OVARIAN PHYSIOLOGY IN A NON-DOMESTICATED BIRD: MECHANISMS BEHIND THE ACCUMULATION AND VARIATION OF YOLK ANDROGENS IN HOUSE SPARROWS

Jeremy R. Egbert

In birds and other oviparous species, the accumulation of maternally-derived yolk androgens may represent an adaptive transgenerational information system. Researchers have found multiple effects of these yolk androgens on offspring in diverse oviparous species, in addition to many social and environmental factors that act on females to alter their yolk androgen levels. What is missing, however, is an understanding of the mechanism by which androgens accumulate in yolk, as well as knowledge of the physiological cause(s) of the immense intra-specific variation in yolk androgens. Using the drug tamoxifen to reduce levels of vitellogenin yolk precursors in house sparrows (Passer domesticus), I tested the hypothesis that androgens enter the yolk by binding to vitellogenin, which is then taken up by the oocyte via receptor-mediated endocytosis. Tamoxifen-treated females laid eggs that were lighter in overall mass and yolk mass, but there was no difference in yolk androgen concentrations or total androgen content between treatments. This suggests that yolk androgen accumulation is not dependent upon vitellogenin uptake. I also tested the hypothesis that between-female variation in yolk androgen concentrations is explained by variation in their capacity to produce steroids. This was accomplished by treating females with gonadotropin releasing hormone (GnRH) to induce maximum androgen production and then determining the expression of four steroid-producing enzymes using quantitative real-time PCR. In house sparrow females, plasma testosterone levels and yolk androgen concentrations of pre-ovulatory follicles collected 30 minutes following injection of GnRH were significantly elevated compared to controls. There was decreased expression of the measured steroidogenic enzymes in the largest follicles relative to less mature follicles, a change that was not affected by GnRH treatment. Variation in enzyme expression in mid-stage pre-ovulatory follicles significantly predicted androgen concentrations in yolks at more advanced stages of development including eggs. This suggests that inter-individual differences in yolk androgen concentrations are explained by differential expression of steroidogenic enzymes between females. While more work needs to be done to determine the mechanisms of yolk androgen accumulation, this is the first study to explain between-female variation in yolk androgen concentrations at the molecular level.

OVARIAN PHYSIOLOGY IN A NON-DOMESTICATED BIRD: MECHANISMS BEHIND THE ACCUMULATION AND VARIATION OF YOLK ANDROGENS IN HOUSE SPARROWS

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