Maternal social environment shapes yolk testosterone allocation and embryonic neural gene expression in tree swallows
A female's behavioral and physiological response to her environment can have lasting effects on the development of her offspring. This type of non-genetic inheritance (or maternal effect) occurs across taxa but is particularly well-studied in birds due to the relative ease of measuring maternal hormones in externally developing eggs. For example, females breeding in areas of high social competition allocate more of the hormone testosterone (T) to their egg yolks. These high-T offspring exhibit numerous phenotypic changes, including faster juvenile growth and enhanced aggression well into adulthood, which are potentially beneficial traits in high-competition areas. Thus, females may be able to communicate the current social environment to their offspring and prepare them for adverse conditions. Yet, the proximate mechanisms underlying how maternal hormones affect offspring are largely ignored, impairing our understanding of an ecologically important source of phenotypic plasticity. Here, we explored the effects of the maternal social environment on yolk T allocation and genome-wide patterns of neural gene expression in male and female tree swallow (Tachycineta bicolor) embryos. We observed a population of free-living tree swallows breeding at sites with variable breeding densities. We performed behavioral observations to record the rate of physical aggression at these sites, which was significantly higher at high density sites. We collected eggs at two developmental timepoints: the day laid to measure maternally derived T concentrations and embryonic day 11 (ED11) to measure gene expression patterns in whole brains using RNA-seq. We took a genome-wide approach to help clarify the potentially diverse mechanisms regulated by early T exposure across variable social environments. We additionally molecularly sexed the embryos to explore sex-specific effects. Overall design: Whole brains collected from embryos incubated to embryonic day 11 exposed to natural variation in maternal breeding density.