Blastocyst development and conceptus sex selection in red deer Cervus elaphus: studies of a free-living population on the Isle of Rum.

Skewing of the sex ratio at birth occurs in red deer in response to dominance status, with dominant hinds giving birth to a higher proportion of male calves than subordinates. To investigate the physiological basis for this phenomenon, reproductive tracts were collected from red deer during a cull for management purposes carried out on the Island of Rum, Scotland. Blastocysts were flushed from the uterus and sexed by polymerase chain reaction using Y chromosome-specific primers. Concentrations of interferon (measured as antiviral activity) in uterine flushings, of oxytocin receptors in endometrium, and of progesterone in jugular venous blood were measured, and ovarian morphology was recorded. Times of mating were determined retrospectively from calving dates observed during the following spring. Changes in uterine and fetal weights and sizes confirmed the degree of reproductive synchrony. Intervals between stages of blastocyst development (spherical, tubular, filamentous, and attached) derived from the observed incidence of each form showed that approximate times of blastocyst elongation and attachment were 13 and 30 days after conception, respectively. Hinds carrying male blastocysts were in better body condition (higher kidney fat weights, P = 0.025) than those carrying females. Interferon was detectable in uterine flushings from 1 of 7 hinds carrying early filamentous blastocysts and 5 of 12 hinds carrying late filamentous blastocysts, but in no case where the blastocysts were male (P = 0.035). Oxytocin receptor concentrations in caruncular endometrium (but not in intercaruncular endometrium) were lower in pregnant than in nonpregnant hinds (P < 0.05), but there was no correlation with interferon concentrations in flushings. Corpora luteal concentrations of oxytocin ranged from 1.8 to 51.2 micrograms/g tissue and declined with advancing blastocyst development. The data are consistent with the hypothesis that sexual dimorphism in trophoblast interferon production leads to differential blastocyst loss and hence to sex ratio skewing on the basis of dominance status.