PARENT SESSION
Preimplantation Embryo/Fetal Development

SEX BIAS IN BLASTOCYST FORMATION AND EXPRESSION OF INTERFERON- BY BOVINE EMBRYOS.

Larson, Melissa¹, Kubisch, H. Michael¹, Roberts, R. Michael¹, ¹

ABSTRACT- Conceptus-derived interferon- (IFN-) is responsible for preventing the regression of the CL during early pregnancy in cattle. The first detectable production of IFN- by bovine embryos begins at the blastocyst stage prior to hatching. Preliminary data from this laboratory indicated that female blastocysts release more IFN- than male blastocysts during in vitro culture. Given the role that IFN- plays in maternal recognition of pregnancy, and the fact that some female mammals can bias the sex ratio of their offspring, the goal of this work has been to provide more information about this difference between the sexes. Standard IVM/IVF of slaughterhouse-derived oocytes and standard culture procedures were used throughout. Beginning on d 7 (d 0 = insemination), groups of developing embryos were observed every 12 h, and expanded blastocysts that had formed were removed and placed in individual culture. Medium was replaced every 24 h for assaying the production of IFN-. On d 11, blastocysts were collected and sexed retrospectively with Y-specific primers. The experiment was replicated four times in this study, resulting in 625 medium samples (six replicates comprised of 379 medium samples in an earlier study gave comparable results). Overall means for IFN- production differed by day of oocyte collection (p < 0.01). Of the 269 blastocysts collected, 171 were male and 98 were female (ratio 0.64: 0.36; p < 0.001). No difference was observed in the mean day of blastocyst formation between the sexes (8.4 ± 0.1 d for both male and female embryos; p = 0.5). Male blastocysts were scored of higher quality (p < 0.001) and were more likely to hatch in culture than female blastocysts (p < 0.001). Significantly, female blastocysts produced almost twice as much IFN- as male blastocysts when all samples were combined for analysis (mean IFN- production 883 ± 82 vs. 478 ± 32 pM, respectively, p < 0.001), with production by females exceeding that by males in all four replicates (p < 0.01). When the data were analyzed by hour of culture, female blastocysts secreted more IFN- than males at each time point examined (after 24 h, 48 h and 72 h in culture; p < 0.02). These data confirm that more male bovine IVP embryos reach the expanded blastocyst stage than females, but that female blastocysts produce more IFN- than males. Supported by NIH grant HD 21896.

KEY WORDS: blastocyst, sex ratio, interferon-, bovine