Differential effects of steroid hormones on levels of broad-sense heritability in a wild bird: possible mechanism of environment × genetic variance interaction?

Abstract

Genetic variation is one of the key concepts in evolutionary biology and an important prerequisite of evolutionary change. However, we know very little about processes that modulate its levels in wild populations. In particular, we still are to understand why genetic variances often depend on environmental conditions. One of possible environment-sensitive modulators of observed levels of genetic variance are maternal effects. In this study we attempt to experimentally test the hypothesis that maternally transmitted agents (e.g. hormones) may influence the expression of genetic variance in quantitative traits in the offspring. We manipulated the levels of steroid hormones (testosterone and corticosterone) in eggs laid by blue tits in a wild population. Our experimental setup allowed for full crossing of genetic and rearing effects with the experimental manipulation. We observed that birds treated with corticosterone exhibited a significant decrease in broad-sense genetic variance of tarsus length, and an increase in this component in body mass on the 2nd day post-hatching. Our study indicates, that maternally transmitted substances such as hormones may have measurable impact on the levels of genetic variance and hence, on the evolutionary potential of quantitative traits.

Fig. 1. Schematic summary of the experiment and different measurements performed during its course.

The top time axis is not to scale. Represented egg/chick numbers may differ between nests; also, hatching failure or nestling mortality may lead to some individuals dropping out

Fig. 2. Heritabilities and genetic variances of the studied traits (symbol-coded) split by the treatment group (colour-coded).

Genetic variances (A) are presented on trait-specific scales. Heritabilities (B) come from models with fully heterogenous genetic variance structures. Whiskers represent SE (estimated via the delta method for heritability or estimated by the model for variances)

Fig. 3. Results of bootstrapping simulations of the tarsus length variable mixed models.

Histograms represent simulated distributions of differences in genetic variances calculated between treatment groups indicated on the x-axes. Overlaid on the histograms are: kernel density estimators (blue solid line), zero lines (blue dotted) and differences in variances from original mixed models (red solid lines). Panels represent differences in genetic variance between control and CORT groups (A), TESTO and CORT groups (B), and control and TESTO groups (C)

Fig. 4. Possible scenarios of hormonal treatment affecting the variance in maternal effects, depending on distribution of hormone concentrations and the function mapping hormone levels to phenotype values.

Symmetrical distributions – with linear (A), sigmoidal (saturating; B) or heterogenous linear (concentration-dependent) mapping functions (C), and a linear mapping function with skewed (log-normal) distributions (D)