MATERNAL EFFECTS MEDIATED BY EGG QUALITY IN THE YELLOW-LEGGED GULL (LARUS MICHAHELLIS)

The genetic component and the environmental conditions have cascading effects on individual phenotypic values. However, the most pervasive influences, which are potentially both genetic and environmental, are provided by ‘maternal effects’ that arise when the phenotype and environment of mothers markedly affect individual developmental trajectories of their progeny, potentially exerting carrying-over effects on the subsequent generation and maximizing their own fitness. In birds, mothers can adjust offspring phenotype by modulation of egg size and quality, in terms of variation in the concentration of quantitatively major (i.e. lipids, albumen) or minor (e.g. antioxidants, steroid hormones) components. Because antioxidants and steroid hormones are pivotal for the development of morphological and behavioural traits and their concentrations can vary according to laying order and in relation to maternal environmental experience, they are fundamental to interpret maternal allocation strategies to the eggs in an evolutionary perspective. The studies presented in this thesis aimed at assessing maternal effects mediated by egg quality in the yellow-legged gull (Larus michahellis), manipulating egg composition by directly injecting in the yolk a physiological dose of different maternal compounds, including vitamin E, testosterone and corticosterone. The consequences of variation in egg composition were investigated on markers of oxidative status, telomere length, morphology and behavioural lateralization of both embryos and hatchlings. Moreover, because egg antioxidants are thought to operate in an integrated way in the complex defence system against oxidative stress, I scrutinized the patterns of correlation between several egg antioxidants, markers of oxidative status in liver and brain, and morphology of embryos. In the same study, I also analysed the consequences of an experimental supplementation of vitamin E concentration on the distribution of other antioxidants and their effects on embryo phenotypic traits. Finally, the last work of this thesis aimed at testing non-consumptive effects of predation, which was experimentally increased by presenting stuffed predators to breeding adults before egg laying, on egg concentration of steroid hormones, and clutch and egg size. The results on the administration of vitamin E in egg yolk showed a positive effect on somatic growth during embryonic development and on oxidative status soon after hatching, although the supplemental dose did not affect oxidative damage and telomere length during early life stages. From the study focused on the correlations among egg antioxidants and phenotypic embryo traits emerged that embryo morphology was positively associated with the concentration of antioxidants and negatively associated with markers of oxidative status. While antioxidant concentrations were positively correlated both within and between organs, this was not the case for markers of oxidative status; in addition, weak relationships existed between antioxidants and markers of oxidative status. Moreover, the experimental increase of vitamin E concentration did not affect the distribution of other antioxidants and their effects on embryo phenotypic traits. The evidence on the consequences of testosterone supplementation was complex and differed in relation to phenotypic traits and ontogenetic stage. Indeed, testosterone had contrasting effects on embryo oxidative status depending on the focal organ, ameliorating oxidative status in the liver but not in the brain. In addition, although the supplemental dose of testosterone was observed to boost body size during embryonic development, it had a negative effect on embryo brain size and on somatic growth soon after hatching. Furthermore, the present thesis adds information to the scant knowledge about the organizational effects of steroid hormones on the development of lateralization; indeed, the physiological dose of testosterone was found to increase the consistency of lateralization whereas the direction of lateralization was affected by corticosterone treatment. Finally, the last work of this thesis, where non-consumptive predator effects were tested, showed that mothers exposed to increased predation risk laid larger eggs compared to control ones but did not modulate egg steroid hormone concentration and clutch size. In conclusion, these studies shed new light on the consequences of maternal effects mediated by egg quality during both pre- and postnatal periods in the yellow-legged gull, showing that maternal transmission to the eggs may entail substantial costs and benefits for the progeny.