Meeting Abstract
In many vertebrates, tradeoffs between competitive and parental strategies are well-understood to be mediated by sex steroids. Disruptive selection leading to alternative behavioral strategies may thus act on the mechanisms underlying steroid signaling and metabolism. White-throated sparrows exhibit two color morphs with different responses to territorial intrusion; white-striped (WS) birds sing more than tan-striped (TS) birds. Although WS males have higher levels of plasma testosterone (T) and estradiol than TS males, experimental equalization of these hormones does not abolish morph differences in singing. We recently showed that the expression of estrogen receptor alpha (ERα) in the brain, which could confer sensitivity to sex steroids, differs between the morphs and may drive the behavioral polymorphism. First, the ERα promoter region contains fixed polymorphisms that affect transcription efficiency in vitro. Second, in a free-living population, local expression of ERα depends strongly on morph and predicts territorial singing. Differential ERα expression is particularly striking in the medial amygdala; WS birds have three times more ERα mRNA than TS birds. This difference persists during the non-breeding season and is unaffected by exogenous T treatment. Finally, weighted gene co-expression analysis of the medial amygdala identified one module of genes significantly correlated with both morph and song. Within this module, the gene most highly correlated with territorial singing was ESR1, which encodes ER&alpha. Together, these results suggest that ERα plays a pivotal role in territorial displays and may be a target of disruptive selection leading to alternative behavioral strategies. Our future directions include a more detailed analysis of the ERα promoter regions to determine the molecular basis of differential expression.
