Early embryonic modification of maternal hormones differs systematically among embryos of different laying order: A study in birds

Abstract

Vertebrate embryos are exposed to maternal hormones that can profoundly affect their later phenotype. Although it is known that the embryo can metabolize these maternal hormones, the metabolic outcomes, their quantitative dynamics and timing are poorly understood. Moreover, it is unknown whether embryos can adjust their metabolic activity to, for example, hormones or other maternal signals. We studied the dynamics of maternal steroids in fertilized and unfertilized rock pigeon eggs during early incubation. Embryos of this species are naturally exposed to different amounts of maternal steroids in the egg according to their laying position, which provides a natural context to study differential embryonic regulation of the maternal signals. We used mass spectrometric analyses to map changes in the androgen and estrogen pathways of conversion. We show that the active hormones are heavily metabolized only in fertilized eggs, with a corresponding increase in supposedly less potent metabolites already within one-fourth of total incubation period. Interestingly, the rate of androgen metabolism was different between embryos in different laying positions. The results also warrant a re-interpretation of the timing of hormone mediated maternal effects and the role of the supposedly biologically inactive metabolites. Furthermore, the results also provide a potential solution as to how the embryo can prevent maternal steroids in the egg from interfering with its sexual differentiation processes as we show that the embryo can metabolize most of the maternal steroids before sexual differentiation starts.

Avian yolk androgens are metabolized instead of taken up by the embryo during the first days of incubation

“…Since the decline of androgen concentrations was found throughout the egg, it cannot be due to dilution with albumen or water, or embryonic uptake. This indicates that the maternal T and A4 are metabolized by embryonic and/or maternal enzymes, which is in line with the suggestions made in other studies (Gilbert et al, 2007;Kumar et al, 2018a;Parsons, 1970;Vassallo et al, 2014), as explained below. This raises the intriguing question as to which metabolic products these androgens are converted into, as this will determine the biological significance of maternal hormone deposition.…”

“…2) at such an early stage of embryonic development as aromatase is expressed only after day 5 of incubation in chicken (Nomura et al, 1999), which is also experimentally supported by the lack of such early androgen conversion to estrogens (Parsons, 1970). Parsons (1970) suggested 5β-androstane-3α,17β-diol as the main metabolite in an in vitro study on metabolism of radiolabelled T by the 2-dayold chicken embryo; suggested conjugated etiocholanolone as the main metabolite in the yolk albumen homogenates of European starling eggs incubated for 6 days; and Kumar et al (2018a) showed etiocholanolone and its conjugate as main androgenic metabolites in rock pigeon eggs incubated for 4.5 days. These studies indicate a 5β-reduction, followed by 3α-hydroxylation metabolic route for maternal T and A4 (Fig.…”

“…The majority of maternal hormones are deposited in the yolk, but the yolk becomes mixed with albumen early during incubation. Therefore, the decline in yolk hormone concentrations reported in the literature (Bowden et al, 2002;Eising et al, 2003;Elf and Fivizzani, 2002;Feist and Schreck, 1996;Paitz and Bowden, 2009;Wilson and McNabb, 1997) might be due not to metabolism but rather to the dilution of yolk, for which no evidence was found in European starling and rock pigeon (Kumar et al, 2018a) eggs, but is as yet unknown for chicken eggs. Therefore, we performed this study in the yolk albumen homogenate, enabling us to distinguish between both hypotheses.…”

Revisiting mechanisms and functions of prenatal hormone-mediated maternal effects using avian species as a model

“…Although the machinery is thus in place to perceive maternal hormonal cues, there is also growing evidence that embryos rapidly and heavily metabolize maternal hormones in the egg [62,78,79]. Intriguingly, most of these metabolites are (supposedly) inactive forms of the free steroids, such as conjugated forms or etiocholanolone in the case of androgens [80].…”

Maternal Temperature, Corticosterone, and Body Condition as Mediators of Maternal Effects in Jacky Dragons (Amphibolurus muricatus)

“…Furthermore, variation in maternal corticosterone may not be reflected in embryos if mothers regulate corticosterone deposition into eggs or if embryos metabolize maternal hormones (Moore and Johnston 2008;Paitz et al 2012), both of which may be more true for natural than experimental levels of corticosterone. Recent studies on reptiles and birds have shown that hormones such as estradiol and testosterone are metabolized early in development and that embryonic uptake of metabolites occurs much later (Paitz et al 2012;Kumar et al 2018Kumar et al , 2019. This, coupled with the possibility that embryos may actively control their endocrine environment, suggests that embryos can be buffered from the effects of maternal corticosterone Kumar et al 2018Kumar et al , 2019.…”