EFFECT OF CRYOPROTECTANT AND GENETIC SELECTION FOR BODY-FAT CONTENT ON EMBRYONIC CRYOSURVIVAL IN MICE

Lines of mice selected for high (HF) or low (LF) 12-week epididymal fa t pad weight as a percentage of body weight were used to investigate t he effects of genotype, two cryoprotectants [glycerol (GLY) and propyl ene glycol (PG)] and genotype x cryoprotectant interaction on cryosurv ival of four and eight-cell embryos. Embryos were collected from selec tion lines and reciprocal crosses of selection lines (HFLF and LFHF) a nd frozen by established slow-cool methods. Embryos were thawed for 40 s at room temperature and then placed in a 37 degrees C waterbath for 1 min. Cryoprotectant was diluted from embryos with either 0.5 M sucro se (GLY-treated) or 1.0 M sucrose (PG-treated). Post-thaw survival was measured as the percentage of embryos developing to 36 h (PTS36), 48 h (PTS48) and hatched blastocyst (PTSHB), respectively. Non-frozen con trols were cultured concurrently with frozen embryos. No significant g enotype or genotype x cryoprotectant interaction effects were found. R esults of the embryo freezing study indicated that selection for high or low fat content did not affect the ability of embryos to survive cr yopreservation. There was no indication of embryo heterosis for post-t haw survial. Embryos frozen with GLY survived the freeze-thaw stress s ignificantly better than those frozen in PG (P < 0.05). In vitro devel opment of non-frozen controls at 36 and 48 h did not vary significantl y among lines, but in vitro development was significantly different am ong lines at the hatched blastocyst stage (P < 0.05). Linear contrasts showed that the embryonic genome was responsible for differential in vitro development at the hatched blastocyst stage between these select ed lines (HF > LF; P < 0.05); asymmetric response also occurred in tha t both HF and LF exceeded the unselected control line (P < 0.05).