Effects of rate of development of in vitro-produced ovine embryos on sex ratio and in vivo survival after embryo transfer

Two experiments were designed to determine the effects of embryonic sex on the rate of development of in vitro-produced ovine embryos and the effects of transferring early developing ovine blastocysts on the sex ratio of the lambs born. In Experiment 1, in vitro matured and fertilized (IVM/IVF) ovine oocytes were cultured in synthetic oviduct fluid (SOF) supplemented with BSA and amino acids, and assessed for blastocyst formation daily from 6 to 9 d post insemination. The embryos were assigned to 5 developmental stages: early blastocyst (EB), blastocyst (B), expanding or expanded blastocyst (ExB), hatching blastocyst (HgB) and hatched blastocyst (HB). A total of 99 blastocysts were produced, 76 of which were successfully sexed by polymerase chain reaction (PCR) and 80 assessed for cell number. Overall the male:female sex ratio (45:31) of in vitro produced blastocysts was not different from the expected 1:1 ratio. Twenty-six of the 50 embryos (52%) first observed as blastocysts on Day 6 were at the advanced stages (ExB, HgB and HB) and were designated Early Day 6; of these, 18 were sexed and 83.3% were male (significantly different from the expected 1:1 sex ratio; P<0.01). Slower developing Day 6 blastocysts (B and EB; Late Day 6), and all blastocyst stages first observed on Days 7, 8 and 9 did not differ from the 1:1 sex ratio. The mean cell number of blastocysts from the Early Day 6 group was higher than that of blastocysts first seen on Day 7 (P<0.05) or Days 8 and 9 (P<0.01). Overall, male blastocysts had a higher mean cell number than the female blastocysts (85.7 +/- 8.54 and 54.5 +/- 4.67, mean +/- SEM, respectively; P<0.05). In Experiment 2, 90 blastocysts first seen on Day 6 or 7 were transferred in pairs to recipient sheep and their in vivo survival and the resultant sex of lambs recorded. Of the 48 blastocysts derived from the Early Day 6 group that were transferred, 22 (45.8%) survived to birth, 8 (36.4%) of which were males. Twelve lambs were born derived from transfer of 31 Late Day 6 blastocysts (38.9% survival), 4 of which were males (33.3%). One female lamb was born from 11 (9.1% survival) embryos derived from blastocysts first seen on Day 7, In all 3 groups, more females than males were born, bur the ratios were not significantly different from the expected 1:1 ratio. It would appear from these findings that our in vitro culture system enhances the growth rate in male compared with female ovine embryos. However, the transfer of early developing in vitro-produced embryos did not alter the sex ratio of offspring in favor of the males. (C) 1997 by Elsevier Science Inc.