Potential for sexual conflict assessed via testosterone-mediated transcriptional changes in liver and muscle of a songbird

Abstract

Males and females can be highly dimorphic in metabolism and physiology despite sharing nearly identical genomes, and both sexes respond phenotypically to elevated testosterone, a steroid hormone that alters gene expression. Only recently has it become possible to learn how a hormone such as testosterone affects global gene expression in non-model systems, and whether it affects the same genes in males and females. To investigate the transcriptional mechanisms by which testosterone exerts its metabolic and physiological effects on the periphery, we compared gene expression by sex and in response to experimentally elevated testosterone in a well-studied bird species, the dark-eyed junco (Junco hyemalis). We identified 291 genes in the liver and 658 in the pectoralis muscle that were differentially expressed between males and females. In addition, we identified 1727 genes that were differentially expressed between testosterone-treated and control individuals in at least one tissue and sex. Testosterone treatment altered the expression of only 128 genes in both males and females in the same tissue, and 847 genes were affected significantly differently by testosterone treatment in the two sexes. These substantial differences in transcriptional response to testosterone suggest that males and females may employ different pathways when responding to elevated testosterone, despite the fact that many phenotypic effects of experimentally elevated testosterone are similar in both sexes. In contrast, of the 121 genes that were affected by testosterone treatment in both sexes, 78% were regulated in the same direction (e.g. either higher or lower in testosterone-treated than control individuals) in both males and females. Thus, it appears that testosterone acts through both unique and shared transcriptional pathways in males and females, suggesting multiple mechanisms by which sexual conflict can be mediated.

Keywords: Gene expression; Hormones; Sexual conflict.

Fig. 1.

Sex differences in gene expression. Differences in gene expression between the sexes are represented by heat maps that show scaled individual expression scores (Z-scores) for significantly differentially expressed genes in the liver (A) and pectoralis (B). Yellow represents high gene expression; blue represents low expression scaled to the levels of expression for each gene. (C) Venn diagram shows the overlap in significant genes between the two tissues. Each column represents an individual, and each row a gene.

Fig. 2.

Gene expression in response to testosterone treatment in each sex. Differences in gene expression between testosterone-treated and control individuals in both the liver (left column) and the pectoralis (middle column) in females (A–C) and males (D–F). Heat maps show scaled individual expression scores (Z-scores) for genes that were significantly differentially expressed between testosterone treated and control individuals in each sex (A,B,D,E). Each column represents an individual, and each row a gene. Yellow represents high gene expression, and blue represents low expression scaled to the levels of expression for each gene. (C,F) Venn diagrams show the overlap of significant genes within each contrast between the tissues. See Results and supplementary material Table S1 for more information.

Fig. 3.

Comparing the effect of testosterone treatment in males and females. Venn diagrams for (A) liver and (B) pectoralis showing the number of genes significantly differentially expressed between testosterone-treated and control individuals in males and females, and those with a significant sex-by-treatment interaction effect.