In birds, exposure to maternal (yolk) testosterone affects a diversity of offspring post-hatching traits, which eventually affect offspring competitiveness. However, maternal testosterone is heavily metabolized at very early embryonic developmental stages to hydrophilic metabolites that are often assumed to be much less biologically potent. Either the rapid metabolism could either keep the maternal testosterone from reaching the embryos, opening the possibility for a mother–offspring conflict or the metabolites may facilitate the uptake of the lipophilic testosterone from the yolk into the embryonic circulation after which they are either converted back to the testosterone or functioning directly as metabolites. To test these possibilities, we injected isotope-labeled testosterone (T-[D5]) into the yolk of freshly laid Rock pigeon (Columba livia) eggs and determined the concentration and distribution of T-[D5] and its labeled metabolites within different egg fractions by liquid chromatography combined with tandem mass spectrometry at day 2, 5 and 10 of incubation. Although under a supraphysiological dosage injection, yolk testosterone decreased within 2 days and was metabolized into androstenedione, conjugated testosterone, etiocholanolone and other components that were unidentifiable due to methodological limitation. We show for the first time that testosterone, androstenedione and conjugated testosterone, but not etiocholanolone, reached the embryo including its brain. Their high concentrations in the yolk and extraembryonic membranes suggest that conversion takes place here. We also found no sex-specific metabolism, explaining why maternal testosterone does not affect sexual differentiation. Our findings showed that maternal testosterone is quickly converted by the embryo, with several but not all metabolites reaching the embryo providing evidence for both hypotheses.
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