Abstract
Exposure of the vertebrate embryo to maternal hormones can have long-lasting effects on its phenotype, which has been studied extensively by experimentally manipulating maternal steroids, mostly androgens, in bird eggs. Yet, there is a severe lack of understanding of how and when these effects are actually mediated, hampering both underlying proximate and ultimate explanations. Here we report a novel finding that the embryo expresses androgen receptor (AR) and estrogen receptor (ERα) mRNA in its extraembryonic membranes (EMs) as early as before its own hormone production starts, suggesting a novel substrate for action of maternal hormones on the offspring. We also report the first experimental evidence for steroid receptor regulation in the avian embryo in response to yolk steroid levels: the level of AR is dependent on yolk androgen levels only in the EMs but not in body tissues, suggesting embryonic adaptation to maternal hormones. The results also solve the problem of uptake of lipophilic steroids from the yolk, why they affect multiple traits, and how they could mediate maternal effects without affecting embryonic sexual differentiation.
Highlights
In many animal taxa, including vertebrates, the embryo is exposed to maternal hormones, which can have long-lasting effects on its phenotype
We report that both androgen receptor (AR) and ERs are expressed in avian extraembryonic membranes (EMs) (Fig. 2) as early as approximately one-fourth of the entire egg incubation period until hatching, before the embryo’s own hormone production starts[15,16,17], opening up a novel, potential pathway for hormone mediated maternal effects
We found that AR expression is dependent on yolk A4 levels only in the EMs, suggesting embryonic adaptation to its exposure to maternal androgens in the egg as the EMs are right at the interface of maternal yolk environment and embryonic circulation
Summary
In many animal taxa, including vertebrates, the embryo is exposed to maternal hormones, which can have long-lasting effects on its phenotype (fish[1], reptiles[2], birds[3,4,5], mammals[6,7]). We assessed the effect of elevated yolk testosterone (T) and, in other eggs, androstenedione (A4), within the physiological range of the species on AR and ERα expression in the EMs and in embryonic body tissues (the head and the decapitated body) analysed by quantitative PCR (qPCR), using chicken eggs incubated for five days. This time-period was chosen because the gonadal differentiation[15,16] and the surge of the endogenous steroid production[17] in the chicken embryo starts only after this period
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