Yolk hormones in birds

BY JAIME MURIEL

Currently there is strong evidence that mothers can adjust the development and phenotype of their offspring based on environmental conditions. These adjusting mechanisms, named “maternal effects”, consist in an epigenetic modification on the offspring phenotype that results in an increase of transgenerational plasticity, ultimately affecting individual fitness (Mousseau and Fox, 1998).

In the context of evolutionary ecology, there is increasing interest in the mechanisms behind these maternal effects, and specifically, in the mediating role of steroid hormones (reviewed in Groothuis et al., 2005; Groothuis and Schwabl, 2008; Gil, 2008). Steroid hormones are great candidates for studying these adaptive changes, as they regulate key developmental processes and also mediate adult responses to environmental changes (Groothuis and Schwabl, 2008). Birds, and in our particular case the Spotless starling (Sturnus unicolor), are excellent models to study maternal effects because their embryos develop outside the mother's body in a sealed environment. This facilitates sample collection for descriptive studies and performing experimental manipulations of maternal effects by modifying yolk composition (Groothuis et al. 2005).

Our studies deep into the causes and consequences of the phenotypic plasticity caused by the variation in the levels of androgens in the egg with the aim of understanding how these processes mediate individual responses to life-history trade-offs (Groothuis et al. 2005; Gil, 2008; Williams, 2012). For the moment, the main results we have obtained during my PhD have allowed us to disentangle the separate and combined effects of the main steroids in the egg yolks of starlings (testosterone and androstenedione, Muriel et al. 2013). Also, we have discovered the existence of complex dose-response effects of these hormones on several traits of nestlings (Muriel et al. 2015a), and how the androgen effects vary according to the environmental context (Muriel et al. 2015b). In addition, we have observed that the cost of androgen levels, often emerge as sex-specific immunosuppression, as high hormone levels negative impact some components of the innate and adaptive axes of the immune system (e.g. lysozyme activity of the plasma and cell-mediated immunoresponsiveness) in male nestlings but not in females (Muriel et al. 2017). In the near future we will explore the effects of these hormones on adult phenotype and several life history traits in this model species.

References 

• Gil D. (2008). Hormones in avian eggs: physiology, ecology and behavior. Adv Study Behav. 38, 337–398.
• Groothuis, T. G. G., Müller, W., von Engelhardt, N., Carere, C., and Eising, C. (2005). Maternal hormones as a tool to adjust offspring phenotype in avian species. Neurosci. Biobehav. Rev. 29, 329–352.
• Groothuis TGG, Schwabl H. (2008). Hormone-mediated maternal effect in birds: mechanisms matter but what do we know of them? Philos Trans R Soc Lond B Biol Sci. 363:1647–1661.
• Mousseau, T. A., and Fox, C.W. (1998). ‘‘Maternal Effects as Adaptations. ’’ Oxford University Press, New York, NY.
• Muriel J, Pérez-Rodríguez L, Puerta M, Gil D (2013) Differential effects of yolk testosterone and androstenedione in embryo development and nestling growth in the spotless starling (Sturnus unicolor). Gen. Comp. Endocrinol. 194: 175-182.
• Muriel  J., Pérez-Rodríguez L., Puerta M., Gil D. (2015a). Diverse dose-response effects of yolk androgens on embryo development and nestling growth in a wild passerine. J. Exp. Biol. 218: 2241-2249.
• Muriel J., Salmon P., Nunez-Buiza A., de Salas F., Pérez-Rodríguez L., Puerta M., Gil D. (2015b). Context-dependent effects of yolk androgens on nestling growth and immune function in a multibrooded passerine. J. Evol. Biol. 28: 1476-1488.
• Muriel J., Pérez-Rodríguez L., Ortiz-Santaliestra M.E., Puerta M., Gil D. (2017). Sex-specific effects of high yolk androgen levels on constitutive and cell-mediated immune responses in nestlings of an altricial passerine. Physiol. Biochem. Zool. 90(1): 106-117.
• Williams TD (2012) Hormones, life-history, and phenotypic variation: Opportunities in evolutionary avian endocrinology. Gen. Comp. Endocrinol, 176(3): 286-295.