the oocytes were activated by electric pulsing followed by 30-min culture in cytochelasin B. They were reactivated using ionomycin (5 min) followed by 2-h culture in 6-DMAP or CHX. In contrast, NT embryos were produced using standard NT procedures using male sheep fetal fibroblasts. Reconstructed embryos were activated using the same methods described earlier. The embryos (compact morulae and blastocysts) were fixed and subjected to FISH analysis using cattle X and Y chromosome painting probes. The data were analyzed using Fisher's exact test. Of the parthenogenetic embryos (6-DMAP, n = 28; CHX, n = 32) analyzed, none of the embryos was totally haploid (X) or totally polyploid. When all of the nuclei per embryo were considered, normal (XX) genotype embryos were 6.2% and 0.0% in CHX and 6-DMAP groups, respectively. The rest of the embryos were abnormal due to mixoploidy (100% vs. 93.8%, P < 0.05) in 6-DMAP and CHX treatment groups, respectively. The abnormal nuclei per embryo ranged from 7.3% to 72.2%. The mean total cell number of parthenogenetic blastocysts was 91.2 ± 4.3 and 81.8 ± 6.2 (mean ± SE) in 6-DMAP and CHX, respectively. Among NT embryos analyzed, (6-DMAP, n = 30; CHX, n = 32) only 40.0% and 43.8% of embryos were completely normal for XY chromosomes in 6-DMAP- and CHX-treated groups, respectively. The rest of the embryos were abnormal due to mixoploidy (60.0% vs. 56.2%, P > 0.05) in 6-DMAP and CHX groups, respectively. Monosomy (XO or OY), trisomy (XXY), and tetrasomy (XXYY) were the common abnormalities detected in mixoploid embryos. The abnormal cells per embryo ranged from 3.8% to 41.8% in both treatment groups. The mean total cell number of NT blastocysts was 71.2 ± 9.8 and 63.8 ± 8.4, in 6-DMAP and CHX treatment groups, respectively. In conclusion, the 6-DMAP-treated embryos derived from parthenogenetic activation had significantly higher chromosomal abnormalities than CHX-treated embryo groups (P < 0.05). In contrast, the NT embryos derived from either 6-DMAP or CHX treatment did not show any significant difference in producing chromosomally abnormal embryos at the blastocyst stage. This study also highlights the feasibility of using bovine chromosome painting probes on ovine embryo spreads.

This work was supported by NSERC, OMAFRA, and ICCS.