Divergence in expression of a singing-related neuroplasticity gene in the brains of 2 <i>Ficedula</i> flycatchers and their hybrids

Species-specific sexual traits facilitate species-assortative mating by reducing mating across species and reducing hybrid sexual attractiveness. For learned sexual traits, such as song in oscine birds, species distinctiveness can be eroded when species co-occur. Transcriptional regulatory divergence in brain regions involved in sensory learning is hypothesized to maintain species distinctiveness, but relatively few studies have compared gene expression in relevant brain regions between closely related species. Species differences in song are an important premating reproductive barrier between the collared (Ficedula albicollis) and pied flycatcher (F. hypoleuca). Here, we compare brain gene expression in adult males from each species and their naturally occurring F1 hybrids. We report overall conserved expression across species in a portion of the brain containing regions and nuclei known to be involved in song responses and learning. Further, among those genes that were differentially expressed between species, we find largely intermediate expression in hybrids. A single gene, SYT4 (synaptotagmin 4), known to be singing-associated, both was differentially expressed and has a putative upstream transcriptional regulatory factor containing fixed differences between the 2 species. Although a finer-scale investigation limited to song-specific regions may reveal further species differences, our findings provide insight into regulatory divergence in the brain between closely related species.