Evidence of altered primary sex ratios in birds shows that mothers can manipulate the sex of their offspring before oviposition. In birds, females are the heterogametic sex (ZW) and males are homogametic (ZZ). Sex is determined in the first meiotic division, when one sex chromosome is retained in the oocyte and the other segregates to the polar body. Altered primary sex ratios suggest that birds may be capable of biasing the segregation of sex chromosomes during meiosis I. During the time of meiosis I, follicular steroid production is limited primarily to progesterone (P4). We experimentally manipulated the levels of P4 in female domestic chickens during the approximate time of meiosis I. We advanced the ovulation of the first egg of a sequence (or clutch) with a subcutaneous injection of P4. We found a significant effect of P4 dose on the sex of the resulting egg. The high progesterone group produced 25% males whereas the low progesterone group produced 61% males and the control group produced 63% males in the first ovulation of the sequence. We propose that variation in maternal progesterone during the critical time for genetic sex determination is the mechanism for primary sex ratio manipulation in birds.
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