Hollmen, L. | Search bioRxiv

bioRxiv2024

The effects of early-life ambient temperature on the gut microbiome of a wild bird? an experimental approach

Vertebrate gut microbiome has significant effects on host development, health, and fitness. Multiple external factors contribute to gut microbiome variation, and the role of ambient temperature has gained increasing attention. Yet, temperature effects are often tested in extremes and in captive systems. We experimentally studied the effect of subtle temperature decreases during post-natal development on gut microbiome diversity and composition in wild pied flycatchers (Ficedula hypoleuca). We also performed partial cross-fostering to study the relative contribution of genetic and rearing environment on microbiome. Nest-box cold treatment did not influence gut microbiome diversity or composition, which may be due to the small temperature change, ontogenetic stage, or other factors, such as diet, causing large variation in the data. Rearing environment explained more of the variation in gut microbiome than genetic background, but the variance explained was relatively small. Future studies need to further address the drivers of the large intraspecific variation in microbiome in natural populations.

ecology↗

bioRxiv2023

From maternal glucocorticoid and thyroid hormones to epigenetic regulation of gene expression: an experimental study in a wild bird species

Offspring phenotype at birth is determined by its genotype and the prenatal environment including exposure to maternal hormones. Variation in both maternal glucocorticoids and thyroid hormones can affect offspring phenotype. However, the underlying molecular mechanisms shaping the offspring phenotype, especially those contributing to long-lasting effects, remain unclear. Epigenetic changes (such as DNA methylation) have been postulated as mediators of long-lasting effects of early-life environment. In this study, we determined the effects of elevated prenatal glucocorticoid and thyroid hormones on handling stress response (breath rate), DNA methylation and gene expression of glucocorticoid receptor (GCR) and thyroid hormone receptor (THR) in great tit (Parus major). Eggs were injected before incubation onset with corticosterone (main avian glucocorticoid) and/or thyroid hormones (thyroxine and triiodothyronine) to simulate variation in maternal hormone deposition. Breath rate during handling and gene expression of GCR and THR were evaluated 14 days after hatching. Methylation status of GCR and THR genes were analyzed from the longitudinal blood samples taken 7 and 14 days after hatching, as well as in the following autumn. Elevated prenatal corticosterone level significantly increased the breath rate during handling, indicating enhanced stress response and/or metabolism. Prenatal corticosterone manipulation had CpG-site-specific effects on DNA methylation at the GCR putative promoter region, while it did not significantly affect GCR gene expression. GCR expression was negatively associated with earlier hatching date and chick size. THR methylation or expression did not exhibit any significant relationship with the hormonal treatments or the examined covariates, suggesting that TH signaling may be more robust due to its crucial role in development. This study supports the view that maternal corticosterone may influence offspring metabolism and stress response via epigenetic alterations, yet their possible adaptive role in optimizing offspring phenotype to the prevailing conditions, context-dependency, and the underlying molecular interplay needs further research.

evolutionary biology↗