Sexual dimorphism in interferon‐τ production by in vivo‐derived bovine embryos

Interferon‐tau (IFN‐τ) is an anti‐luteolytic factor responsible for preventing regression of the maternal corpus luteum (CL) during early pregnancy of cattle. In vitro‐produced (IVP) bovine embryos first produce IFN‐τ as blastocysts. In the present study, we have examined whether sexually dimorphic production of IFN‐τ, which is observed among IVP blastocysts, also occurs among in vivo‐produced blastocysts, and whether this difference between the sexes persists to day 14 when silencing of one of the X‐chromosomes in the trophectoderm is complete. Embryos were flushed from cattle that had been superovulated and bred by AI. Blastocysts (63 male, 62 female) recovered between days 8.5 and 9.5 of pregnancy, were cultured individually. No differences were observed between males and females in either their developmental stage or quality at the beginning, during, and at the end of culture. Female embryos produced more IFN‐τ than males by 24 hr (mean values, males: 16.6 ± 3.7, females: 49.4 ± 9.0 pg per embryo; P < 0.05) and 48 hr (male: 189.8 ± 37.1, female: 410.9 ± 66.6 pg per embryo; P < 0.05). However, the variability in IFN‐τ production between individual blastocysts was so great that IFN‐τ secretion is unlikely to be of value as a non‐invasive means to predict embryo sex. When conceptuses were recovered at day 14, elongating males (n = 25) and females (n = 24) were similar in dimension and did not differ in their IFN‐τ production after 4.5 hr (male: 2,550 ± 607, female: 2,376 ± 772 ng per conceptus) and 24 hr (male: 12,056 ± 2,438, female: 8,447 ± 1,630 ng per conceptus) of culture. Thus, sexual dimorphism in IFN‐τ production is observed in both IVP and in vivo‐produced expanded blastocysts, but is lost by day 14 of in vivo development. Mol. Reprod. Dev. 67:193–199, 2004. © 2004 Wiley‐Liss, Inc.