Changes in Sex-Bias in Gene Expression in Migratory and Resident Oncorhynchus mykiss during the First Two Years of Development

Many organisms show patterns of sex-bias in gene expression. However, few studies have measured how sex-bias in expression varies over development, and how it varies between populations under different selection regimes. To that end we used an RNA-seq approach to measure sex-bias in gene expression in the brain transcriptome of migratory steelhead and resident rainbow trout (Oncorhynchus mykiss). Samples were taken from six time points during the first two years of development. A total of 1,716 (4.6% of assembly) genes showed evidence of sex-bias in gene expression in at least one time point, with a similar number of genes upregulated in males (883) and females (857). The majority (96. 7%) of sex-biased genes were differentially expressed in the second year of development. This coincides with the point in development when migratory O. mykiss are undergoing saltwater adaptation (smoltification). Moreover, more genes showed sex-bias in expression in progeny from the migrant population (1,638) than progeny from the resident population (87) suggesting these genes may also be important in the decision to migrate or remain resident. Mapping of differentially expressed genes to the O. mykiss genome revealed the X chromosome was enriched for female upregulated genes, and this may indicate a lack of dosage compensation in rainbow trout. Pathway analysis suggested that the functions of sex-biased genes varies, with some genes involved in functions such as spermatogenesis, as well as others with less obvious sex-biased functions such as calcium signaling, and phototransduction. Ultimately, this study adds to a growing body of literature that documents sex-bias in gene expression. Our study suggests that patterns of sex-bias vary between different populations, and changes rapidly during development.