A new mouse line based on the Gpr101 promoter drives expression of Cre in medium spiny neurons of the striatum.

In recent times, major advances have been made towards understanding sexual dimorphism in the brain on a molecular basis. This thesis summarises my modest contributions to these endeavours. Sexual dimorphisms are manifested throughout the spectrum of biological complexity, and can be studied by numerous approaches. The approach of this thesis is to explore sex-biased gene expression in mammalian somatic tissues. Paper I describes an evolutionarily conserved sexual gene expression pattern in the primate brain. Conserved sex-biased genes may underlie important sex differences in neurobiology. In Paper II, Y-chromosome genes expressed across several regions of the human male brain during mid-gestation are identified. Such genes may play male-specific roles during brain development. The studies of Papers III and IV explore sex-biased gene expression in several somatic tissues from mouse. The amount of genes with sex-biased expression varied in different brain regions. The striatum was particularly interesting, with an order of magnitude increase in the number of sex-biased genes as compared to the other included brain regions. Of potentially wider significance are my observations regarding the transcriptional regulation of domains that escape X-chromosome inactivation (XCI). Specifically, I provide the first evidence that long non-coding RNAs (lncRNAs) transcribe together with protein-coding genes in XCI-escaping domains. This raises the possibility that lncRNAs mediate the transcriptional regulation of XCI-escaping domains. I also present evidence that the mouse X-chromosome has undergone both feminisation and de-masculinisation during evolution, as indicated by the sex-skewed regulation of genes on this chromosome. This finding is relevant for understanding the selective forces that shaped the mammalian X-chromosome. In the final chapter, Paper V, the generation of a novel transgenic mouse line, Gpr101-Cre, is described. Its progeny can be used for functional studies of striatum, a brain structure with major sexual dimorphism, as is further demonstrated in the Papers of this thesis.