Dynamics of sex-biased gene expression over development in the stick insect Timema californicum

Sexually dimorphic phenotypes are thought to arise primarily from sex-biased gene expression during development. Major changes in developmental strategies, such as the shift from hemimetabolous to holometabolous development, are therefore expected to have profound consequences for the dynamics of sex-biased gene expression. However, no studies have previously examined sex-biased gene expression over development in hemimetabolous insects, precluding comparisons between developmental strategies. Here we characterized sex-biased gene expression at three developmental stages in a hemimetabolous stick insect (Timema californicum): hatchlings, juveniles, and adults. As expected, the proportion of sex-biased genes gradually increased over development. Sex-biased genes identified at early developmental stages were generally consistently male- or female- biased at later stages, suggesting their importance in sexual differentiation. We then compared the dynamics of sex-biased gene expression over development in T. californicum to those of the holometabolous fly Drosophila melanogaster by reanalyzing publicly available RNA-seq data from third instar larval, pupal and adult stages. In D. melanogaster, sex-biased gene expression increases abruptly at the adult stage when morphological sexual dimorphism is manifested. This supports the prediction that sex-biased gene expression mirrors phenotypic sexual dimorphism. Our study details for the first time the dynamics of sex-biased gene expression over development in a hemimetabolous insect and suggests that these dynamics differ extensively between holometabolous and hemimetabolous species.