Evolutionary origin of sex-related genes in the mouse brain

With the aim of elucidating the evolutionary process of sexual dimorphism in the brain at the molecular level, we conducted genomic comparisons of a set of genes expressed in a sexually different manner in the mouse brain with all genes from other species of eukaryotes. First, seventeen protein-coding genes whose levels of mRNA expression in the brain differed between male and female mice have been known according to the currently available microarray data, and we designated these genes operationally as "sex-related genes in the mouse brain". Next, we estimated the time when these sex-related genes in the mouse brain emerged in the evolutionary process of eukaryotes by examining the presence or absence of the orthologues in the 26 eukaryotic species whose genome sequences are available. As a result, we found that the ten sex-related genes in the mouse brain emerged after the divergence of urochordates and mammals whereas the other seven sex-related genes in the mouse brain emerged before the divergence of urochordates and mammals. In particular, five sex-related genes out of the ten genes in the mouse brain emerged just before the appearance of bony fish which have phenotypic sexual dimorphism in the brain. Interestingly, three of these five sex-related genes that emerged during this period were classified into the "protein binding" function category. Moreover, all of these three genes were expected to have the functions that are related to cell-cell communications in the brain according to the gene expression patterns and/or functional information of these genes. These findings suggest that the orthologues of the sex-related genes in the mouse brain that emerged just before the divergence of bony fish might have essential roles in the evolution of the sexual dimorphism in the brain forming protein-protein interactions.