The effects of steroid hormone treatment during meiotic segregation on primary sex ratios in the domestic chicken
Pinson, Sara Elizabeth
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Evidence suggests that birds bias sex ratios prior to hatch. It is likely that females, as the heterogametic sex, can facultatively control offspring sex ratios through a variety of potential mechanisms. Hormones, as important regulators of reproductive behavior and physiology, have been suggested as potential mediators of primary sex ratio adjustment, but the roles of steroid hormones, such as the stress hormone corticosterone and the reproductive hormone testosterone, are not well understood. Chronic corticosterone had stimulated breeding female birds to produce more female offspring, while chronic or non-targeted increases in testosterone stimulated females to produce more male offspring; however the roles of corticosterone and testosterone or the potential mechanisms had not been determined. We administered corticosterone and testosterone injections to laying hens during meiotic segregation and quantified sexes from the subsequently laid eggs to determine the effects of these hormones on primary sex ratios. Results of these experiments provided a further understanding of the role of steroid hormones in primary sex ratio manipulation in birds. Results presented here suggest that an acute pharmacological elevation of corticosterone beginning at 5 h prior to ovulation stimulated females to produce more male offspring, likely through non-random chromosome segregation; however, if the elevation was not within the pharmacological range or did not occur until 4 h prior to ovulation, there was no significant change in sex ratios, suggesting a critical role of the timing and magnitude of the hormone elevation. Also, an acute increase of testosterone during meiotic segregation stimulated females to produce more male offspring and the sex ratio adjustment likely occurred through non-random chromosome segregation. To our knowledge, until these experiments were conducted, the effects of acute corticosterone or testosterone elevations during meiotic segregation were unknown in the chicken system. The ability to intentionally bias sex ratios or understand the range of factors influencing primary sex ratios, such as circulating maternal steroid hormones, could have profound impacts on the poultry industry as well as captive breeding and conservation programs.