Genetic control of survival of frozen mouse embryos

Lines of mice selected for increased litter size (L+), increased body weight (W+), or randomly (K) were used to study genetic variation in embryo cryosurvival in response to standard cryopreservation protocols. A total of 60528-cell embryos from 400 females were used in two studies. In Study 1, embryos from L+, W+, and K were frozen by slow-cool and ultrarapid (direct-plunge) methods to evaluate effects of selection on cryosurvival and genotype X freezing method interaction. Post-thaw survival (PTS) was measured as percentage of recovered embryos developing in vitro to blastocyst per donor female. Nonfrozen control embryos developed similarly for each line. Within slow-cool freezing, lines differed (W+ greater than K, W+ = L+, L+ = K; p less than 0.05); no differences were observed within the ultrarapid freezing. However, line X method interaction effects on PTS were not significant. In Study 2, reciprocal crosses were made between L+ and K and between W+ and K. Hybrid and pure line embryos were frozen by slow-cooling. Control embryos developed similarly for all genotypes. Selection lines did not differ for overall PTS. However, hybrid embryos from L+ dams were superior to those from K dams (84 vs. 61%; p less than .001). No overall embryo heterosis was observed. Differences were not significant among embryo genotypes or treatments for cell number or in vivo survival. These results demonstrate significant correlated responses in embryo post-thaw cryosurvival due to selection, and implicate both maternal and embryonic genomes as controlling mouse embryo cryosurvival.