Yolk androstenedione in domestic chicks (Gallus gallus domesticus): Uptake and sex-dependent alteration of growth and behavior

• Despite interest in yolk androgens, uptake and effects of yolk androstenedione (“A4”) have been neglected.
• Yolk A4 was detectable in male and female embryos at days 7 and 16 of incubation.
• Elevated yolk A4 increased skeletal growth in females but not males.
• A4 reduced comb size in males only; androstenedione reduced tonic immobility in both sexes.
• Sex-dependent enzyme expression and activity may explain sexually dimorphic responses.

In birds, causes and consequences of variation in maternally-derived steroids in egg yolk have been the subject of intense experimentation. Many studies have quantified or manipulated testosterone (“T”) and one of its immediate precursors, androstenedione (“A4”) – often lumping the two steroids as “androgens” and treating them as functionally equivalent. However, yolk A4 is deposited in substantially higher concentrations than T, binds only weakly to the androgen receptor, and is readily converted into either T or estrone by steroidogenic enzymes present during embryonic development. Thus it may not be appropriate to assume that A4 has the same effect as T. In addition, A4’s metabolic fate is likely to differ between females and males. The goals of this study were to examine the sex-specific uptake and metabolism of yolk A4 and consequences of elevated levels of yolk A4 on development and behavior of domestic chicks. Eggs were injected with 2μ Ci of tritiated androstenedione; radioactivity was detected in all tissues of day 7 and day 16 embryos and found in both aqueous and organics phases of day 7 yolk, with no difference between sexes. A second set of eggs was injected with 125 ng of A4. A4 increased growth of morphological traits (tarsus, beak) in females, but not males. A4 males had smaller combs than controls; there was no treatment effect in females. A4 reduced tonic immobility behavior in both sexes. The results of this study illustrate the importance of distinguishing both between androgens and between sexes when investigating avian endocrine maternal effects.