Meeting Abstract
P2.169 Monday, Jan. 5 Embryonic modulation of yolk steroids in European starlings (Sturnus vulgaris) PAITZ, R.T.; BOWDEN, R.M.; CASTO, J.M.*; Illinois State Univ; Illinois State Univ; Illinois State Univ jmcasto@ilstu.edu
Sex steroids influence sexual differentiation during critical stages of vertebrate embryonic development. In birds, steroids of maternal origin are present in the yolk at oviposition and affect various aspects of offspring development, but do not influence sexual differentiation. Steroids in bird egg yolks decline markedly during early incubation via unknown mechanisms. Our goal was to determine if embryos actively modulate yolk steroid signals during early incubation. First, testosterone (T) and estradiol (E2) levels in yolk and albumen of European starling eggs were characterized at oviposition. Next, other eggs were incubated and yolk/albumen (YA) and whole embryo homogenates were sampled throughout embryonic development to track changes in progesterone (PROG), T, and E2 levels. Finally, tritiated T was injected into eggs and radioactivity was measured in extracted organic and aqueous fractions of both YA and embryo homegenates on embryonic day 6 (E6). Results indicate that at oviposition, T and E2 occur in yolk but not albumen, and yolk T concentration varies significantly among clutches. Across development, E2 remains low or undetectable, PROG and T concentrations decline in YA homogenates, and are negatively correlated with embryo mass. Presence of radioactivity in aqueous fractions of both YA and embryo homogenates on E6 suggests that some tritiated T was converted to a water-soluble form, the amount of which positively correlates with embryo mass. Collectively the results indicate that starling embryos do actively modulate yolk testosterone during early development via metabolism of lipid-soluble steroid into a water-soluble metabolite. This steroid shunt likely buffers embryos from the influence of yolk steroids during periods when sexual differentiation could otherwise be influenced.
