ABSTRACT: Despite sharing much of their genomes, males and females are often highly dimorphic, reflecting at least in part the resolution of sexual conflict in response to sexually antagonistic selection. Sexual dimorphism arises owing to sex differences in gene expression, and steroid hormones are often invoked as a proximate cause of sexual dimorphism. Experimental elevation of androgens can lead to masculinization of behavior, physiology, and gene expression, but knowledge of the role of hormones remains incomplete, including how the sexes differ in their gene expression in response to exposure to hormones. We addressed these questions in a bird species with a long history of behavioral endocrinological and ecological study, the dark-eyed junco (Junco hyemalis), using a species-specific microarray. Focusing on two brain regions involved in sexually dimorphic behavior and regulation of hormone secretion, we identified 1,639 genes that differed in expression by sex in the ventromedial telencephalon and 768 in hypothalamus. In response to experimentally elevated testosterone, females exhibited a more “male-like” expression pattern than control females; unexpectedly, male expression patterns became more “female-like” rather than hyper-masculinized when compared to control males. This sex difference in pattern arose both because testosterone altered regulation of different genes in each sex and because testosterone altered regulation of the same genes differentially, i.e., up in one sex, down in the other. Hormonally regulated gene expression is a key genetic and physiological mechanism underlying sexual dimorphism, and further study should help to explain how it relates to the resolution of sexual conflict. Hypothalamus: 24 samples were analyzed, all were biological (not technical) replicates. 6 from males treated with testosterone [MT], 6 from control males [MC], 6 from females treated with testosterone [FT], and 6 from control females [FC]. All hybridizations were paired, and all treatment groups were compared, but no sample was analyzed more than once. Ventromedial telencephalon: 24 samples were analyzed, all were biological (not technical) replicates. 6 from males treated with testosterone [MT], 6 from control males [MC], 6 from females treated with testosterone [FT], and 6 from control females [FC]. All hybridizations were paired, and all treatment groups were compared, but no sample was analyzed more than once.
