Seasonal neurogenomic changes provide genetic links between male song and testosterone-mediated neurogenesis in a wild songbird

Seasonal changes in testosterone mediate the transition from reduced singing in the non-breeding season to high song rates in the early breeding season in male temperate songbirds, a process accompanied by marked neurogenesis in song control regions of the brain. However, the resulting genetic changes and their association with the subsequent behavioral, neuroanatomical and reproductive changes remain poorly understood. Here, we compared gene expression in HVC, a major song control nucleus, between the non-breeding and breeding season in male northern house wrens ( Troglodytes aedon ) and we examined associations with HVC volume, plasma testosterone concentrations, and testes size. Differential gene expression analysis identified three genes (CLIP4, FAM169A, and TTR) consistently linked to seasonal transitions from non-breeding to three breeding stages (pre-laying, egg-laying and incubation). Notably, TTR, which transports thyroid hormone (TH), was highly expressed in the nonbreeding season, consistent with a possible role of TH in regulating seasonal shifts in song output and structure. We also identified seasonal changes in networks of genes related to neural connectivity, cellular restructuring, and cell migration. Weighted gene co-expression network analysis (WGCNA) revealed gene clusters specifically correlated with testosterone and HVC volume. Testosterone-associated genes included genes involved in neural circuit remodeling, chromatin organization, and cytoskeletal dynamics, consistent with testosterone-mediated regulation of these neuroanatomical changes. Genes linked to seasonal increases in HVC volume were involved in neuronal restructuring and neuron migration, implicating these genes in seasonal neurogenesis. Together, our findings link novel gene expression patterns to hormone regulation and neurogenesis underlying seasonal transitions in birdsong.