Head Color Morph- and Sex-Specific Differences in Follistatin Gene Expression in the Gouldian Finch Brain.

The Gouldian finch exhibits a head color polymorphism, which is tightly coupled to distinct differences in aggression, stress responses, and parental care. In competitive environments, red-headed birds are more aggressive, are less parental, and exhibit a heightened stress response relative to black-headed birds. The head color polymorphism has been associated with genetic variation in a small noncoding region near the follistatin (FST) gene. Given the regulatory nature of this gene, we hypothesized that FST mRNA would be differentially expressed in association with morph- and sex-specific differences in the brains of red- and black-headed morphs. To test this hypothesis, we analyzed FST gene expression in the Gouldian finch brain using RNAscope in situ hybridization assay. Our results revealed significant differences in FST gene expression between morphs and sex. Specifically, black-headed morphs, regardless of sex, displayed higher FST mRNA levels across multiple brain regions associated with aggression, stress responses, and parental care compared to red-headed morphs. Furthermore, males consistently showed greater FST mRNA levels within the same morph type than females. These findings suggest that head color morph- and sex-specific differences in FST gene expression may underlie the observed morph- and sex-specific differences in aggression, stress responses, and parental care in Gouldian finches.